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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="X-UA-Compatible" content="IE=Edge" /> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>6. Simple statements — Python 2.7.16 documentation</title> <link rel="stylesheet" href="../_static/classic.css" type="text/css" /> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" /> <script type="text/javascript" id="documentation_options" data-url_root="../" src="../_static/documentation_options.js"></script> <script type="text/javascript" src="../_static/jquery.js"></script> <script type="text/javascript" src="../_static/underscore.js"></script> <script type="text/javascript" src="../_static/doctools.js"></script> <script type="text/javascript" src="../_static/sidebar.js"></script> <link rel="search" type="application/opensearchdescription+xml" title="Search within Python 2.7.16 documentation" href="../_static/opensearch.xml"/> <link rel="author" title="About these documents" href="../about.html" /> <link rel="index" title="Index" href="../genindex.html" /> <link rel="search" title="Search" href="../search.html" /> <link rel="copyright" title="Copyright" href="../copyright.html" /> <link rel="next" title="7. Compound statements" href="compound_stmts.html" /> <link rel="prev" title="5. Expressions" href="expressions.html" /> <link rel="shortcut icon" type="image/png" href="../_static/py.png" /> <link rel="canonical" href="https://docs.python.org/2/reference/simple_stmts.html" /> <script type="text/javascript" src="../_static/copybutton.js"></script> </head><body> <div class="related" role="navigation" aria-label="related navigation"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" accesskey="I">index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="compound_stmts.html" title="7. Compound statements" accesskey="N">next</a> |</li> <li class="right" > <a href="expressions.html" title="5. Expressions" accesskey="P">previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="https://www.python.org/">Python</a> »</li> <li> <a href="../index.html">Python 2.7.16 documentation</a> » </li> <li class="nav-item nav-item-1"><a href="index.html" accesskey="U">The Python Language Reference</a> »</li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body" role="main"> <div class="section" id="simple-statements"> <span id="simple"></span><h1>6. Simple statements<a class="headerlink" href="#simple-statements" title="Permalink to this headline">¶</a></h1> <p id="index-0">Simple statements are comprised within a single logical line. Several simple statements may occur on a single line separated by semicolons. The syntax for simple statements is:</p> <pre> <strong id="grammar-token-simple_stmt">simple_stmt</strong> ::= <a class="reference internal" href="#grammar-token-expression_stmt"><code class="xref docutils literal notranslate"><span class="pre">expression_stmt</span></code></a> | <a class="reference internal" href="#grammar-token-assert_stmt"><code class="xref docutils literal notranslate"><span class="pre">assert_stmt</span></code></a> | <a class="reference internal" href="#grammar-token-assignment_stmt"><code class="xref docutils literal notranslate"><span class="pre">assignment_stmt</span></code></a> | <a class="reference internal" href="#grammar-token-augmented_assignment_stmt"><code class="xref docutils literal notranslate"><span class="pre">augmented_assignment_stmt</span></code></a> | <a class="reference internal" href="#grammar-token-pass_stmt"><code class="xref docutils literal notranslate"><span class="pre">pass_stmt</span></code></a> | <a class="reference internal" href="#grammar-token-del_stmt"><code class="xref docutils literal notranslate"><span class="pre">del_stmt</span></code></a> | <a class="reference internal" href="#grammar-token-print_stmt"><code class="xref docutils literal notranslate"><span class="pre">print_stmt</span></code></a> | <a class="reference internal" href="#grammar-token-return_stmt"><code class="xref docutils literal notranslate"><span class="pre">return_stmt</span></code></a> | <a class="reference internal" href="#grammar-token-yield_stmt"><code class="xref docutils literal notranslate"><span class="pre">yield_stmt</span></code></a> | <a class="reference internal" href="#grammar-token-raise_stmt"><code class="xref docutils literal notranslate"><span class="pre">raise_stmt</span></code></a> | <a class="reference internal" href="#grammar-token-break_stmt"><code class="xref docutils literal notranslate"><span class="pre">break_stmt</span></code></a> | <a class="reference internal" href="#grammar-token-continue_stmt"><code class="xref docutils literal notranslate"><span class="pre">continue_stmt</span></code></a> | <a class="reference internal" href="#grammar-token-import_stmt"><code class="xref docutils literal notranslate"><span class="pre">import_stmt</span></code></a> | <code class="xref docutils literal notranslate"><span class="pre">future_stmt</span></code> | <a class="reference internal" href="#grammar-token-global_stmt"><code class="xref docutils literal notranslate"><span class="pre">global_stmt</span></code></a> | <a class="reference internal" href="#grammar-token-exec_stmt"><code class="xref docutils literal notranslate"><span class="pre">exec_stmt</span></code></a> </pre> <div class="section" id="expression-statements"> <span id="exprstmts"></span><h2>6.1. Expression statements<a class="headerlink" href="#expression-statements" title="Permalink to this headline">¶</a></h2> <p id="index-1">Expression statements are used (mostly interactively) to compute and write a value, or (usually) to call a procedure (a function that returns no meaningful result; in Python, procedures return the value <code class="docutils literal notranslate"><span class="pre">None</span></code>). Other uses of expression statements are allowed and occasionally useful. The syntax for an expression statement is:</p> <pre> <strong id="grammar-token-expression_stmt">expression_stmt</strong> ::= <a class="reference internal" href="expressions.html#grammar-token-expression_list"><code class="xref docutils literal notranslate"><span class="pre">expression_list</span></code></a> </pre> <p>An expression statement evaluates the expression list (which may be a single expression).</p> <p id="index-2">In interactive mode, if the value is not <code class="docutils literal notranslate"><span class="pre">None</span></code>, it is converted to a string using the built-in <a class="reference internal" href="../library/functions.html#repr" title="repr"><code class="xref py py-func docutils literal notranslate"><span class="pre">repr()</span></code></a> function and the resulting string is written to standard output (see section <a class="reference internal" href="#print"><span class="std std-ref">The print statement</span></a>) on a line by itself. (Expression statements yielding <code class="docutils literal notranslate"><span class="pre">None</span></code> are not written, so that procedure calls do not cause any output.)</p> </div> <div class="section" id="assignment-statements"> <span id="assignment"></span><h2>6.2. Assignment statements<a class="headerlink" href="#assignment-statements" title="Permalink to this headline">¶</a></h2> <p id="index-3">Assignment statements are used to (re)bind names to values and to modify attributes or items of mutable objects:</p> <pre> <strong id="grammar-token-assignment_stmt">assignment_stmt</strong> ::= (<a class="reference internal" href="#grammar-token-target_list"><code class="xref docutils literal notranslate"><span class="pre">target_list</span></code></a> "=")+ (<a class="reference internal" href="expressions.html#grammar-token-expression_list"><code class="xref docutils literal notranslate"><span class="pre">expression_list</span></code></a> | <a class="reference internal" href="expressions.html#grammar-token-yield_expression"><code class="xref docutils literal notranslate"><span class="pre">yield_expression</span></code></a>) <strong id="grammar-token-target_list">target_list </strong> ::= <a class="reference internal" href="#grammar-token-target"><code class="xref docutils literal notranslate"><span class="pre">target</span></code></a> ("," <a class="reference internal" href="#grammar-token-target"><code class="xref docutils literal notranslate"><span class="pre">target</span></code></a>)* [","] <strong id="grammar-token-target">target </strong> ::= <a class="reference internal" href="lexical_analysis.html#grammar-token-identifier"><code class="xref docutils literal notranslate"><span class="pre">identifier</span></code></a> | "(" <a class="reference internal" href="#grammar-token-target_list"><code class="xref docutils literal notranslate"><span class="pre">target_list</span></code></a> ")" | "[" [<a class="reference internal" href="#grammar-token-target_list"><code class="xref docutils literal notranslate"><span class="pre">target_list</span></code></a>] "]" | <a class="reference internal" href="expressions.html#grammar-token-attributeref"><code class="xref docutils literal notranslate"><span class="pre">attributeref</span></code></a> | <a class="reference internal" href="expressions.html#grammar-token-subscription"><code class="xref docutils literal notranslate"><span class="pre">subscription</span></code></a> | <a class="reference internal" href="expressions.html#grammar-token-slicing"><code class="xref docutils literal notranslate"><span class="pre">slicing</span></code></a> </pre> <p>(See section <a class="reference internal" href="expressions.html#primaries"><span class="std std-ref">Primaries</span></a> for the syntax definitions for the last three symbols.)</p> <p id="index-4">An assignment statement evaluates the expression list (remember that this can be a single expression or a comma-separated list, the latter yielding a tuple) and assigns the single resulting object to each of the target lists, from left to right.</p> <p id="index-5">Assignment is defined recursively depending on the form of the target (list). When a target is part of a mutable object (an attribute reference, subscription or slicing), the mutable object must ultimately perform the assignment and decide about its validity, and may raise an exception if the assignment is unacceptable. The rules observed by various types and the exceptions raised are given with the definition of the object types (see section <a class="reference internal" href="datamodel.html#types"><span class="std std-ref">The standard type hierarchy</span></a>).</p> <p id="index-6">Assignment of an object to a target list is recursively defined as follows.</p> <ul class="simple"> <li>If the target list is a single target: The object is assigned to that target.</li> <li>If the target list is a comma-separated list of targets: The object must be an iterable with the same number of items as there are targets in the target list, and the items are assigned, from left to right, to the corresponding targets.</li> </ul> <p>Assignment of an object to a single target is recursively defined as follows.