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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>2. Lexical analysis — Python 3.6.7 documentation</title> <link rel="stylesheet" href="../_static/pydoctheme.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 3.6.7 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="3. 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Introduction" 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">3.6.7 Documentation</a> » </li> <li class="nav-item nav-item-1"><a href="index.html" accesskey="U">The Python Language Reference</a> »</li> <li class="right"> <div class="inline-search" style="display: none" role="search"> <form class="inline-search" action="../search.html" method="get"> <input placeholder="Quick search" 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> <script type="text/javascript">$('.inline-search').show(0);</script> | </li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body" role="main"> <div class="section" id="lexical-analysis"> <span id="lexical"></span><h1>2. Lexical analysis<a class="headerlink" href="#lexical-analysis" title="Permalink to this headline">¶</a></h1> <p id="index-0">A Python program is read by a <em>parser</em>. Input to the parser is a stream of <em>tokens</em>, generated by the <em>lexical analyzer</em>. This chapter describes how the lexical analyzer breaks a file into tokens.</p> <p>Python reads program text as Unicode code points; the encoding of a source file can be given by an encoding declaration and defaults to UTF-8, see <span class="target" id="index-1"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3120"><strong>PEP 3120</strong></a> for details. If the source file cannot be decoded, a <a class="reference internal" href="../library/exceptions.html#SyntaxError" title="SyntaxError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">SyntaxError</span></code></a> is raised.</p> <div class="section" id="line-structure"> <span id="id1"></span><h2>2.1. Line structure<a class="headerlink" href="#line-structure" title="Permalink to this headline">¶</a></h2> <p id="index-2">A Python program is divided into a number of <em>logical lines</em>.</p> <div class="section" id="logical-lines"> <span id="id2"></span><h3>2.1.1. Logical lines<a class="headerlink" href="#logical-lines" title="Permalink to this headline">¶</a></h3> <p id="index-3">The end of a logical line is represented by the token NEWLINE. Statements cannot cross logical line boundaries except where NEWLINE is allowed by the syntax (e.g., between statements in compound statements). A logical line is constructed from one or more <em>physical lines</em> by following the explicit or implicit <em>line joining</em> rules.</p> </div> <div class="section" id="physical-lines"> <span id="id3"></span><h3>2.1.2. Physical lines<a class="headerlink" href="#physical-lines" title="Permalink to this headline">¶</a></h3> <p>A physical line is a sequence of characters terminated by an end-of-line sequence. In source files and strings, any of the standard platform line termination sequences can be used - the Unix form using ASCII LF (linefeed), the Windows form using the ASCII sequence CR LF (return followed by linefeed), or the old Macintosh form using the ASCII CR (return) character. All of these forms can be used equally, regardless of platform. The end of input also serves as an implicit terminator for the final physical line.</p> <p>When embedding Python, source code strings should be passed to Python APIs using the standard C conventions for newline characters (the <code class="docutils literal notranslate"><span class="pre">\n</span></code> character, representing ASCII LF, is the line terminator).</p> </div> <div class="section" id="comments"> <span id="id4"></span><h3>2.1.3. Comments<a class="headerlink" href="#comments" title="Permalink to this headline">¶</a></h3> <p id="index-4">A comment starts with a hash character (<code class="docutils literal notranslate"><span class="pre">#</span></code>) that is not part of a string literal, and ends at the end of the physical line. A comment signifies the end of the logical line unless the implicit line joining rules are invoked. Comments are ignored by the syntax; they are not tokens.</p> </div> <div class="section" id="encoding-declarations"> <span id="encodings"></span><h3>2.1.4. Encoding declarations<a class="headerlink" href="#encoding-declarations" title="Permalink to this headline">¶</a></h3> <p id="index-5">If a comment in the first or second line of the Python script matches the regular expression <code class="docutils literal notranslate"><span class="pre">coding[=:]\s*([-\w.]+)</span></code>, this comment is processed as an encoding declaration; the first group of this expression names the encoding of the source code file. The encoding declaration must appear on a line of its own. If it is the second line, the first line must also be a comment-only line. The recommended forms of an encoding expression are</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="c1"># -*- coding: <encoding-name> -*-</span> </pre></div> </div> <p>which is recognized also by GNU Emacs, and</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="c1"># vim:fileencoding=<encoding-name></span> </pre></div> </div> <p>which is recognized by Bram Moolenaar’s VIM.</p> <p>If no encoding declaration is found, the default encoding is UTF-8. In addition, if the first bytes of the file are the UTF-8 byte-order mark (<code class="docutils literal notranslate"><span class="pre">b'\xef\xbb\xbf'</span></code>), the declared file encoding is UTF-8 (this is supported, among others, by Microsoft’s <strong class="program">notepad</strong>).</p> <p>If an encoding is declared, the encoding name must be recognized by Python. The encoding is used for all lexical analysis, including string literals, comments and identifiers.</p> </div> <div class="section" id="explicit-line-joining"> <span id="explicit-joining"></span><h3>2.1.5. Explicit line joining<a class="headerlink" href="#explicit-line-joining" title="Permalink to this headline">¶</a></h3> <p id="index-6">Two or more physical lines may be joined into logical lines using backslash characters (<code class="docutils literal notranslate"><span class="pre">\</span></code>), as follows: when a physical line ends in a backslash that is not part of a string literal or comment, it is joined with the following forming a single logical line, deleting the backslash and the following end-of-line character. For example:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="mi">1900</span> <span class="o"><</span> <span class="n">year</span> <span class="o"><</span> <span class="mi">2100</span> <span class="ow">and</span> <span class="mi">1</span> <span class="o"><=</span> <span class="n">month</span> <span class="o"><=</span> <span class="mi">12</span> \ <span class="ow">and</span> <span class="mi">1</span> <span class="o"><=</span> <span class="n">day</span> <span class="o"><=</span> <span class="mi">31</span> <span class="ow">and</span> <span class="mi">0</span> <span class="o"><=</span> <span class="n">hour</span> <span class="o"><</span> <span class="mi">24</span> \ <span class="ow">and</span> <span class="mi">0</span> <span class="o"><=</span> <span class="n">minute</span> <span class="o"><</span> <span class="mi">60</span> <span class="ow">and</span> <span class="mi">0</span> <span class="o"><=</span> <span class="n">second</span> <span class="o"><</span> <span class="mi">60</span><span class="p">:</span> <span class="c1"># Looks like a valid date</span> <span class="k">return</span> <span class="mi">1</span> </pre></div> </div> <p>A line ending in a backslash cannot carry a comment. A backslash does not continue a comment. A backslash does not continue a token except for string literals (i.e., tokens other than string literals cannot be split across physical lines using a backslash). A backslash is illegal elsewhere on a line outside a string literal.