</p> <ul> <li><p class="first">If the target is an identifier (name):</p> <blockquote> <div></div></blockquote> <ul class="simple" id="index-7"> <li>If the name does not occur in a <a class="reference internal" href="#global"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code></a> statement in the current code block: the name is bound to the object in the current local namespace.</li> <li>Otherwise: the name is bound to the object in the current global namespace.</li> </ul> <p id="index-8">The name is rebound if it was already bound. This may cause the reference count for the object previously bound to the name to reach zero, causing the object to be deallocated and its destructor (if it has one) to be called.</p> </li> <li><p class="first">If the target is a target list enclosed in parentheses or in square brackets: The object must be an iterable with the same number of items as there are targets in the target list, and its items are assigned, from left to right, to the corresponding targets.</p> </li> <li id="index-9"><p class="first">If the target is an attribute reference: The primary expression in the reference is evaluated. It should yield an object with assignable attributes; if this is not the case, <a class="reference internal" href="../library/exceptions.html#exceptions.TypeError" title="exceptions.TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> is raised. That object is then asked to assign the assigned object to the given attribute; if it cannot perform the assignment, it raises an exception (usually but not necessarily <a class="reference internal" href="../library/exceptions.html#exceptions.AttributeError" title="exceptions.AttributeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">AttributeError</span></code></a>).</p> <p id="attr-target-note">Note: If the object is a class instance and the attribute reference occurs on both sides of the assignment operator, the RHS expression, <code class="docutils literal notranslate"><span class="pre">a.x</span></code> can access either an instance attribute or (if no instance attribute exists) a class attribute. The LHS target <code class="docutils literal notranslate"><span class="pre">a.x</span></code> is always set as an instance attribute, creating it if necessary. Thus, the two occurrences of <code class="docutils literal notranslate"><span class="pre">a.x</span></code> do not necessarily refer to the same attribute: if the RHS expression refers to a class attribute, the LHS creates a new instance attribute as the target of the assignment:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Cls</span><span class="p">:</span> <span class="n">x</span> <span class="o">=</span> <span class="mi">3</span> <span class="c1"># class variable</span> <span class="n">inst</span> <span class="o">=</span> <span class="n">Cls</span><span class="p">()</span> <span class="n">inst</span><span class="o">.</span><span class="n">x</span> <span class="o">=</span> <span class="n">inst</span><span class="o">.</span><span class="n">x</span> <span class="o">+</span> <span class="mi">1</span> <span class="c1"># writes inst.x as 4 leaving Cls.x as 3</span> </pre></div> </div> <p>This description does not necessarily apply to descriptor attributes, such as properties created with <a class="reference internal" href="../library/functions.html#property" title="property"><code class="xref py py-func docutils literal notranslate"><span class="pre">property()</span></code></a>.</p> </li> <li id="index-10"><p class="first">If the target is a subscription: The primary expression in the reference is evaluated. It should yield either a mutable sequence object (such as a list) or a mapping object (such as a dictionary). Next, the subscript expression is evaluated.</p> <p id="index-11">If the primary is a mutable sequence object (such as a list), the subscript must yield a plain integer. If it is negative, the sequence’s length is added to it. The resulting value must be a nonnegative integer less than the sequence’s length, and the sequence is asked to assign the assigned object to its item with that index. If the index is out of range, <a class="reference internal" href="../library/exceptions.html#exceptions.IndexError" title="exceptions.IndexError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">IndexError</span></code></a> is raised (assignment to a subscripted sequence cannot add new items to a list).</p> <p id="index-12">If the primary is a mapping object (such as a dictionary), the subscript must have a type compatible with the mapping’s key type, and the mapping is then asked to create a key/datum pair which maps the subscript to the assigned object. This can either replace an existing key/value pair with the same key value, or insert a new key/value pair (if no key with the same value existed).</p> </li> <li id="index-13"><p class="first">If the target is a slicing: The primary expression in the reference is evaluated. It should yield a mutable sequence object (such as a list). The assigned object should be a sequence object of the same type. Next, the lower and upper bound expressions are evaluated, insofar they are present; defaults are zero and the sequence’s length. The bounds should evaluate to (small) integers. If either bound is negative, the sequence’s length is added to it. The resulting bounds are clipped to lie between zero and the sequence’s length, inclusive. Finally, the sequence object is asked to replace the slice with the items of the assigned sequence. The length of the slice may be different from the length of the assigned sequence, thus changing the length of the target sequence, if the object allows it.</p> </li> </ul> <div class="impl-detail compound"> <p><strong>CPython implementation detail:</strong> In the current implementation, the syntax for targets is taken to be the same as for expressions, and invalid syntax is rejected during the code generation phase, causing less detailed error messages.</p> </div> <p>WARNING: Although the definition of assignment implies that overlaps between the left-hand side and the right-hand side are ‘safe’ (for example <code class="docutils literal notranslate"><span class="pre">a,</span> <span class="pre">b</span> <span class="pre">=</span> <span class="pre">b,</span> <span class="pre">a</span></code> swaps two variables), overlaps <em>within</em> the collection of assigned-to variables are not safe! For instance, the following program prints <code class="docutils literal notranslate"><span class="pre">[0,</span> <span class="pre">2]</span></code>:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">x</span> <span class="o">=</span> <span class="p">[</span><span class="mi">0</span><span class="p">,</span> <span class="mi">1</span><span class="p">]</span> <span class="n">i</span> <span class="o">=</span> <span class="mi">0</span> <span class="n">i</span><span class="p">,</span> <span class="n">x</span><span class="p">[</span><span class="n">i</span><span class="p">]</span> <span class="o">=</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">2</span> <span class="nb">print</span> <span class="n">x</span> </pre></div> </div> <div class="section" id="augmented-assignment-statements"> <span id="augassign"></span><h3>6.2.1. Augmented assignment statements<a class="headerlink" href="#augmented-assignment-statements" title="Permalink to this headline">¶</a></h3> <p id="index-14">Augmented assignment is the combination, in a single statement, of a binary operation and an assignment statement:</p> <pre> <strong id="grammar-token-augmented_assignment_stmt">augmented_assignment_stmt</strong> ::= <a class="reference internal" href="#grammar-token-augtarget"><code class="xref docutils literal notranslate"><span class="pre">augtarget</span></code></a> <a class="reference internal" href="#grammar-token-augop"><code class="xref docutils literal notranslate"><span class="pre">augop</span></code></a> (<a class="reference internal" href="expressions.html#grammar-token-expression_list"><code class="xref docutils literal notranslate"><span class="pre">expression_list</span></code></a> | <a class="reference internal" href="expressions.html#grammar-token-yield_expression"><code class="xref docutils literal notranslate"><span class="pre">yield_expression</span></code></a>) <strong id="grammar-token-augtarget">augtarget </strong> ::= <a class="reference internal" href="lexical_analysis.html#grammar-token-identifier"><code class="xref docutils literal notranslate"><span class="pre">identifier</span></code></a> | <a class="reference internal" href="expressions.html#grammar-token-attributeref"><code class="xref docutils literal notranslate"><span class="pre">attributeref</span></code></a> | <a class="reference internal" href="expressions.html#grammar-token-subscription"><code class="xref docutils literal notranslate"><span class="pre">subscription</span></code></a> | <a class="reference internal" href="expressions.html#grammar-token-slicing"><code class="xref docutils literal notranslate"><span class="pre">slicing</span></code></a> <strong id="grammar-token-augop">augop </strong> ::= "+=" | "-=" | "*=" | "/=" | "//=" | "%=" | "**=" | ">>=" | "<<=" | "&=" | "^=" | "|=" </pre> <p>(See section <a class="reference internal" href="expressions.html#primaries"><span class="std std-ref">Primaries</span></a> for the syntax definitions for the last three symbols.)</p> <p>An augmented assignment evaluates the target (which, unlike normal assignment statements, cannot be an unpacking) and the expression list, performs the binary operation specific to the type of assignment on the two operands, and assigns the result to the original target. The target is only evaluated once.</p> <p>An augmented assignment expression like <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">+=</span> <span class="pre">1</span></code> can be rewritten as <code class="docutils literal notranslate"><span class="pre">x</span> <span class="pre">=</span> <span class="pre">x</span> <span class="pre">+</span> <span class="pre">1</span></code> to achieve a similar, but not exactly equal effect. In the augmented version, <code class="docutils literal notranslate"><span class="pre">x</span></code> is only evaluated once. Also, when possible, the actual operation is performed <em>in-place</em>, meaning that rather than creating a new object and assigning that to the target, the old object is modified instead.</p> <p>With the exception of assigning to tuples and multiple targets in a single statement, the assignment done by augmented assignment statements is handled the same way as normal assignments. Similarly, with the exception of the possible <em>in-place</em> behavior, the binary operation performed by augmented assignment is the same as the normal binary operations.</p> <p>For targets which are attribute references, the same <a class="reference internal" href="#attr-target-note"><span class="std std-ref">caveat about class and instance attributes</span></a> applies as for regular assignments.