</p> </div> <div class="section" id="implicit-line-joining"> <span id="implicit-joining"></span><h3>2.1.6. Implicit line joining<a class="headerlink" href="#implicit-line-joining" title="Permalink to this headline">¶</a></h3> <p>Expressions in parentheses, square brackets or curly braces can be split over more than one physical line without using backslashes. For example:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">month_names</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'Januari'</span><span class="p">,</span> <span class="s1">'Februari'</span><span class="p">,</span> <span class="s1">'Maart'</span><span class="p">,</span> <span class="c1"># These are the</span> <span class="s1">'April'</span><span class="p">,</span> <span class="s1">'Mei'</span><span class="p">,</span> <span class="s1">'Juni'</span><span class="p">,</span> <span class="c1"># Dutch names</span> <span class="s1">'Juli'</span><span class="p">,</span> <span class="s1">'Augustus'</span><span class="p">,</span> <span class="s1">'September'</span><span class="p">,</span> <span class="c1"># for the months</span> <span class="s1">'Oktober'</span><span class="p">,</span> <span class="s1">'November'</span><span class="p">,</span> <span class="s1">'December'</span><span class="p">]</span> <span class="c1"># of the year</span> </pre></div> </div> <p>Implicitly continued lines can carry comments. The indentation of the continuation lines is not important. Blank continuation lines are allowed. There is no NEWLINE token between implicit continuation lines. Implicitly continued lines can also occur within triple-quoted strings (see below); in that case they cannot carry comments.</p> </div> <div class="section" id="blank-lines"> <span id="id5"></span><h3>2.1.7. Blank lines<a class="headerlink" href="#blank-lines" title="Permalink to this headline">¶</a></h3> <p id="index-7">A logical line that contains only spaces, tabs, formfeeds and possibly a comment, is ignored (i.e., no NEWLINE token is generated). During interactive input of statements, handling of a blank line may differ depending on the implementation of the read-eval-print loop. In the standard interactive interpreter, an entirely blank logical line (i.e. one containing not even whitespace or a comment) terminates a multi-line statement.</p> </div> <div class="section" id="indentation"> <span id="id6"></span><h3>2.1.8. Indentation<a class="headerlink" href="#indentation" title="Permalink to this headline">¶</a></h3> <p id="index-8">Leading whitespace (spaces and tabs) at the beginning of a logical line is used to compute the indentation level of the line, which in turn is used to determine the grouping of statements.</p> <p>Tabs are replaced (from left to right) by one to eight spaces such that the total number of characters up to and including the replacement is a multiple of eight (this is intended to be the same rule as used by Unix). The total number of spaces preceding the first non-blank character then determines the line’s indentation. Indentation cannot be split over multiple physical lines using backslashes; the whitespace up to the first backslash determines the indentation.</p> <p>Indentation is rejected as inconsistent if a source file mixes tabs and spaces in a way that makes the meaning dependent on the worth of a tab in spaces; a <a class="reference internal" href="../library/exceptions.html#TabError" title="TabError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TabError</span></code></a> is raised in that case.</p> <p><strong>Cross-platform compatibility note:</strong> because of the nature of text editors on non-UNIX platforms, it is unwise to use a mixture of spaces and tabs for the indentation in a single source file. It should also be noted that different platforms may explicitly limit the maximum indentation level.</p> <p>A formfeed character may be present at the start of the line; it will be ignored for the indentation calculations above. Formfeed characters occurring elsewhere in the leading whitespace have an undefined effect (for instance, they may reset the space count to zero).</p> <p id="index-9">The indentation levels of consecutive lines are used to generate INDENT and DEDENT tokens, using a stack, as follows.</p> <p>Before the first line of the file is read, a single zero is pushed on the stack; this will never be popped off again. The numbers pushed on the stack will always be strictly increasing from bottom to top. At the beginning of each logical line, the line’s indentation level is compared to the top of the stack. If it is equal, nothing happens. If it is larger, it is pushed on the stack, and one INDENT token is generated. If it is smaller, it <em>must</em> be one of the numbers occurring on the stack; all numbers on the stack that are larger are popped off, and for each number popped off a DEDENT token is generated. At the end of the file, a DEDENT token is generated for each number remaining on the stack that is larger than zero.</p> <p>Here is an example of a correctly (though confusingly) indented piece of Python code:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">perm</span><span class="p">(</span><span class="n">l</span><span class="p">):</span> <span class="c1"># Compute the list of all permutations of l</span> <span class="k">if</span> <span class="nb">len</span><span class="p">(</span><span class="n">l</span><span class="p">)</span> <span class="o"><=</span> <span class="mi">1</span><span class="p">:</span> <span class="k">return</span> <span class="p">[</span><span class="n">l</span><span class="p">]</span> <span class="n">r</span> <span class="o">=</span> <span class="p">[]</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="nb">len</span><span class="p">(</span><span class="n">l</span><span class="p">)):</span> <span class="n">s</span> <span class="o">=</span> <span class="n">l</span><span class="p">[:</span><span class="n">i</span><span class="p">]</span> <span class="o">+</span> <span class="n">l</span><span class="p">[</span><span class="n">i</span><span class="o">+</span><span class="mi">1</span><span class="p">:]</span> <span class="n">p</span> <span class="o">=</span> <span class="n">perm</span><span class="p">(</span><span class="n">s</span><span class="p">)</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="n">p</span><span class="p">:</span> <span class="n">r</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">l</span><span class="p">[</span><span class="n">i</span><span class="p">:</span><span class="n">i</span><span class="o">+</span><span class="mi">1</span><span class="p">]</span> <span class="o">+</span> <span class="n">x</span><span class="p">)</span> <span class="k">return</span> <span class="n">r</span> </pre></div> </div> <p>The following example shows various indentation errors:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span> <span class="k">def</span> <span class="nf">perm</span><span class="p">(</span><span class="n">l</span><span class="p">):</span> <span class="c1"># error: first line indented</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="nb">len</span><span class="p">(</span><span class="n">l</span><span class="p">)):</span> <span class="c1"># error: not indented</span> <span class="n">s</span> <span class="o">=</span> <span class="n">l</span><span class="p">[:</span><span class="n">i</span><span class="p">]</span> <span class="o">+</span> <span class="n">l</span><span class="p">[</span><span class="n">i</span><span class="o">+</span><span class="mi">1</span><span class="p">:]</span> <span class="n">p</span> <span class="o">=</span> <span class="n">perm</span><span class="p">(</span><span class="n">l</span><span class="p">[:</span><span class="n">i</span><span class="p">]</span> <span class="o">+</span> <span class="n">l</span><span class="p">[</span><span class="n">i</span><span class="o">+</span><span class="mi">1</span><span class="p">:])</span> <span class="c1"># error: unexpected indent</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="n">p</span><span class="p">:</span> <span class="n">r</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">l</span><span class="p">[</span><span class="n">i</span><span class="p">:</span><span class="n">i</span><span class="o">+</span><span class="mi">1</span><span class="p">]</span> <span class="o">+</span> <span class="n">x</span><span class="p">)</span> <span class="k">return</span> <span class="n">r</span> <span class="c1"># error: inconsistent dedent</span> </pre></div> </div> <p>(Actually, the first three errors are detected by the parser; only the last error is found by the lexical analyzer — the indentation of <code class="docutils literal notranslate"><span class="pre">return</span> <span class="pre">r</span></code> does not match a level popped off the stack.)