</p> </div> </div> <div class="section" id="the-assert-statement"> <span id="assert"></span><h2>6.3. The <a class="reference internal" href="#assert"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">assert</span></code></a> statement<a class="headerlink" href="#the-assert-statement" title="Permalink to this headline">¶</a></h2> <p id="index-15">Assert statements are a convenient way to insert debugging assertions into a program:</p> <pre> <strong id="grammar-token-assert_stmt">assert_stmt</strong> ::= "assert" <a class="reference internal" href="expressions.html#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> ["," <a class="reference internal" href="expressions.html#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a>] </pre> <p>The simple form, <code class="docutils literal notranslate"><span class="pre">assert</span> <span class="pre">expression</span></code>, is equivalent to</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="n">__debug__</span><span class="p">:</span> <span class="k">if</span> <span class="ow">not</span> <span class="n">expression</span><span class="p">:</span> <span class="k">raise</span> <span class="ne">AssertionError</span> </pre></div> </div> <p>The extended form, <code class="docutils literal notranslate"><span class="pre">assert</span> <span class="pre">expression1,</span> <span class="pre">expression2</span></code>, is equivalent to</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="n">__debug__</span><span class="p">:</span> <span class="k">if</span> <span class="ow">not</span> <span class="n">expression1</span><span class="p">:</span> <span class="k">raise</span> <span class="ne">AssertionError</span><span class="p">(</span><span class="n">expression2</span><span class="p">)</span> </pre></div> </div> <p id="index-16">These equivalences assume that <a class="reference internal" href="../library/constants.html#__debug__" title="__debug__"><code class="xref py py-const docutils literal notranslate"><span class="pre">__debug__</span></code></a> and <a class="reference internal" href="../library/exceptions.html#exceptions.AssertionError" title="exceptions.AssertionError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">AssertionError</span></code></a> refer to the built-in variables with those names. In the current implementation, the built-in variable <a class="reference internal" href="../library/constants.html#__debug__" title="__debug__"><code class="xref py py-const docutils literal notranslate"><span class="pre">__debug__</span></code></a> is <code class="docutils literal notranslate"><span class="pre">True</span></code> under normal circumstances, <code class="docutils literal notranslate"><span class="pre">False</span></code> when optimization is requested (command line option -O). The current code generator emits no code for an assert statement when optimization is requested at compile time. Note that it is unnecessary to include the source code for the expression that failed in the error message; it will be displayed as part of the stack trace.</p> <p>Assignments to <a class="reference internal" href="../library/constants.html#__debug__" title="__debug__"><code class="xref py py-const docutils literal notranslate"><span class="pre">__debug__</span></code></a> are illegal. The value for the built-in variable is determined when the interpreter starts.</p> </div> <div class="section" id="the-pass-statement"> <span id="pass"></span><h2>6.4. The <a class="reference internal" href="#pass"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">pass</span></code></a> statement<a class="headerlink" href="#the-pass-statement" title="Permalink to this headline">¶</a></h2> <pre id="index-17"> <strong id="grammar-token-pass_stmt">pass_stmt</strong> ::= "pass" </pre> <p><a class="reference internal" href="#pass"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">pass</span></code></a> is a null operation — when it is executed, nothing happens. It is useful as a placeholder when a statement is required syntactically, but no code needs to be executed, for example:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">arg</span><span class="p">):</span> <span class="k">pass</span> <span class="c1"># a function that does nothing (yet)</span> <span class="k">class</span> <span class="nc">C</span><span class="p">:</span> <span class="k">pass</span> <span class="c1"># a class with no methods (yet)</span> </pre></div> </div> </div> <div class="section" id="the-del-statement"> <span id="del"></span><h2>6.5. The <a class="reference internal" href="#del"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">del</span></code></a> statement<a class="headerlink" href="#the-del-statement" title="Permalink to this headline">¶</a></h2> <pre id="index-18"> <strong id="grammar-token-del_stmt">del_stmt</strong> ::= "del" <a class="reference internal" href="#grammar-token-target_list"><code class="xref docutils literal notranslate"><span class="pre">target_list</span></code></a> </pre> <p>Deletion is recursively defined very similar to the way assignment is defined. Rather than spelling it out in full details, here are some hints.</p> <p>Deletion of a target list recursively deletes each target, from left to right.</p> <p id="index-19">Deletion of a name removes the binding of that name from the local or global namespace, depending on whether the name occurs in a <a class="reference internal" href="#global"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code></a> statement in the same code block. If the name is unbound, a <a class="reference internal" href="../library/exceptions.html#exceptions.NameError" title="exceptions.NameError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">NameError</span></code></a> exception will be raised.</p> <p id="index-20">It is illegal to delete a name from the local namespace if it occurs as a free variable in a nested block.</p> <p id="index-21">Deletion of attribute references, subscriptions and slicings is passed to the primary object involved; deletion of a slicing is in general equivalent to assignment of an empty slice of the right type (but even this is determined by the sliced object).</p> </div> <div class="section" id="the-print-statement"> <span id="print"></span><h2>6.6. The <a class="reference internal" href="#print"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">print</span></code></a> statement<a class="headerlink" href="#the-print-statement" title="Permalink to this headline">¶</a></h2> <pre id="index-22"> <strong id="grammar-token-print_stmt">print_stmt</strong> ::= "print" ([<a class="reference internal" href="expressions.html#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> ("," <a class="reference internal" href="expressions.html#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a>)* [","]] | ">>" <a class="reference internal" href="expressions.html#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> [("," <a class="reference internal" href="expressions.html#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a>)+ [","]]) </pre> <p><a class="reference internal" href="#print"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">print</span></code></a> evaluates each expression in turn and writes the resulting object to standard output (see below). If an object is not a string, it is first converted to a string using the rules for string conversions. The (resulting or original) string is then written. A space is written before each object is (converted and) written, unless the output system believes it is positioned at the beginning of a line. This is the case (1) when no characters have yet been written to standard output, (2) when the last character written to standard output is a whitespace character except <code class="docutils literal notranslate"><span class="pre">'</span> <span class="pre">'</span></code>, or (3) when the last write operation on standard output was not a <a class="reference internal" href="#print"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">print</span></code></a> statement. (In some cases it may be functional to write an empty string to standard output for this reason.)</p> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last">Objects which act like file objects but which are not the built-in file objects often do not properly emulate this aspect of the file object’s behavior, so it is best not to rely on this.</p> </div> <p id="index-23">A <code class="docutils literal notranslate"><span class="pre">'\n'</span></code> character is written at the end, unless the <a class="reference internal" href="#print"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">print</span></code></a> statement ends with a comma. This is the only action if the statement contains just the keyword <a class="reference internal" href="#print"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">print</span></code></a>.</p> <p id="index-24">Standard output is defined as the file object named <code class="docutils literal notranslate"><span class="pre">stdout</span></code> in the built-in module <a class="reference internal" href="../library/sys.html#module-sys" title="sys: Access system-specific parameters and functions."><code class="xref py py-mod docutils literal notranslate"><span class="pre">sys</span></code></a>. If no such object exists, or if it does not have a <code class="xref py py-meth docutils literal notranslate"><span class="pre">write()</span></code> method, a <a class="reference internal" href="../library/exceptions.html#exceptions.RuntimeError" title="exceptions.RuntimeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">RuntimeError</span></code></a> exception is raised.</p> <p id="index-25"><a class="reference internal" href="#print"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">print</span></code></a> also has an extended form, defined by the second portion of the syntax described above. This form is sometimes referred to as “<a class="reference internal" href="#print"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">print</span></code></a> chevron.” In this form, the first expression after the <code class="docutils literal notranslate"><span class="pre">>></span></code> must evaluate to a “file-like” object, specifically an object that has a <code class="xref py py-meth docutils literal notranslate"><span class="pre">write()</span></code> method as described above. With this extended form, the subsequent expressions are printed to this file object. If the first expression evaluates to <code class="docutils literal notranslate"><span class="pre">None</span></code>, then <code class="docutils literal notranslate"><span class="pre">sys.stdout</span></code> is used as the file for output.</p> </div> <div class="section" id="the-return-statement"> <span id="return"></span><h2>6.7. The <a class="reference internal" href="#return"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code></a> statement<a class="headerlink" href="#the-return-statement" title="Permalink to this headline">¶</a></h2> <pre id="index-26"> <strong id="grammar-token-return_stmt">return_stmt</strong> ::= "return" [<a class="reference internal" href="expressions.html#grammar-token-expression_list"><code class="xref docutils literal notranslate"><span class="pre">expression_list</span></code></a>] </pre> <p><a class="reference internal" href="#return"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code></a> may only occur syntactically nested in a function definition, not within a nested class definition.</p> <p>If an expression list is present, it is evaluated, else <code class="docutils literal notranslate"><span class="pre">None</span></code> is substituted.</p> <p><a class="reference internal" href="#return"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code></a> leaves the current function call with the expression list (or <code class="docutils literal notranslate"><span class="pre">None</span></code>) as return value.</p> <p id="index-27">When <a class="reference internal" href="#return"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code></a> passes control out of a <a class="reference internal" href="compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> statement with a <a class="reference internal" href="compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> clause, that <a class="reference internal" href="compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> clause is executed before really leaving the function.</p> <p>In a generator function, the <a class="reference internal" href="#return"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code></a> statement is not allowed to include an <a class="reference internal" href="expressions.html#grammar-token-expression_list"><code class="xref std std-token docutils literal notranslate"><span class="pre">expression_list</span></code></a>. In that context, a bare <a class="reference internal" href="#return"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code></a> indicates that the generator is done and will cause <a class="reference internal" href="../library/exceptions.html#exceptions.StopIteration" title="exceptions.StopIteration"><code class="xref py py-exc docutils literal notranslate"><span class="pre">StopIteration</span></code></a> to be raised.</p> </div> <div class="section" id="the-yield-statement"> <span id="yield"></span><h2>6.8. The <a class="reference internal" href="#yield"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code></a> statement<a class="headerlink" href="#the-yield-statement" title="Permalink to this headline">¶</a></h2> <pre id="index-28"> <strong id="grammar-token-yield_stmt">yield_stmt</strong> ::= <a class="reference internal" href="expressions.html#grammar-token-yield_expression"><code class="xref docutils literal notranslate"><span class="pre">yield_expression</span></code></a> </pre> <p>The <a class="reference internal" href="#yield"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code></a> statement is only used when defining a generator function, and is only used in the body of the generator function. Using a <a class="reference internal" href="#yield"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code></a> statement in a function definition is sufficient to cause that definition to create a generator function instead of a normal function.</p> <p>When a generator function is called, it returns an iterator known as a generator iterator, or more commonly, a generator. The body of the generator function is executed by calling the generator’s <a class="reference internal" href="expressions.html#generator.next" title="generator.next"><code class="xref py py-meth docutils literal notranslate"><span class="pre">next()</span></code></a> method repeatedly until it raises an exception.</p> <p>When a <a class="reference internal" href="#yield"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code></a> statement is executed, the state of the generator is frozen and the value of <a class="reference internal" href="expressions.html#grammar-token-expression_list"><code class="xref std std-token docutils literal notranslate"><span class="pre">expression_list</span></code></a> is returned to <a class="reference internal" href="expressions.html#generator.next" title="generator.next"><code class="xref py py-meth docutils literal notranslate"><span class="pre">next()</span></code></a>’s caller. By “frozen” we mean that all local state is retained, including the current bindings of local variables, the instruction pointer, and the internal evaluation stack: enough information is saved so that the next time <a class="reference internal" href="expressions.html#generator.next" title="generator.next"><code class="xref py py-meth docutils literal notranslate"><span class="pre">next()</span></code></a> is invoked, the function can proceed exactly as if the <a class="reference internal" href="#yield"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code></a> statement were just another external call.</p> <p>As of Python version 2.5, the <a class="reference internal" href="#yield"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code></a> statement is now allowed in the <a class="reference internal" href="compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> clause of a <a class="reference internal" href="compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> … <a class="reference internal" href="compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> construct. If the generator is not resumed before it is finalized (by reaching a zero reference count or by being garbage collected), the generator-iterator’s <code class="xref py py-meth docutils literal notranslate"><span class="pre">close()</span></code> method will be called, allowing any pending <a class="reference internal" href="compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> clauses to execute.</p> <p>For full details of <a class="reference internal" href="#yield"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code></a> semantics, refer to the <a class="reference internal" href="expressions.html#yieldexpr"><span class="std std-ref">Yield expressions</span></a> section.</p> <div class="admonition note"> <p class="first admonition-title">Note</p> <p>In Python 2.2, the <a class="reference internal" href="#yield"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code></a> statement was only allowed when the <code class="docutils literal notranslate"><span class="pre">generators</span></code> feature has been enabled. This <code class="docutils literal notranslate"><span class="pre">__future__</span></code> import statement was used to enable the feature:</p> <div class="last highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">__future__</span> <span class="k">import</span> <span class="n">generators</span> </pre></div> </div> </div> <div class="admonition seealso"> <p class="first admonition-title">See also</p> <dl class="last docutils"> <dt><span class="target" id="index-29"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0255"><strong>PEP 255</strong></a> - Simple Generators</dt> <dd>The proposal for adding generators and the <a class="reference internal" href="#yield"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code></a> statement to Python.</dd> <dt><span class="target" id="index-30"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0342"><strong>PEP 342</strong></a> - Coroutines via Enhanced Generators</dt> <dd>The proposal that, among other generator enhancements, proposed allowing <a class="reference internal" href="#yield"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code></a> to appear inside a <a class="reference internal" href="compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> … <a class="reference internal" href="compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> block.</dd> </dl> </div> </div> <div class="section" id="the-raise-statement"> <span id="raise"></span><h2>6.9. The <a class="reference internal" href="#raise"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">raise</span></code></a> statement<a class="headerlink" href="#the-raise-statement" title="Permalink to this headline">¶</a></h2> <pre id="index-31"> <strong id="grammar-token-raise_stmt">raise_stmt</strong> ::= "raise" [<a class="reference internal" href="expressions.html#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> ["," <a class="reference internal" href="expressions.html#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> ["," <a class="reference internal" href="expressions.html#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a>]]] </pre> <p>If no expressions are present, <a class="reference internal" href="#raise"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">raise</span></code></a> re-raises the last exception that was active in the current scope. If no exception is active in the current scope, a <a class="reference internal" href="../library/exceptions.html#exceptions.TypeError" title="exceptions.TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> exception is raised indicating that this is an error (if running under IDLE, a <a class="reference internal" href="../library/queue.html#Queue.Empty" title="Queue.Empty"><code class="xref py py-exc docutils literal notranslate"><span class="pre">Queue.Empty</span></code></a> exception is raised instead).</p> <p>Otherwise, <a class="reference internal" href="#raise"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">raise</span></code></a> evaluates the expressions to get three objects, using <code class="docutils literal notranslate"><span class="pre">None</span></code> as the value of omitted expressions. The first two objects are used to determine the <em>type</em> and <em>value</em> of the exception.</p> <p>If the first object is an instance, the type of the exception is the class of the instance, the instance itself is the value, and the second object must be <code class="docutils literal notranslate"><span class="pre">None</span></code>.</p> <p>If the first object is a class, it becomes the type of the exception. The second object is used to determine the exception value: If it is an instance of the class, the instance becomes the exception value. If the second object is a tuple, it is used as the argument list for the class constructor; if it is <code class="docutils literal notranslate"><span class="pre">None</span></code>, an empty argument list is used, and any other object is treated as a single argument to the constructor. The instance so created by calling the constructor is used as the exception value.</p> <p id="index-32">If a third object is present and not <code class="docutils literal notranslate"><span class="pre">None</span></code>, it must be a traceback object (see section <a class="reference internal" href="datamodel.html#types"><span class="std std-ref">The standard type hierarchy</span></a>), and it is substituted instead of the current location as the place where the exception occurred. If the third object is present and not a traceback object or <code class="docutils literal notranslate"><span class="pre">None</span></code>, a <a class="reference internal" href="../library/exceptions.html#exceptions.TypeError" title="exceptions.TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> exception is raised. The three-expression form of <a class="reference internal" href="#raise"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">raise</span></code></a> is useful to re-raise an exception transparently in an except clause, but <a class="reference internal" href="#raise"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">raise</span></code></a> with no expressions should be preferred if the exception to be re-raised was the most recently active exception in the current scope.