</p> </div> <div class="section" id="whitespace-between-tokens"> <span id="whitespace"></span><h3>2.1.9. Whitespace between tokens<a class="headerlink" href="#whitespace-between-tokens" title="Permalink to this headline">¶</a></h3> <p>Except at the beginning of a logical line or in string literals, the whitespace characters space, tab and formfeed can be used interchangeably to separate tokens. Whitespace is needed between two tokens only if their concatenation could otherwise be interpreted as a different token (e.g., ab is one token, but a b is two tokens).</p> </div> </div> <div class="section" id="other-tokens"> <span id="id7"></span><h2>2.2. Other tokens<a class="headerlink" href="#other-tokens" title="Permalink to this headline">¶</a></h2> <p>Besides NEWLINE, INDENT and DEDENT, the following categories of tokens exist: <em>identifiers</em>, <em>keywords</em>, <em>literals</em>, <em>operators</em>, and <em>delimiters</em>. Whitespace characters (other than line terminators, discussed earlier) are not tokens, but serve to delimit tokens. Where ambiguity exists, a token comprises the longest possible string that forms a legal token, when read from left to right.</p> </div> <div class="section" id="identifiers"> <span id="identifiers-and-keywords"></span><h2>2.3. Identifiers and keywords<a class="headerlink" href="#identifiers" title="Permalink to this headline">¶</a></h2> <p id="index-10">Identifiers (also referred to as <em>names</em>) are described by the following lexical definitions.</p> <p>The syntax of identifiers in Python is based on the Unicode standard annex UAX-31, with elaboration and changes as defined below; see also <span class="target" id="index-11"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3131"><strong>PEP 3131</strong></a> for further details.</p> <p>Within the ASCII range (U+0001..U+007F), the valid characters for identifiers are the same as in Python 2.x: the uppercase and lowercase letters <code class="docutils literal notranslate"><span class="pre">A</span></code> through <code class="docutils literal notranslate"><span class="pre">Z</span></code>, the underscore <code class="docutils literal notranslate"><span class="pre">_</span></code> and, except for the first character, the digits <code class="docutils literal notranslate"><span class="pre">0</span></code> through <code class="docutils literal notranslate"><span class="pre">9</span></code>.</p> <p>Python 3.0 introduces additional characters from outside the ASCII range (see <span class="target" id="index-12"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3131"><strong>PEP 3131</strong></a>). For these characters, the classification uses the version of the Unicode Character Database as included in the <a class="reference internal" href="../library/unicodedata.html#module-unicodedata" title="unicodedata: Access the Unicode Database."><code class="xref py py-mod docutils literal notranslate"><span class="pre">unicodedata</span></code></a> module.</p> <p>Identifiers are unlimited in length. Case is significant.</p> <pre> <strong id="grammar-token-identifier">identifier </strong> ::= <a class="reference internal" href="#grammar-token-xid_start"><code class="xref docutils literal notranslate"><span class="pre">xid_start</span></code></a> <a class="reference internal" href="#grammar-token-xid_continue"><code class="xref docutils literal notranslate"><span class="pre">xid_continue</span></code></a>* <strong id="grammar-token-id_start">id_start </strong> ::= <all characters in general categories Lu, Ll, Lt, Lm, Lo, Nl, the underscore, and characters with the Other_ID_Start property> <strong id="grammar-token-id_continue">id_continue </strong> ::= <all characters in <a class="reference internal" href="#grammar-token-id_start"><code class="xref docutils literal notranslate"><span class="pre">id_start</span></code></a>, plus characters in the categories Mn, Mc, Nd, Pc and others with the Other_ID_Continue property> <strong id="grammar-token-xid_start">xid_start </strong> ::= <all characters in <a class="reference internal" href="#grammar-token-id_start"><code class="xref docutils literal notranslate"><span class="pre">id_start</span></code></a> whose NFKC normalization is in "id_start xid_continue*"> <strong id="grammar-token-xid_continue">xid_continue</strong> ::= <all characters in <a class="reference internal" href="#grammar-token-id_continue"><code class="xref docutils literal notranslate"><span class="pre">id_continue</span></code></a> whose NFKC normalization is in "id_continue*"> </pre> <p>The Unicode category codes mentioned above stand for:</p> <ul class="simple"> <li><em>Lu</em> - uppercase letters</li> <li><em>Ll</em> - lowercase letters</li> <li><em>Lt</em> - titlecase letters</li> <li><em>Lm</em> - modifier letters</li> <li><em>Lo</em> - other letters</li> <li><em>Nl</em> - letter numbers</li> <li><em>Mn</em> - nonspacing marks</li> <li><em>Mc</em> - spacing combining marks</li> <li><em>Nd</em> - decimal numbers</li> <li><em>Pc</em> - connector punctuations</li> <li><em>Other_ID_Start</em> - explicit list of characters in <a class="reference external" href="http://www.unicode.org/Public/9.0.0/ucd/PropList.txt">PropList.txt</a> to support backwards compatibility</li> <li><em>Other_ID_Continue</em> - likewise</li> </ul> <p>All identifiers are converted into the normal form NFKC while parsing; comparison of identifiers is based on NFKC.</p> <p>A non-normative HTML file listing all valid identifier characters for Unicode 4.1 can be found at <a class="reference external" href="https://www.dcl.hpi.uni-potsdam.de/home/loewis/table-3131.html">https://www.dcl.hpi.uni-potsdam.de/home/loewis/table-3131.html</a>.</p> <div class="section" id="keywords"> <span id="id8"></span><h3>2.3.1. Keywords<a class="headerlink" href="#keywords" title="Permalink to this headline">¶</a></h3> <p id="index-13">The following identifiers are used as reserved words, or <em>keywords</em> of the language, and cannot be used as ordinary identifiers. They must be spelled exactly as written here:</p> <div class="highlight-text notranslate"><div class="highlight"><pre><span></span>False class finally is return None continue for lambda try True def from nonlocal while and del global not with as elif if or yield assert else import pass break except in raise </pre></div> </div> </div> <div class="section" id="reserved-classes-of-identifiers"> <span id="id-classes"></span><h3>2.3.2. Reserved classes of identifiers<a class="headerlink" href="#reserved-classes-of-identifiers" title="Permalink to this headline">¶</a></h3> <p>Certain classes of identifiers (besides keywords) have special meanings. These classes are identified by the patterns of leading and trailing underscore characters:</p> <dl class="docutils"> <dt><code class="docutils literal notranslate"><span class="pre">_*</span></code></dt> <dd><p class="first">Not imported by <code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">module</span> <span class="pre">import</span> <span class="pre">*</span></code>. The special identifier <code class="docutils literal notranslate"><span class="pre">_</span></code> is used in the interactive interpreter to store the result of the last evaluation; it is stored in the <a class="reference internal" href="../library/builtins.html#module-builtins" title="builtins: The module that provides the built-in namespace."><code class="xref py py-mod docutils literal notranslate"><span class="pre">builtins</span></code></a> module. When not in interactive mode, <code class="docutils literal notranslate"><span class="pre">_</span></code> has no special meaning and is not defined. See section <a class="reference internal" href="simple_stmts.html#import"><span class="std std-ref">The import statement</span></a>.</p> <div class="last admonition note"> <p class="first admonition-title">Note</p> <p class="last">The name <code class="docutils literal notranslate"><span class="pre">_</span></code> is often used in conjunction with internationalization; refer to the documentation for the <a class="reference internal" href="../library/gettext.html#module-gettext" title="gettext: Multilingual internationalization services."><code class="xref py py-mod docutils literal notranslate"><span class="pre">gettext</span></code></a> module for more information on this convention.</p> </div> </dd> <dt><code class="docutils literal notranslate"><span class="pre">__*__</span></code></dt> <dd>System-defined names. These names are defined by the interpreter and its implementation (including the standard library). Current system names are discussed in the <a class="reference internal" href="datamodel.html#specialnames"><span class="std std-ref">Special method names</span></a> section and elsewhere. More will likely be defined in future versions of Python. <em>Any</em> use of <code class="docutils literal notranslate"><span class="pre">__*__</span></code> names, in any context, that does not follow explicitly documented use, is subject to breakage without warning.