</p> <p>Additional information on exceptions can be found in section <a class="reference internal" href="executionmodel.html#exceptions"><span class="std std-ref">Exceptions</span></a>, and information about handling exceptions is in section <a class="reference internal" href="compound_stmts.html#try"><span class="std std-ref">The try statement</span></a>.</p> </div> <div class="section" id="the-break-statement"> <span id="break"></span><h2>6.10. The <a class="reference internal" href="#break"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">break</span></code></a> statement<a class="headerlink" href="#the-break-statement" title="Permalink to this headline">¶</a></h2> <pre id="index-33"> <strong id="grammar-token-break_stmt">break_stmt</strong> ::= "break" </pre> <p><a class="reference internal" href="#break"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">break</span></code></a> may only occur syntactically nested in a <a class="reference internal" href="compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code></a> or <a class="reference internal" href="compound_stmts.html#while"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">while</span></code></a> loop, but not nested in a function or class definition within that loop.</p> <p id="index-34">It terminates the nearest enclosing loop, skipping the optional <a class="reference internal" href="compound_stmts.html#else"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">else</span></code></a> clause if the loop has one.</p> <p id="index-35">If a <a class="reference internal" href="compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code></a> loop is terminated by <a class="reference internal" href="#break"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">break</span></code></a>, the loop control target keeps its current value.</p> <p id="index-36">When <a class="reference internal" href="#break"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">break</span></code></a> passes control out of a <a class="reference internal" href="compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> statement with a <a class="reference internal" href="compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> clause, that <a class="reference internal" href="compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> clause is executed before really leaving the loop.</p> </div> <div class="section" id="the-continue-statement"> <span id="continue"></span><h2>6.11. The <a class="reference internal" href="#continue"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">continue</span></code></a> statement<a class="headerlink" href="#the-continue-statement" title="Permalink to this headline">¶</a></h2> <pre id="index-37"> <strong id="grammar-token-continue_stmt">continue_stmt</strong> ::= "continue" </pre> <p><a class="reference internal" href="#continue"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">continue</span></code></a> may only occur syntactically nested in a <a class="reference internal" href="compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code></a> or <a class="reference internal" href="compound_stmts.html#while"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">while</span></code></a> loop, but not nested in a function or class definition or <a class="reference internal" href="compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> clause within that loop. It continues with the next cycle of the nearest enclosing loop.</p> <p>When <a class="reference internal" href="#continue"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">continue</span></code></a> passes control out of a <a class="reference internal" href="compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> statement with a <a class="reference internal" href="compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> clause, that <a class="reference internal" href="compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> clause is executed before really starting the next loop cycle.</p> </div> <div class="section" id="the-import-statement"> <span id="from"></span><span id="import"></span><h2>6.12. The <a class="reference internal" href="#import"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">import</span></code></a> statement<a class="headerlink" href="#the-import-statement" title="Permalink to this headline">¶</a></h2> <pre id="index-38"> <strong id="grammar-token-import_stmt">import_stmt </strong> ::= "import" <a class="reference internal" href="#grammar-token-module"><code class="xref docutils literal notranslate"><span class="pre">module</span></code></a> ["as" <a class="reference internal" href="#grammar-token-name"><code class="xref docutils literal notranslate"><span class="pre">name</span></code></a>] ( "," <a class="reference internal" href="#grammar-token-module"><code class="xref docutils literal notranslate"><span class="pre">module</span></code></a> ["as" <a class="reference internal" href="#grammar-token-name"><code class="xref docutils literal notranslate"><span class="pre">name</span></code></a>] )* | "from" <a class="reference internal" href="#grammar-token-relative_module"><code class="xref docutils literal notranslate"><span class="pre">relative_module</span></code></a> "import" <a class="reference internal" href="lexical_analysis.html#grammar-token-identifier"><code class="xref docutils literal notranslate"><span class="pre">identifier</span></code></a> ["as" <a class="reference internal" href="#grammar-token-name"><code class="xref docutils literal notranslate"><span class="pre">name</span></code></a>] ( "," <a class="reference internal" href="lexical_analysis.html#grammar-token-identifier"><code class="xref docutils literal notranslate"><span class="pre">identifier</span></code></a> ["as" <a class="reference internal" href="#grammar-token-name"><code class="xref docutils literal notranslate"><span class="pre">name</span></code></a>] )* | "from" <a class="reference internal" href="#grammar-token-relative_module"><code class="xref docutils literal notranslate"><span class="pre">relative_module</span></code></a> "import" "(" <a class="reference internal" href="lexical_analysis.html#grammar-token-identifier"><code class="xref docutils literal notranslate"><span class="pre">identifier</span></code></a> ["as" <a class="reference internal" href="#grammar-token-name"><code class="xref docutils literal notranslate"><span class="pre">name</span></code></a>] ( "," <a class="reference internal" href="lexical_analysis.html#grammar-token-identifier"><code class="xref docutils literal notranslate"><span class="pre">identifier</span></code></a> ["as" <a class="reference internal" href="#grammar-token-name"><code class="xref docutils literal notranslate"><span class="pre">name</span></code></a>] )* [","] ")" | "from" <a class="reference internal" href="#grammar-token-module"><code class="xref docutils literal notranslate"><span class="pre">module</span></code></a> "import" "*" <strong id="grammar-token-module">module </strong> ::= (<a class="reference internal" href="lexical_analysis.html#grammar-token-identifier"><code class="xref docutils literal notranslate"><span class="pre">identifier</span></code></a> ".")* <a class="reference internal" href="lexical_analysis.html#grammar-token-identifier"><code class="xref docutils literal notranslate"><span class="pre">identifier</span></code></a> <strong id="grammar-token-relative_module">relative_module</strong> ::= "."* <a class="reference internal" href="#grammar-token-module"><code class="xref docutils literal notranslate"><span class="pre">module</span></code></a> | "."+ <strong id="grammar-token-name">name </strong> ::= <a class="reference internal" href="lexical_analysis.html#grammar-token-identifier"><code class="xref docutils literal notranslate"><span class="pre">identifier</span></code></a> </pre> <p>Import statements are executed in two steps: (1) find a module, and initialize it if necessary; (2) define a name or names in the local namespace (of the scope where the <a class="reference internal" href="#import"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">import</span></code></a> statement occurs). The statement comes in two forms differing on whether it uses the <a class="reference internal" href="#from"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">from</span></code></a> keyword. The first form (without <a class="reference internal" href="#from"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">from</span></code></a>) repeats these steps for each identifier in the list. The form with <a class="reference internal" href="#from"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">from</span></code></a> performs step (1) once, and then performs step (2) repeatedly.</p> <p id="index-39">To understand how step (1) occurs, one must first understand how Python handles hierarchical naming of modules. To help organize modules and provide a hierarchy in naming, Python has a concept of packages. A package can contain other packages and modules while modules cannot contain other modules or packages. From a file system perspective, packages are directories and modules are files.</p> <p id="index-40">Once the name of the module is known (unless otherwise specified, the term “module” will refer to both packages and modules), searching for the module or package can begin. The first place checked is <a class="reference internal" href="../library/sys.html#sys.modules" title="sys.modules"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.modules</span></code></a>, the cache of all modules that have been imported previously. If the module is found there then it is used in step (2) of import.</p> <p id="index-41">If the module is not found in the cache, then <a class="reference internal" href="../library/sys.html#sys.meta_path" title="sys.meta_path"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.meta_path</span></code></a> is searched (the specification for <a class="reference internal" href="../library/sys.html#sys.meta_path" title="sys.meta_path"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.meta_path</span></code></a> can be found in <span class="target" id="index-42"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0302"><strong>PEP 302</strong></a>). The object is a list of <a class="reference internal" href="../glossary.html#term-finder"><span class="xref std std-term">finder</span></a> objects which are queried in order as to whether they know how to load the module by calling their <code class="xref py py-meth docutils literal notranslate"><span class="pre">find_module()</span></code> method with the name of the module. If the module happens to be contained within a package (as denoted by the existence of a dot in the name), then a second argument to <code class="xref py py-meth docutils literal notranslate"><span class="pre">find_module()</span></code> is given as the value of the <code class="xref py py-attr docutils literal notranslate"><span class="pre">__path__</span></code> attribute from the parent package (everything up to the last dot in the name of the module being imported). If a finder can find the module it returns a <a class="reference internal" href="../glossary.html#term-loader"><span class="xref std std-term">loader</span></a> (discussed later) or returns <code class="docutils literal notranslate"><span class="pre">None</span></code>.