</dd> <dt><code class="docutils literal notranslate"><span class="pre">__*</span></code></dt> <dd>Class-private names. Names in this category, when used within the context of a class definition, are re-written to use a mangled form to help avoid name clashes between “private” attributes of base and derived classes. See section <a class="reference internal" href="expressions.html#atom-identifiers"><span class="std std-ref">Identifiers (Names)</span></a>.</dd> </dl> </div> </div> <div class="section" id="literals"> <span id="id9"></span><h2>2.4. Literals<a class="headerlink" href="#literals" title="Permalink to this headline">¶</a></h2> <p id="index-14">Literals are notations for constant values of some built-in types.</p> <div class="section" id="string-and-bytes-literals"> <span id="strings"></span><h3>2.4.1. String and Bytes literals<a class="headerlink" href="#string-and-bytes-literals" title="Permalink to this headline">¶</a></h3> <p id="index-15">String literals are described by the following lexical definitions:</p> <pre> <strong id="grammar-token-stringliteral">stringliteral </strong> ::= [<a class="reference internal" href="#grammar-token-stringprefix"><code class="xref docutils literal notranslate"><span class="pre">stringprefix</span></code></a>](<a class="reference internal" href="#grammar-token-shortstring"><code class="xref docutils literal notranslate"><span class="pre">shortstring</span></code></a> | <a class="reference internal" href="#grammar-token-longstring"><code class="xref docutils literal notranslate"><span class="pre">longstring</span></code></a>) <strong id="grammar-token-stringprefix">stringprefix </strong> ::= "r" | "u" | "R" | "U" | "f" | "F" | "fr" | "Fr" | "fR" | "FR" | "rf" | "rF" | "Rf" | "RF" <strong id="grammar-token-shortstring">shortstring </strong> ::= "'" <a class="reference internal" href="#grammar-token-shortstringitem"><code class="xref docutils literal notranslate"><span class="pre">shortstringitem</span></code></a>* "'" | '"' <a class="reference internal" href="#grammar-token-shortstringitem"><code class="xref docutils literal notranslate"><span class="pre">shortstringitem</span></code></a>* '"' <strong id="grammar-token-longstring">longstring </strong> ::= "'''" <a class="reference internal" href="#grammar-token-longstringitem"><code class="xref docutils literal notranslate"><span class="pre">longstringitem</span></code></a>* "'''" | '"""' <a class="reference internal" href="#grammar-token-longstringitem"><code class="xref docutils literal notranslate"><span class="pre">longstringitem</span></code></a>* '"""' <strong id="grammar-token-shortstringitem">shortstringitem</strong> ::= <a class="reference internal" href="#grammar-token-shortstringchar"><code class="xref docutils literal notranslate"><span class="pre">shortstringchar</span></code></a> | <a class="reference internal" href="#grammar-token-stringescapeseq"><code class="xref docutils literal notranslate"><span class="pre">stringescapeseq</span></code></a> <strong id="grammar-token-longstringitem">longstringitem </strong> ::= <a class="reference internal" href="#grammar-token-longstringchar"><code class="xref docutils literal notranslate"><span class="pre">longstringchar</span></code></a> | <a class="reference internal" href="#grammar-token-stringescapeseq"><code class="xref docutils literal notranslate"><span class="pre">stringescapeseq</span></code></a> <strong id="grammar-token-shortstringchar">shortstringchar</strong> ::= <any source character except "\" or newline or the quote> <strong id="grammar-token-longstringchar">longstringchar </strong> ::= <any source character except "\"> <strong id="grammar-token-stringescapeseq">stringescapeseq</strong> ::= "\" <any source character> </pre> <pre> <strong id="grammar-token-bytesliteral">bytesliteral </strong> ::= <a class="reference internal" href="#grammar-token-bytesprefix"><code class="xref docutils literal notranslate"><span class="pre">bytesprefix</span></code></a>(<a class="reference internal" href="#grammar-token-shortbytes"><code class="xref docutils literal notranslate"><span class="pre">shortbytes</span></code></a> | <a class="reference internal" href="#grammar-token-longbytes"><code class="xref docutils literal notranslate"><span class="pre">longbytes</span></code></a>) <strong id="grammar-token-bytesprefix">bytesprefix </strong> ::= "b" | "B" | "br" | "Br" | "bR" | "BR" | "rb" | "rB" | "Rb" | "RB" <strong id="grammar-token-shortbytes">shortbytes </strong> ::= "'" <a class="reference internal" href="#grammar-token-shortbytesitem"><code class="xref docutils literal notranslate"><span class="pre">shortbytesitem</span></code></a>* "'" | '"' <a class="reference internal" href="#grammar-token-shortbytesitem"><code class="xref docutils literal notranslate"><span class="pre">shortbytesitem</span></code></a>* '"' <strong id="grammar-token-longbytes">longbytes </strong> ::= "'''" <a class="reference internal" href="#grammar-token-longbytesitem"><code class="xref docutils literal notranslate"><span class="pre">longbytesitem</span></code></a>* "'''" | '"""' <a class="reference internal" href="#grammar-token-longbytesitem"><code class="xref docutils literal notranslate"><span class="pre">longbytesitem</span></code></a>* '"""' <strong id="grammar-token-shortbytesitem">shortbytesitem</strong> ::= <a class="reference internal" href="#grammar-token-shortbyteschar"><code class="xref docutils literal notranslate"><span class="pre">shortbyteschar</span></code></a> | <a class="reference internal" href="#grammar-token-bytesescapeseq"><code class="xref docutils literal notranslate"><span class="pre">bytesescapeseq</span></code></a> <strong id="grammar-token-longbytesitem">longbytesitem </strong> ::= <a class="reference internal" href="#grammar-token-longbyteschar"><code class="xref docutils literal notranslate"><span class="pre">longbyteschar</span></code></a> | <a class="reference internal" href="#grammar-token-bytesescapeseq"><code class="xref docutils literal notranslate"><span class="pre">bytesescapeseq</span></code></a> <strong id="grammar-token-shortbyteschar">shortbyteschar</strong> ::= <any ASCII character except "\" or newline or the quote> <strong id="grammar-token-longbyteschar">longbyteschar </strong> ::= <any ASCII character except "\"> <strong id="grammar-token-bytesescapeseq">bytesescapeseq</strong> ::= "\" <any ASCII character> </pre> <p>One syntactic restriction not indicated by these productions is that whitespace is not allowed between the <a class="reference internal" href="#grammar-token-stringprefix"><code class="xref std std-token docutils literal notranslate"><span class="pre">stringprefix</span></code></a> or <a class="reference internal" href="#grammar-token-bytesprefix"><code class="xref std std-token docutils literal notranslate"><span class="pre">bytesprefix</span></code></a> and the rest of the literal. The source character set is defined by the encoding declaration; it is UTF-8 if no encoding declaration is given in the source file; see section <a class="reference internal" href="#encodings"><span class="std std-ref">Encoding declarations</span></a>.</p> <p id="index-16">In plain English: Both types of literals can be enclosed in matching single quotes (<code class="docutils literal notranslate"><span class="pre">'</span></code>) or double quotes (<code class="docutils literal notranslate"><span class="pre">"</span></code>). They can also be enclosed in matching groups of three single or double quotes (these are generally referred to as <em>triple-quoted strings</em>). The backslash (<code class="docutils literal notranslate"><span class="pre">\</span></code>) character is used to escape characters that otherwise have a special meaning, such as newline, backslash itself, or the quote character.</p> <p>Bytes literals are always prefixed with <code class="docutils literal notranslate"><span class="pre">'b'</span></code> or <code class="docutils literal notranslate"><span class="pre">'B'</span></code>; they produce an instance of the <a class="reference internal" href="../library/stdtypes.html#bytes" title="bytes"><code class="xref py py-class docutils literal notranslate"><span class="pre">bytes</span></code></a> type instead of the <a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-class docutils literal notranslate"><span class="pre">str</span></code></a> type. They may only contain ASCII characters; bytes with a numeric value of 128 or greater must be expressed with escapes.</p> <p>Both string and bytes literals may optionally be prefixed with a letter <code class="docutils literal notranslate"><span class="pre">'r'</span></code> or <code class="docutils literal notranslate"><span class="pre">'R'</span></code>; such strings are called <em class="dfn">raw strings</em> and treat backslashes as literal characters. As a result, in string literals, <code class="docutils literal notranslate"><span class="pre">'\U'</span></code> and <code class="docutils literal notranslate"><span class="pre">'\u'</span></code> escapes in raw strings are not treated specially. Given that Python 2.x’s raw unicode literals behave differently than Python 3.x’s the <code class="docutils literal notranslate"><span class="pre">'ur'</span></code> syntax is not supported.</p> <div class="versionadded"> <p><span class="versionmodified">New in version 3.3: </span>The <code class="docutils literal notranslate"><span class="pre">'rb'</span></code> prefix of raw bytes literals has been added as a synonym of <code class="docutils literal notranslate"><span class="pre">'br'</span></code>.</p> </div> <div class="versionadded"> <p><span class="versionmodified">New in version 3.3: </span>Support for the unicode legacy literal (<code class="docutils literal notranslate"><span class="pre">u'value'</span></code>) was reintroduced to simplify the maintenance of dual Python 2.x and 3.x codebases. See <span class="target" id="index-17"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0414"><strong>PEP 414</strong></a> for more information.</p> </div> <p>A string literal with <code class="docutils literal notranslate"><span class="pre">'f'</span></code> or <code class="docutils literal notranslate"><span class="pre">'F'</span></code> in its prefix is a <em class="dfn">formatted string literal</em>; see <a class="reference internal" href="#f-strings"><span class="std std-ref">Formatted string literals</span></a>. The <code class="docutils literal notranslate"><span class="pre">'f'</span></code> may be combined with <code class="docutils literal notranslate"><span class="pre">'r'</span></code>, but not with <code class="docutils literal notranslate"><span class="pre">'b'</span></code> or <code class="docutils literal notranslate"><span class="pre">'u'</span></code>, therefore raw formatted strings are possible, but formatted bytes literals are not.</p> <p>In triple-quoted literals, unescaped newlines and quotes are allowed (and are retained), except that three unescaped quotes in a row terminate the literal. (A “quote” is the character used to open the literal, i.e. either <code class="docutils literal notranslate"><span class="pre">'</span></code> or <code class="docutils literal notranslate"><span class="pre">"</span></code>.)</p> <p id="index-18">Unless an <code class="docutils literal notranslate"><span class="pre">'r'</span></code> or <code class="docutils literal notranslate"><span class="pre">'R'</span></code> prefix is present, escape sequences in string and bytes literals are interpreted according to rules similar to those used by Standard C. The recognized escape sequences are:</p> <table border="1" class="docutils"> <colgroup> <col width="30%" /> <col width="58%" /> <col width="12%" /> </colgroup> <thead valign="bottom"> <tr class="row-odd"><th class="head">Escape Sequence</th> <th class="head">Meaning</th> <th class="head">Notes</th> </tr> </thead> <tbody valign="top"> <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">\newline</span></code></td> <td>Backslash and newline ignored</td> <td> </td> </tr> <tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">\\</span></code></td> <td>Backslash (<code class="docutils literal notranslate"><span class="pre">\</span></code>)</td> <td> </td> </tr> <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">\'</span></code></td> <td>Single quote (<code class="docutils literal notranslate"><span class="pre">'</span></code>)</td> <td> </td> </tr> <tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">\"</span></code></td> <td>Double quote (<code class="docutils literal notranslate"><span class="pre">"</span></code>)</td> <td> </td> </tr> <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">\a</span></code></td> <td>ASCII Bell (BEL)</td> <td> </td> </tr> <tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">\b</span></code></td> <td>ASCII Backspace (BS)</td> <td> </td> </tr> <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">\f</span></code></td> <td>ASCII Formfeed (FF)</td> <td> </td> </tr> <tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">\n</span></code></td> <td>ASCII Linefeed (LF)</td> <td> </td> </tr> <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">\r</span></code></td> <td>ASCII Carriage Return (CR)</td> <td> </td> </tr> <tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">\t</span></code></td> <td>ASCII Horizontal Tab (TAB)</td> <td> </td> </tr> <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">\v</span></code></td> <td>ASCII Vertical Tab (VT)</td> <td> </td> </tr> <tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">\ooo</span></code></td> <td>Character with octal value <em>ooo</em></td> <td>(1,3)</td> </tr> <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">\xhh</span></code></td> <td>Character with hex value <em>hh</em></td> <td>(2,3)</td> </tr> </tbody> </table> <p>Escape sequences only recognized in string literals are:</p> <table border="1" class="docutils"> <colgroup> <col width="30%" /> <col width="58%" /> <col width="12%" /> </colgroup> <thead valign="bottom"> <tr class="row-odd"><th class="head">Escape Sequence</th> <th class="head">Meaning</th> <th class="head">Notes</th> </tr> </thead> <tbody valign="top"> <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">\N{name}</span></code></td> <td>Character named <em>name</em> in the Unicode database</td> <td>(4)</td> </tr> <tr class="row-odd"><td><code class="docutils literal notranslate"><span class="pre">\uxxxx</span></code></td> <td>Character with 16-bit hex value <em>xxxx</em></td> <td>(5)</td> </tr> <tr class="row-even"><td><code class="docutils literal notranslate"><span class="pre">\Uxxxxxxxx</span></code></td> <td>Character with 32-bit hex value <em>xxxxxxxx</em></td> <td>(6)</td> </tr> </tbody> </table> <p>Notes:</p> <ol class="arabic"> <li><p class="first">As in Standard C, up to three octal digits are accepted.</p> </li> <li><p class="first">Unlike in Standard C, exactly two hex digits are required.</p> </li> <li><p class="first">In a bytes literal, hexadecimal and octal escapes denote the byte with the given value. In a string literal, these escapes denote a Unicode character with the given value.</p> </li> <li><div class="first versionchanged"> <p><span class="versionmodified">Changed in version 3.3: </span>Support for name aliases <a class="footnote-reference" href="#id13" id="id10">[1]</a> has been added.</p> </div> </li> <li><p class="first">Exactly four hex digits are required.</p> </li> <li><p class="first">Any Unicode character can be encoded this way. Exactly eight hex digits are required.</p> </li> </ol> <p id="index-19">Unlike Standard C, all unrecognized escape sequences are left in the string unchanged, i.e., <em>the backslash is left in the result</em>. (This behavior is useful when debugging: if an escape sequence is mistyped, the resulting output is more easily recognized as broken.) It is also important to note that the escape sequences only recognized in string literals fall into the category of unrecognized escapes for bytes literals.</p> <blockquote> <div><div class="versionchanged"> <p><span class="versionmodified">Changed in version 3.6: </span>Unrecognized escape sequences produce a DeprecationWarning. In some future version of Python they will be a SyntaxError.</p> </div> </div></blockquote> <p>Even in a raw literal, quotes can be escaped with a backslash, but the backslash remains in the result; for example, <code class="docutils literal notranslate"><span class="pre">r"\""</span></code> is a valid string literal consisting of two characters: a backslash and a double quote; <code class="docutils literal notranslate"><span class="pre">r"\"</span></code> is not a valid string literal (even a raw string cannot end in an odd number of backslashes). Specifically, <em>a raw literal cannot end in a single backslash</em> (since the backslash would escape the following quote character). Note also that a single backslash followed by a newline is interpreted as those two characters as part of the literal, <em>not</em> as a line continuation.