</p> <p id="index-43">If none of the finders on <a class="reference internal" href="../library/sys.html#sys.meta_path" title="sys.meta_path"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.meta_path</span></code></a> are able to find the module then some implicitly defined finders are queried. Implementations of Python vary in what implicit meta path finders are defined. The one they all do define, though, is one that handles <a class="reference internal" href="../library/sys.html#sys.path_hooks" title="sys.path_hooks"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.path_hooks</span></code></a>, <a class="reference internal" href="../library/sys.html#sys.path_importer_cache" title="sys.path_importer_cache"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.path_importer_cache</span></code></a>, and <a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.path</span></code></a>.</p> <p>The implicit finder searches for the requested module in the “paths” specified in one of two places (“paths” do not have to be file system paths). If the module being imported is supposed to be contained within a package then the second argument passed to <code class="xref py py-meth docutils literal notranslate"><span class="pre">find_module()</span></code>, <code class="xref py py-attr docutils literal notranslate"><span class="pre">__path__</span></code> on the parent package, is used as the source of paths. If the module is not contained in a package then <a class="reference internal" href="../library/sys.html#sys.path" title="sys.path"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.path</span></code></a> is used as the source of paths.</p> <p>Once the source of paths is chosen it is iterated over to find a finder that can handle that path. The dict at <a class="reference internal" href="../library/sys.html#sys.path_importer_cache" title="sys.path_importer_cache"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.path_importer_cache</span></code></a> caches finders for paths and is checked for a finder. If the path does not have a finder cached then <a class="reference internal" href="../library/sys.html#sys.path_hooks" title="sys.path_hooks"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.path_hooks</span></code></a> is searched by calling each object in the list with a single argument of the path, returning a finder or raises <a class="reference internal" href="../library/exceptions.html#exceptions.ImportError" title="exceptions.ImportError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ImportError</span></code></a>. If a finder is returned then it is cached in <a class="reference internal" href="../library/sys.html#sys.path_importer_cache" title="sys.path_importer_cache"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.path_importer_cache</span></code></a> and then used for that path entry. If no finder can be found but the path exists then a value of <code class="docutils literal notranslate"><span class="pre">None</span></code> is stored in <a class="reference internal" href="../library/sys.html#sys.path_importer_cache" title="sys.path_importer_cache"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.path_importer_cache</span></code></a> to signify that an implicit, file-based finder that handles modules stored as individual files should be used for that path. If the path does not exist then a finder which always returns <code class="docutils literal notranslate"><span class="pre">None</span></code> is placed in the cache for the path.</p> <p id="index-44">If no finder can find the module then <a class="reference internal" href="../library/exceptions.html#exceptions.ImportError" title="exceptions.ImportError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ImportError</span></code></a> is raised. Otherwise some finder returned a loader whose <code class="xref py py-meth docutils literal notranslate"><span class="pre">load_module()</span></code> method is called with the name of the module to load (see <span class="target" id="index-45"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0302"><strong>PEP 302</strong></a> for the original definition of loaders). A loader has several responsibilities to perform on a module it loads. First, if the module already exists in <a class="reference internal" href="../library/sys.html#sys.modules" title="sys.modules"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.modules</span></code></a> (a possibility if the loader is called outside of the import machinery) then it is to use that module for initialization and not a new module. But if the module does not exist in <a class="reference internal" href="../library/sys.html#sys.modules" title="sys.modules"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.modules</span></code></a> then it is to be added to that dict before initialization begins. If an error occurs during loading of the module and it was added to <a class="reference internal" href="../library/sys.html#sys.modules" title="sys.modules"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.modules</span></code></a> it is to be removed from the dict. If an error occurs but the module was already in <a class="reference internal" href="../library/sys.html#sys.modules" title="sys.modules"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.modules</span></code></a> it is left in the dict.</p> <p id="index-46">The loader must set several attributes on the module. <code class="xref py py-data docutils literal notranslate"><span class="pre">__name__</span></code> is to be set to the name of the module. <code class="xref py py-data docutils literal notranslate"><span class="pre">__file__</span></code> is to be the “path” to the file unless the module is built-in (and thus listed in <a class="reference internal" href="../library/sys.html#sys.builtin_module_names" title="sys.builtin_module_names"><code class="xref py py-data docutils literal notranslate"><span class="pre">sys.builtin_module_names</span></code></a>) in which case the attribute is not set. If what is being imported is a package then <code class="xref py py-data docutils literal notranslate"><span class="pre">__path__</span></code> is to be set to a list of paths to be searched when looking for modules and packages contained within the package being imported. <code class="xref py py-data docutils literal notranslate"><span class="pre">__package__</span></code> is optional but should be set to the name of package that contains the module or package (the empty string is used for module not contained in a package). <code class="xref py py-data docutils literal notranslate"><span class="pre">__loader__</span></code> is also optional but should be set to the loader object that is loading the module.</p> <p id="index-47">If an error occurs during loading then the loader raises <a class="reference internal" href="../library/exceptions.html#exceptions.ImportError" title="exceptions.ImportError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ImportError</span></code></a> if some other exception is not already being propagated. Otherwise the loader returns the module that was loaded and initialized.</p> <p>When step (1) finishes without raising an exception, step (2) can begin.</p> <p>The first form of <a class="reference internal" href="#import"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">import</span></code></a> statement binds the module name in the local namespace to the module object, and then goes on to import the next identifier, if any. If the module name is followed by <a class="reference internal" href="compound_stmts.html#as"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">as</span></code></a>, the name following <a class="reference internal" href="compound_stmts.html#as"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">as</span></code></a> is used as the local name for the module.</p> <p id="index-48">The <a class="reference internal" href="#from"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">from</span></code></a> form does not bind the module name: it goes through the list of identifiers, looks each one of them up in the module found in step (1), and binds the name in the local namespace to the object thus found. As with the first form of <a class="reference internal" href="#import"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">import</span></code></a>, an alternate local name can be supplied by specifying “<a class="reference internal" href="compound_stmts.html#as"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">as</span></code></a> localname”. If a name is not found, <a class="reference internal" href="../library/exceptions.html#exceptions.ImportError" title="exceptions.ImportError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ImportError</span></code></a> is raised. If the list of identifiers is replaced by a star (<code class="docutils literal notranslate"><span class="pre">'*'</span></code>), all public names defined in the module are bound in the local namespace of the <a class="reference internal" href="#import"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">import</span></code></a> statement..</p> <p id="index-49">The <em>public names</em> defined by a module are determined by checking the module’s namespace for a variable named <code class="docutils literal notranslate"><span class="pre">__all__</span></code>; if defined, it must be a sequence of strings which are names defined or imported by that module. The names given in <code class="docutils literal notranslate"><span class="pre">__all__</span></code> are all considered public and are required to exist. If <code class="docutils literal notranslate"><span class="pre">__all__</span></code> is not defined, the set of public names includes all names found in the module’s namespace which do not begin with an underscore character (<code class="docutils literal notranslate"><span class="pre">'_'</span></code>). <code class="docutils literal notranslate"><span class="pre">__all__</span></code> should contain the entire public API. It is intended to avoid accidentally exporting items that are not part of the API (such as library modules which were imported and used within the module).</p> <p>The <a class="reference internal" href="#from"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">from</span></code></a> form with <code class="docutils literal notranslate"><span class="pre">*</span></code> may only occur in a module scope. If the wild card form of import — <code class="docutils literal notranslate"><span class="pre">import</span> <span class="pre">*</span></code> — is used in a function and the function contains or is a nested block with free variables, the compiler will raise a <a class="reference internal" href="../library/exceptions.html#exceptions.SyntaxError" title="exceptions.SyntaxError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">SyntaxError</span></code></a>.</p> <p id="index-50">When specifying what module to import you do not have to specify the absolute name of the module. When a module or package is contained within another package it is possible to make a relative import within the same top package without having to mention the package name. By using leading dots in the specified module or package after <a class="reference internal" href="#from"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">from</span></code></a> you can specify how high to traverse up the current package hierarchy without specifying exact names. One leading dot means the current package where the module making the import exists. Two dots means up one package level. Three dots is up two levels, etc. So if you execute <code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">.</span> <span class="pre">import</span> <span class="pre">mod</span></code> from a module in the <code class="docutils literal notranslate"><span class="pre">pkg</span></code> package then you will end up importing <code class="docutils literal notranslate"><span class="pre">pkg.mod</span></code>. If you execute <code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">..subpkg2</span> <span class="pre">import</span> <span class="pre">mod</span></code> from within <code class="docutils literal notranslate"><span class="pre">pkg.subpkg1</span></code> you will import <code class="docutils literal notranslate"><span class="pre">pkg.subpkg2.mod</span></code>. The specification for relative imports is contained within <span class="target" id="index-51"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0328"><strong>PEP 328</strong></a>.</p> <p><a class="reference internal" href="../library/importlib.html#importlib.import_module" title="importlib.import_module"><code class="xref py py-func docutils literal notranslate"><span class="pre">importlib.import_module()</span></code></a> is provided to support applications that determine which modules need to be loaded dynamically.</p> <div class="section" id="future-statements"> <span id="future"></span><h3>6.12.1. Future statements<a class="headerlink" href="#future-statements" title="Permalink to this headline">¶</a></h3> <p id="index-52">A <em class="dfn">future statement</em> is a directive to the compiler that a particular module should be compiled using syntax or semantics that will be available in a specified future release of Python. The future statement is intended to ease migration to future versions of Python that introduce incompatible changes to the language. It allows use of the new features on a per-module basis before the release in which the feature becomes standard.</p> <pre> <strong id="grammar-token-future_statement">future_statement</strong> ::= "from" "__future__" "import" feature ["as" name] ("," feature ["as" name])* | "from" "__future__" "import" "(" feature ["as" name] ("," feature ["as" name])* [","] ")" <strong id="grammar-token-feature">feature </strong> ::= identifier <strong id="id1">name </strong> ::= identifier </pre> <p>A future statement must appear near the top of the module. The only lines that can appear before a future statement are:</p> <ul class="simple"> <li>the module docstring (if any),</li> <li>comments,</li> <li>blank lines, and</li> <li>other future statements.</li> </ul> <p>The features recognized by Python 2.6 are <code class="docutils literal notranslate"><span class="pre">unicode_literals</span></code>, <code class="docutils literal notranslate"><span class="pre">print_function</span></code>, <code class="docutils literal notranslate"><span class="pre">absolute_import</span></code>, <code class="docutils literal notranslate"><span class="pre">division</span></code>, <code class="docutils literal notranslate"><span class="pre">generators</span></code>, <code class="docutils literal notranslate"><span class="pre">nested_scopes</span></code> and <code class="docutils literal notranslate"><span class="pre">with_statement</span></code>. <code class="docutils literal notranslate"><span class="pre">generators</span></code>, <code class="docutils literal notranslate"><span class="pre">with_statement</span></code>, <code class="docutils literal notranslate"><span class="pre">nested_scopes</span></code> are redundant in Python version 2.6 and above because they are always enabled.</p> <p>A future statement is recognized and treated specially at compile time: Changes to the semantics of core constructs are often implemented by generating different code. It may even be the case that a new feature introduces new incompatible syntax (such as a new reserved word), in which case the compiler may need to parse the module differently. Such decisions cannot be pushed off until runtime.</p> <p>For any given release, the compiler knows which feature names have been defined, and raises a compile-time error if a future statement contains a feature not known to it.</p> <p>The direct runtime semantics are the same as for any import statement: there is a standard module <a class="reference internal" href="../library/__future__.html#module-__future__" title="__future__: Future statement definitions"><code class="xref py py-mod docutils literal notranslate"><span class="pre">__future__</span></code></a>, described later, and it will be imported in the usual way at the time the future statement is executed.</p> <p>The interesting runtime semantics depend on the specific feature enabled by the future statement.</p> <p>Note that there is nothing special about the statement:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">__future__</span> <span class="p">[</span><span class="k">as</span> <span class="n">name</span><span class="p">]</span> </pre></div> </div> <p>That is not a future statement; it’s an ordinary import statement with no special semantics or syntax restrictions.</p> <p>Code compiled by an <a class="reference internal" href="#exec"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">exec</span></code></a> statement or calls to the built-in functions <a class="reference internal" href="../library/functions.html#compile" title="compile"><code class="xref py py-func docutils literal notranslate"><span class="pre">compile()</span></code></a> and <a class="reference internal" href="../library/functions.html#execfile" title="execfile"><code class="xref py py-func docutils literal notranslate"><span class="pre">execfile()</span></code></a> that occur in a module <code class="xref py py-mod docutils literal notranslate"><span class="pre">M</span></code> containing a future statement will, by default, use the new syntax or semantics associated with the future statement. This can, starting with Python 2.2 be controlled by optional arguments to <a class="reference internal" href="../library/functions.html#compile" title="compile"><code class="xref py py-func docutils literal notranslate"><span class="pre">compile()</span></code></a> — see the documentation of that function for details.</p> <p>A future statement typed at an interactive interpreter prompt will take effect for the rest of the interpreter session. If an interpreter is started with the <a class="reference internal" href="../using/cmdline.html#cmdoption-i"><code class="xref std std-option docutils literal notranslate"><span class="pre">-i</span></code></a> option, is passed a script name to execute, and the script includes a future statement, it will be in effect in the interactive session started after the script is executed.</p> <div class="admonition seealso"> <p class="first admonition-title">See also</p> <dl class="last docutils"> <dt><span class="target" id="index-53"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0236"><strong>PEP 236</strong></a> - Back to the __future__</dt> <dd>The original proposal for the __future__ mechanism.</dd> </dl> </div> </div> </div> <div class="section" id="the-global-statement"> <span id="global"></span><h2>6.13. The <a class="reference internal" href="#global"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code></a> statement<a class="headerlink" href="#the-global-statement" title="Permalink to this headline">¶</a></h2> <pre id="index-54"> <strong id="grammar-token-global_stmt">global_stmt</strong> ::= "global" <a class="reference internal" href="lexical_analysis.html#grammar-token-identifier"><code class="xref docutils literal notranslate"><span class="pre">identifier</span></code></a> ("," <a class="reference internal" href="lexical_analysis.html#grammar-token-identifier"><code class="xref docutils literal notranslate"><span class="pre">identifier</span></code></a>)* </pre> <p>The <a class="reference internal" href="#global"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code></a> statement is a declaration which holds for the entire current code block. It means that the listed identifiers are to be interpreted as globals. It would be impossible to assign to a global variable without <a class="reference internal" href="#global"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code></a>, although free variables may refer to globals without being declared global.</p> <p>Names listed in a <a class="reference internal" href="#global"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code></a> statement must not be used in the same code block textually preceding that <a class="reference internal" href="#global"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code></a> statement.</p> <p>Names listed in a <a class="reference internal" href="#global"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code></a> statement must not be defined as formal parameters or in a <a class="reference internal" href="compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code></a> loop control target, <a class="reference internal" href="compound_stmts.html#class"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">class</span></code></a> definition, function definition, or <a class="reference internal" href="#import"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">import</span></code></a> statement.</p> <div class="impl-detail compound"> <p><strong>CPython implementation detail:</strong> The current implementation does not enforce the latter two restrictions, but programs should not abuse this freedom, as future implementations may enforce them or silently change the meaning of the program.