</p> </div> <div class="section" id="string-literal-concatenation"> <span id="string-concatenation"></span><h3>2.4.2. String literal concatenation<a class="headerlink" href="#string-literal-concatenation" title="Permalink to this headline">¶</a></h3> <p>Multiple adjacent string or bytes literals (delimited by whitespace), possibly using different quoting conventions, are allowed, and their meaning is the same as their concatenation. Thus, <code class="docutils literal notranslate"><span class="pre">"hello"</span> <span class="pre">'world'</span></code> is equivalent to <code class="docutils literal notranslate"><span class="pre">"helloworld"</span></code>. This feature can be used to reduce the number of backslashes needed, to split long strings conveniently across long lines, or even to add comments to parts of strings, for example:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">re</span><span class="o">.</span><span class="n">compile</span><span class="p">(</span><span class="s2">"[A-Za-z_]"</span> <span class="c1"># letter or underscore</span> <span class="s2">"[A-Za-z0-9_]*"</span> <span class="c1"># letter, digit or underscore</span> <span class="p">)</span> </pre></div> </div> <p>Note that this feature is defined at the syntactical level, but implemented at compile time. The ‘+’ operator must be used to concatenate string expressions at run time. Also note that literal concatenation can use different quoting styles for each component (even mixing raw strings and triple quoted strings), and formatted string literals may be concatenated with plain string literals.</p> </div> <div class="section" id="formatted-string-literals"> <span id="f-strings"></span><span id="index-20"></span><h3>2.4.3. Formatted string literals<a class="headerlink" href="#formatted-string-literals" title="Permalink to this headline">¶</a></h3> <div class="versionadded"> <p><span class="versionmodified">New in version 3.6.</span></p> </div> <p>A <em class="dfn">formatted string literal</em> or <em class="dfn">f-string</em> is a string literal that is prefixed with <code class="docutils literal notranslate"><span class="pre">'f'</span></code> or <code class="docutils literal notranslate"><span class="pre">'F'</span></code>. These strings may contain replacement fields, which are expressions delimited by curly braces <code class="docutils literal notranslate"><span class="pre">{}</span></code>. While other string literals always have a constant value, formatted strings are really expressions evaluated at run time.</p> <p>Escape sequences are decoded like in ordinary string literals (except when a literal is also marked as a raw string). After decoding, the grammar for the contents of the string is:</p> <pre> <strong id="grammar-token-f_string">f_string </strong> ::= (<a class="reference internal" href="#grammar-token-literal_char"><code class="xref docutils literal notranslate"><span class="pre">literal_char</span></code></a> | "{{" | "}}" | <a class="reference internal" href="#grammar-token-replacement_field"><code class="xref docutils literal notranslate"><span class="pre">replacement_field</span></code></a>)* <strong id="grammar-token-replacement_field">replacement_field</strong> ::= "{" <a class="reference internal" href="#grammar-token-f_expression"><code class="xref docutils literal notranslate"><span class="pre">f_expression</span></code></a> ["!" <a class="reference internal" href="#grammar-token-conversion"><code class="xref docutils literal notranslate"><span class="pre">conversion</span></code></a>] [":" <a class="reference internal" href="#grammar-token-format_spec"><code class="xref docutils literal notranslate"><span class="pre">format_spec</span></code></a>] "}" <strong id="grammar-token-f_expression">f_expression </strong> ::= (<a class="reference internal" href="expressions.html#grammar-token-conditional_expression"><code class="xref docutils literal notranslate"><span class="pre">conditional_expression</span></code></a> | "*" <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>) ("," <a class="reference internal" href="expressions.html#grammar-token-conditional_expression"><code class="xref docutils literal notranslate"><span class="pre">conditional_expression</span></code></a> | "," "*" <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>)* [","] | <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-conversion">conversion </strong> ::= "s" | "r" | "a" <strong id="grammar-token-format_spec">format_spec </strong> ::= (<a class="reference internal" href="#grammar-token-literal_char"><code class="xref docutils literal notranslate"><span class="pre">literal_char</span></code></a> | NULL | <a class="reference internal" href="#grammar-token-replacement_field"><code class="xref docutils literal notranslate"><span class="pre">replacement_field</span></code></a>)* <strong id="grammar-token-literal_char">literal_char </strong> ::= <any code point except "{", "}" or NULL> </pre> <p>The parts of the string outside curly braces are treated literally, except that any doubled curly braces <code class="docutils literal notranslate"><span class="pre">'{{'</span></code> or <code class="docutils literal notranslate"><span class="pre">'}}'</span></code> are replaced with the corresponding single curly brace. A single opening curly bracket <code class="docutils literal notranslate"><span class="pre">'{'</span></code> marks a replacement field, which starts with a Python expression. After the expression, there may be a conversion field, introduced by an exclamation point <code class="docutils literal notranslate"><span class="pre">'!'</span></code>. A format specifier may also be appended, introduced by a colon <code class="docutils literal notranslate"><span class="pre">':'</span></code>. A replacement field ends with a closing curly bracket <code class="docutils literal notranslate"><span class="pre">'}'</span></code>.</p> <p>Expressions in formatted string literals are treated like regular Python expressions surrounded by parentheses, with a few exceptions. An empty expression is not allowed, and a <a class="reference internal" href="expressions.html#lambda"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">lambda</span></code></a> expression must be surrounded by explicit parentheses. Replacement expressions can contain line breaks (e.g. in triple-quoted strings), but they cannot contain comments. Each expression is evaluated in the context where the formatted string literal appears, in order from left to right.</p> <p>If a conversion is specified, the result of evaluating the expression is converted before formatting. Conversion <code class="docutils literal notranslate"><span class="pre">'!s'</span></code> calls <a class="reference internal" href="../library/stdtypes.html#str" title="str"><code class="xref py py-func docutils literal notranslate"><span class="pre">str()</span></code></a> on the result, <code class="docutils literal notranslate"><span class="pre">'!r'</span></code> calls <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>, and <code class="docutils literal notranslate"><span class="pre">'!a'</span></code> calls <a class="reference internal" href="../library/functions.html#ascii" title="ascii"><code class="xref py py-func docutils literal notranslate"><span class="pre">ascii()</span></code></a>.</p> <p>The result is then formatted using the <a class="reference internal" href="../library/functions.html#format" title="format"><code class="xref py py-func docutils literal notranslate"><span class="pre">format()</span></code></a> protocol. The format specifier is passed to the <a class="reference internal" href="datamodel.html#object.__format__" title="object.__format__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__format__()</span></code></a> method of the expression or conversion result. An empty string is passed when the format specifier is omitted. The formatted result is then included in the final value of the whole string.</p> <p>Top-level format specifiers may include nested replacement fields. These nested fields may include their own conversion fields and <a class="reference internal" href="../library/string.html#formatspec"><span class="std std-ref">format specifiers</span></a>, but may not include more deeply-nested replacement fields. The <a class="reference internal" href="../library/string.html#formatspec"><span class="std std-ref">format specifier mini-language</span></a> is the same as that used by the string .format() method.</p> <p>Formatted string literals may be concatenated, but replacement fields cannot be split across literals.</p> <p>Some examples of formatted string literals:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">name</span> <span class="o">=</span> <span class="s2">"Fred"</span> <span class="gp">>>> </span><span class="n">f</span><span class="s2">"He said his name is </span><span class="si">{name!r}</span><span class="s2">."</span> <span class="go">"He said his name is 'Fred'."</span> <span class="gp">>>> </span><span class="n">f</span><span class="s2">"He said his name is {repr(name)}."</span> <span class="c1"># repr() is equivalent to !r</span> <span class="go">"He said his name is 'Fred'."</span> <span class="gp">>>> </span><span class="n">width</span> <span class="o">=</span> <span class="mi">10</span> <span class="gp">>>> </span><span class="n">precision</span> <span class="o">=</span> <span class="mi">4</span> <span class="gp">>>> </span><span class="n">value</span> <span class="o">=</span> <span class="n">decimal</span><span class="o">.</span><span class="n">Decimal</span><span class="p">(</span><span class="s2">"12.34567"</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">f</span><span class="s2">"result: {value:</span><span class="si">{width}</span><span class="s2">.</span><span class="si">{precision}</span><span class="s2">}"</span> <span class="c1"># nested fields</span> <span class="go">'result: 12.35'</span> <span class="gp">>>> </span><span class="n">today</span> <span class="o">=</span> <span class="n">datetime</span><span class="p">(</span><span class="n">year</span><span class="o">=</span><span class="mi">2017</span><span class="p">,</span> <span class="n">month</span><span class="o">=</span><span class="mi">1</span><span class="p">,</span> <span class="n">day</span><span class="o">=</span><span class="mi">27</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">f</span><span class="s2">"{today:%B </span><span class="si">%d</span><span class="s2">, %Y}"</span> <span class="c1"># using date format specifier</span> <span class="go">'January 27, 2017'</span> <span class="gp">>>> </span><span class="n">number</span> <span class="o">=</span> <span class="mi">1024</span> <span class="gp">>>> </span><span class="n">f</span><span class="s2">"</span><span class="si">{number:#0x}</span><span class="s2">"</span> <span class="c1"># using integer format specifier</span> <span class="go">'0x400'</span> </pre></div> </div> <p>A consequence of sharing the same syntax as regular string literals is that characters in the replacement fields must not conflict with the quoting used in the outer formatted string literal:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">f</span><span class="s2">"abc </span><span class="si">{a["x"]}</span><span class="s2"> def"</span> <span class="c1"># error: outer string literal ended prematurely</span> <span class="n">f</span><span class="s2">"abc </span><span class="si">{a['x']}</span><span class="s2"> def"</span> <span class="c1"># workaround: use different quoting</span> </pre></div> </div> <p>Backslashes are not allowed in format expressions and will raise an error:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">f</span><span class="s2">"newline: {ord('</span><span class="se">\n</span><span class="s2">')}"</span> <span class="c1"># raises SyntaxError</span> </pre></div> </div> <p>To include a value in which a backslash escape is required, create a temporary variable.</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">newline</span> <span class="o">=</span> <span class="nb">ord</span><span class="p">(</span><span class="s1">'</span><span class="se">\n</span><span class="s1">'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">f</span><span class="s2">"newline: </span><span class="si">{newline}</span><span class="s2">"</span> <span class="go">'newline: 10'</span> </pre></div> </div> <p>Formatted string literals cannot be used as docstrings, even if they do not include expressions.</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span> <span class="nf">foo</span><span class="p">():</span> <span class="gp">... </span> <span class="n">f</span><span class="s2">"Not a docstring"</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="n">foo</span><span class="o">.</span><span class="vm">__doc__</span> <span class="ow">is</span> <span class="kc">None</span> <span class="go">True</span> </pre></div> </div> <p>See also <span class="target" id="index-21"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0498"><strong>PEP 498</strong></a> for the proposal that added formatted string literals, and <a class="reference internal" href="../library/stdtypes.html#str.format" title="str.format"><code class="xref py py-meth docutils literal notranslate"><span class="pre">str.format()</span></code></a>, which uses a related format string mechanism.</p> </div> <div class="section" id="numeric-literals"> <span id="numbers"></span><h3>2.4.4. Numeric literals<a class="headerlink" href="#numeric-literals" title="Permalink to this headline">¶</a></h3> <p id="index-22">There are three types of numeric literals: integers, floating point numbers, and imaginary numbers. There are no complex literals (complex numbers can be formed by adding a real number and an imaginary number).</p> <p>Note that numeric literals do not include a sign; a phrase like <code class="docutils literal notranslate"><span class="pre">-1</span></code> is actually an expression composed of the unary operator ‘<code class="docutils literal notranslate"><span class="pre">-</span></code>‘ and the literal <code class="docutils literal notranslate"><span class="pre">1</span></code>.</p> </div> <div class="section" id="integer-literals"> <span id="integers"></span><h3>2.4.5. Integer literals<a class="headerlink" href="#integer-literals" title="Permalink to this headline">¶</a></h3> <p>Integer literals are described by the following lexical definitions:</p> <pre> <strong id="grammar-token-integer">integer </strong> ::= <a class="reference internal" href="#grammar-token-decinteger"><code class="xref docutils literal notranslate"><span class="pre">decinteger</span></code></a> | <a class="reference internal" href="#grammar-token-bininteger"><code class="xref docutils literal notranslate"><span class="pre">bininteger</span></code></a> | <a class="reference internal" href="#grammar-token-octinteger"><code class="xref docutils literal notranslate"><span class="pre">octinteger</span></code></a> | <a class="reference internal" href="#grammar-token-hexinteger"><code class="xref docutils literal notranslate"><span class="pre">hexinteger</span></code></a> <strong id="grammar-token-decinteger">decinteger </strong> ::= <a class="reference internal" href="#grammar-token-nonzerodigit"><code class="xref docutils literal notranslate"><span class="pre">nonzerodigit</span></code></a> (["_"] <a class="reference internal" href="#grammar-token-digit"><code class="xref docutils literal notranslate"><span class="pre">digit</span></code></a>)* | "0"+ (["_"] "0")* <strong id="grammar-token-bininteger">bininteger </strong> ::= "0" ("b" | "B") (["_"] <a class="reference internal" href="#grammar-token-bindigit"><code class="xref docutils literal notranslate"><span class="pre">bindigit</span></code></a>)+ <strong id="grammar-token-octinteger">octinteger </strong> ::= "0" ("o" | "O") (["_"] <a class="reference internal" href="#grammar-token-octdigit"><code class="xref docutils literal notranslate"><span class="pre">octdigit</span></code></a>)+ <strong id="grammar-token-hexinteger">hexinteger </strong> ::= "0" ("x" | "X") (["_"] <a class="reference internal" href="#grammar-token-hexdigit"><code class="xref docutils literal notranslate"><span class="pre">hexdigit</span></code></a>)+ <strong id="grammar-token-nonzerodigit">nonzerodigit</strong> ::= "1"..."9" <strong id="grammar-token-digit">digit </strong> ::= "0"..."9" <strong id="grammar-token-bindigit">bindigit </strong> ::= "0" | "1" <strong id="grammar-token-octdigit">octdigit </strong> ::= "0"..."7" <strong id="grammar-token-hexdigit">hexdigit </strong> ::= <a class="reference internal" href="#grammar-token-digit"><code class="xref docutils literal notranslate"><span class="pre">digit</span></code></a> | "a"..."f" | "A"..."F" </pre> <p>There is no limit for the length of integer literals apart from what can be stored in available memory.