</p> </div> <p id="index-55"><strong>Programmer’s note:</strong> <a class="reference internal" href="#global"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code></a> is a directive to the parser. It applies only to code parsed at the same time as the <a class="reference internal" href="#global"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code></a> statement. In particular, a <a class="reference internal" href="#global"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code></a> statement contained in an <a class="reference internal" href="#exec"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">exec</span></code></a> statement does not affect the code block <em>containing</em> the <a class="reference internal" href="#exec"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">exec</span></code></a> statement, and code contained in an <a class="reference internal" href="#exec"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">exec</span></code></a> statement is unaffected by <a class="reference internal" href="#global"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">global</span></code></a> statements in the code containing the <a class="reference internal" href="#exec"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">exec</span></code></a> statement. The same applies to the <a class="reference internal" href="../library/functions.html#eval" title="eval"><code class="xref py py-func docutils literal notranslate"><span class="pre">eval()</span></code></a>, <a class="reference internal" href="../library/functions.html#execfile" title="execfile"><code class="xref py py-func docutils literal notranslate"><span class="pre">execfile()</span></code></a> and <a class="reference internal" href="../library/functions.html#compile" title="compile"><code class="xref py py-func docutils literal notranslate"><span class="pre">compile()</span></code></a> functions.</p> </div> <div class="section" id="the-exec-statement"> <span id="exec"></span><h2>6.14. The <a class="reference internal" href="#exec"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">exec</span></code></a> statement<a class="headerlink" href="#the-exec-statement" title="Permalink to this headline">¶</a></h2> <pre id="index-56"> <strong id="grammar-token-exec_stmt">exec_stmt</strong> ::= "exec" <a class="reference internal" href="expressions.html#grammar-token-or_expr"><code class="xref docutils literal notranslate"><span class="pre">or_expr</span></code></a> ["in" <a class="reference internal" href="expressions.html#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a> ["," <a class="reference internal" href="expressions.html#grammar-token-expression"><code class="xref docutils literal notranslate"><span class="pre">expression</span></code></a>]] </pre> <p>This statement supports dynamic execution of Python code. The first expression should evaluate to either a Unicode string, a <em>Latin-1</em> encoded string, an open file object, a code object, or a tuple. If it is a string, the string is parsed as a suite of Python statements which is then executed (unless a syntax error occurs). <a class="footnote-reference" href="#id3" id="id2">[1]</a> If it is an open file, the file is parsed until EOF and executed. If it is a code object, it is simply executed. For the interpretation of a tuple, see below. In all cases, the code that’s executed is expected to be valid as file input (see section <a class="reference internal" href="toplevel_components.html#file-input"><span class="std std-ref">File input</span></a>). Be aware that the <a class="reference internal" href="#return"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code></a> and <a class="reference internal" href="#yield"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code></a> statements may not be used outside of function definitions even within the context of code passed to the <a class="reference internal" href="#exec"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">exec</span></code></a> statement.</p> <p>In all cases, if the optional parts are omitted, the code is executed in the current scope. If only the first expression after <code class="docutils literal notranslate"><span class="pre">in</span></code> is specified, it should be a dictionary, which will be used for both the global and the local variables. If two expressions are given, they are used for the global and local variables, respectively. If provided, <em>locals</em> can be any mapping object. Remember that at module level, globals and locals are the same dictionary. If two separate objects are given as <em>globals</em> and <em>locals</em>, the code will be executed as if it were embedded in a class definition.</p> <p>The first expression may also be a tuple of length 2 or 3. In this case, the optional parts must be omitted. The form <code class="docutils literal notranslate"><span class="pre">exec(expr,</span> <span class="pre">globals)</span></code> is equivalent to <code class="docutils literal notranslate"><span class="pre">exec</span> <span class="pre">expr</span> <span class="pre">in</span> <span class="pre">globals</span></code>, while the form <code class="docutils literal notranslate"><span class="pre">exec(expr,</span> <span class="pre">globals,</span> <span class="pre">locals)</span></code> is equivalent to <code class="docutils literal notranslate"><span class="pre">exec</span> <span class="pre">expr</span> <span class="pre">in</span> <span class="pre">globals,</span> <span class="pre">locals</span></code>. The tuple form of <code class="docutils literal notranslate"><span class="pre">exec</span></code> provides compatibility with Python 3, where <code class="docutils literal notranslate"><span class="pre">exec</span></code> is a function rather than a statement.</p> <div class="versionchanged"> <p><span class="versionmodified">Changed in version 2.4: </span>Formerly, <em>locals</em> was required to be a dictionary.</p> </div> <p id="index-57">As a side effect, an implementation may insert additional keys into the dictionaries given besides those corresponding to variable names set by the executed code. For example, the current implementation may add a reference to the dictionary of the built-in module <a class="reference internal" href="../library/__builtin__.html#module-__builtin__" title="__builtin__: The module that provides the built-in namespace."><code class="xref py py-mod docutils literal notranslate"><span class="pre">__builtin__</span></code></a> under the key <code class="docutils literal notranslate"><span class="pre">__builtins__</span></code> (!).</p> <p id="index-58"><strong>Programmer’s hints:</strong> dynamic evaluation of expressions is supported by the built-in function <a class="reference internal" href="../library/functions.html#eval" title="eval"><code class="xref py py-func docutils literal notranslate"><span class="pre">eval()</span></code></a>. The built-in functions <a class="reference internal" href="../library/functions.html#globals" title="globals"><code class="xref py py-func docutils literal notranslate"><span class="pre">globals()</span></code></a> and <a class="reference internal" href="../library/functions.html#locals" title="locals"><code class="xref py py-func docutils literal notranslate"><span class="pre">locals()</span></code></a> return the current global and local dictionary, respectively, which may be useful to pass around for use by <a class="reference internal" href="#exec"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">exec</span></code></a>.</p> <p class="rubric">Footnotes</p> <table class="docutils footnote" frame="void" id="id3" rules="none"> <colgroup><col class="label" /><col /></colgroup> <tbody valign="top"> <tr><td class="label"><a class="fn-backref" href="#id2">[1]</a></td><td>Note that the parser only accepts the Unix-style end of line convention. If you are reading the code from a file, make sure to use <a class="reference internal" href="../glossary.html#term-universal-newlines"><span class="xref std std-term">universal newlines</span></a> mode to convert Windows or Mac-style newlines.</td></tr> </tbody> </table> </div> </div> </div> </div> </div> <div class="sphinxsidebar" role="navigation" aria-label="main navigation"> <div class="sphinxsidebarwrapper"> <h3><a href="../contents.html">Table Of Contents</a></h3> <ul> <li><a class="reference internal" href="#">6. Simple statements</a><ul> <li><a class="reference internal" href="#expression-statements">6.1. Expression statements</a></li> <li><a class="reference internal" href="#assignment-statements">6.2. Assignment statements</a><ul> <li><a class="reference internal" href="#augmented-assignment-statements">6.2.1. Augmented assignment statements</a></li> </ul> </li> <li><a class="reference internal" href="#the-assert-statement">6.3. The <code class="docutils literal notranslate"><span class="pre">assert</span></code> statement</a></li> <li><a class="reference internal" href="#the-pass-statement">6.4. The <code class="docutils literal notranslate"><span class="pre">pass</span></code> statement</a></li> <li><a class="reference internal" href="#the-del-statement">6.5. The <code class="docutils literal notranslate"><span class="pre">del</span></code> statement</a></li> <li><a class="reference internal" href="#the-print-statement">6.6. The <code class="docutils literal notranslate"><span class="pre">print</span></code> statement</a></li> <li><a class="reference internal" href="#the-return-statement">6.7. The <code class="docutils literal notranslate"><span class="pre">return</span></code> statement</a></li> <li><a class="reference internal" href="#the-yield-statement">6.8. The <code class="docutils literal notranslate"><span class="pre">yield</span></code> statement</a></li> <li><a class="reference internal" href="#the-raise-statement">6.9. The <code class="docutils literal notranslate"><span class="pre">raise</span></code> statement</a></li> <li><a class="reference internal" href="#the-break-statement">6.10. The <code class="docutils literal notranslate"><span class="pre">break</span></code> statement</a></li> <li><a class="reference internal" href="#the-continue-statement">6.11. The <code class="docutils literal notranslate"><span class="pre">continue</span></code> statement</a></li> <li><a class="reference internal" href="#the-import-statement">6.12. The <code class="docutils literal notranslate"><span class="pre">import</span></code> statement</a><ul> <li><a class="reference internal" href="#future-statements">6.12.1. Future statements</a></li> </ul> </li> <li><a class="reference internal" href="#the-global-statement">6.13. The <code class="docutils literal notranslate"><span class="pre">global</span></code> statement</a></li> <li><a class="reference internal" href="#the-exec-statement">6.14. The <code class="docutils literal notranslate"><span class="pre">exec</span></code> statement</a></li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="expressions.html" title="previous chapter">5. Expressions</a></p> <h4>Next topic</h4> <p class="topless"><a href="compound_stmts.html" title="next chapter">7. Compound statements</a></p> <div role="note" aria-label="source link"> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../_sources/reference/simple_stmts.rst.txt" rel="nofollow">Show Source</a></li> </ul> </div> <div id="searchbox" style="display: none" role="search"> <h3>Quick search</h3> <div class="searchformwrapper"> <form class="search" action="../search.html" method="get"> <input type="text" name="q" /> <input type="submit" value="Go" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> </div> </div> <script type="text/javascript">$('#searchbox').show(0);</script> </div> </div> <div class="clearer"></div> </div> <div class="related" role="navigation" aria-label="related navigation"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" >index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="compound_stmts.html" title="7. 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