</p> <p>Underscores are ignored for determining the numeric value of the literal. They can be used to group digits for enhanced readability. One underscore can occur between digits, and after base specifiers like <code class="docutils literal notranslate"><span class="pre">0x</span></code>.</p> <p>Note that leading zeros in a non-zero decimal number are not allowed. This is for disambiguation with C-style octal literals, which Python used before version 3.0.</p> <p>Some examples of integer literals:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="mi">7</span> <span class="mi">2147483647</span> <span class="mo">0o177</span> <span class="mb">0b100110111</span> <span class="mi">3</span> <span class="mi">79228162514264337593543950336</span> <span class="mo">0o377</span> <span class="mh">0xdeadbeef</span> <span class="mi">100</span><span class="n">_000_000_000</span> <span class="mi">0</span><span class="n">b_1110_0101</span> </pre></div> </div> <div class="versionchanged"> <p><span class="versionmodified">Changed in version 3.6: </span>Underscores are now allowed for grouping purposes in literals.</p> </div> </div> <div class="section" id="floating-point-literals"> <span id="floating"></span><h3>2.4.6. Floating point literals<a class="headerlink" href="#floating-point-literals" title="Permalink to this headline">¶</a></h3> <p>Floating point literals are described by the following lexical definitions:</p> <pre> <strong id="grammar-token-floatnumber">floatnumber </strong> ::= <a class="reference internal" href="#grammar-token-pointfloat"><code class="xref docutils literal notranslate"><span class="pre">pointfloat</span></code></a> | <a class="reference internal" href="#grammar-token-exponentfloat"><code class="xref docutils literal notranslate"><span class="pre">exponentfloat</span></code></a> <strong id="grammar-token-pointfloat">pointfloat </strong> ::= [<a class="reference internal" href="#grammar-token-digitpart"><code class="xref docutils literal notranslate"><span class="pre">digitpart</span></code></a>] <a class="reference internal" href="#grammar-token-fraction"><code class="xref docutils literal notranslate"><span class="pre">fraction</span></code></a> | <a class="reference internal" href="#grammar-token-digitpart"><code class="xref docutils literal notranslate"><span class="pre">digitpart</span></code></a> "." <strong id="grammar-token-exponentfloat">exponentfloat</strong> ::= (<a class="reference internal" href="#grammar-token-digitpart"><code class="xref docutils literal notranslate"><span class="pre">digitpart</span></code></a> | <a class="reference internal" href="#grammar-token-pointfloat"><code class="xref docutils literal notranslate"><span class="pre">pointfloat</span></code></a>) <a class="reference internal" href="#grammar-token-exponent"><code class="xref docutils literal notranslate"><span class="pre">exponent</span></code></a> <strong id="grammar-token-digitpart">digitpart </strong> ::= <a class="reference internal" href="#grammar-token-digit"><code class="xref docutils literal notranslate"><span class="pre">digit</span></code></a> (["_"] <a class="reference internal" href="#grammar-token-digit"><code class="xref docutils literal notranslate"><span class="pre">digit</span></code></a>)* <strong id="grammar-token-fraction">fraction </strong> ::= "." <a class="reference internal" href="#grammar-token-digitpart"><code class="xref docutils literal notranslate"><span class="pre">digitpart</span></code></a> <strong id="grammar-token-exponent">exponent </strong> ::= ("e" | "E") ["+" | "-"] <a class="reference internal" href="#grammar-token-digitpart"><code class="xref docutils literal notranslate"><span class="pre">digitpart</span></code></a> </pre> <p>Note that the integer and exponent parts are always interpreted using radix 10. For example, <code class="docutils literal notranslate"><span class="pre">077e010</span></code> is legal, and denotes the same number as <code class="docutils literal notranslate"><span class="pre">77e10</span></code>. The allowed range of floating point literals is implementation-dependent. As in integer literals, underscores are supported for digit grouping.</p> <p>Some examples of floating point literals:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="mf">3.14</span> <span class="mf">10.</span> <span class="o">.</span><span class="mi">001</span> <span class="mf">1e100</span> <span class="mf">3.14e-10</span> <span class="mf">0e0</span> <span class="mf">3.14</span><span class="n">_15_93</span> </pre></div> </div> <div class="versionchanged"> <p><span class="versionmodified">Changed in version 3.6: </span>Underscores are now allowed for grouping purposes in literals.</p> </div> </div> <div class="section" id="imaginary-literals"> <span id="imaginary"></span><h3>2.4.7. Imaginary literals<a class="headerlink" href="#imaginary-literals" title="Permalink to this headline">¶</a></h3> <p>Imaginary literals are described by the following lexical definitions:</p> <pre> <strong id="grammar-token-imagnumber">imagnumber</strong> ::= (<a class="reference internal" href="#grammar-token-floatnumber"><code class="xref docutils literal notranslate"><span class="pre">floatnumber</span></code></a> | <a class="reference internal" href="#grammar-token-digitpart"><code class="xref docutils literal notranslate"><span class="pre">digitpart</span></code></a>) ("j" | "J") </pre> <p>An imaginary literal yields a complex number with a real part of 0.0. Complex numbers are represented as a pair of floating point numbers and have the same restrictions on their range. To create a complex number with a nonzero real part, add a floating point number to it, e.g., <code class="docutils literal notranslate"><span class="pre">(3+4j)</span></code>. Some examples of imaginary literals:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="mf">3.14</span><span class="n">j</span> <span class="mf">10.</span><span class="n">j</span> <span class="mi">10</span><span class="n">j</span> <span class="o">.</span><span class="mi">001</span><span class="n">j</span> <span class="mf">1e100j</span> <span class="mf">3.14e-10</span><span class="n">j</span> <span class="mf">3.14</span><span class="n">_15_93j</span> </pre></div> </div> </div> </div> <div class="section" id="operators"> <span id="id11"></span><h2>2.5. Operators<a class="headerlink" href="#operators" title="Permalink to this headline">¶</a></h2> <p id="index-23">The following tokens are operators:</p> <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>+ - * ** / // % @ << >> & | ^ ~ < > <= >= == != </pre></div> </div> </div> <div class="section" id="delimiters"> <span id="id12"></span><h2>2.6. Delimiters<a class="headerlink" href="#delimiters" title="Permalink to this headline">¶</a></h2> <p id="index-24">The following tokens serve as delimiters in the grammar:</p> <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>( ) [ ] { } , : . ; @ = -> += -= *= /= //= %= @= &= |= ^= >>= <<= **= </pre></div> </div> <p>The period can also occur in floating-point and imaginary literals. A sequence of three periods has a special meaning as an ellipsis literal. The second half of the list, the augmented assignment operators, serve lexically as delimiters, but also perform an operation.</p> <p>The following printing ASCII characters have special meaning as part of other tokens or are otherwise significant to the lexical analyzer:</p> <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>' " # \ </pre></div> </div> <p>The following printing ASCII characters are not used in Python. Their occurrence outside string literals and comments is an unconditional error:</p> <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>$ ? ` </pre></div> </div> <p class="rubric">Footnotes</p> <table class="docutils footnote" frame="void" id="id13" rules="none"> <colgroup><col class="label" /><col /></colgroup> <tbody valign="top"> <tr><td class="label"><a class="fn-backref" href="#id10">[1]</a></td><td><a class="reference external" href="http://www.unicode.org/Public/9.0.0/ucd/NameAliases.txt">http://www.unicode.org/Public/9.0.0/ucd/NameAliases.txt</a></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="#">2. Lexical analysis</a><ul> <li><a class="reference internal" href="#line-structure">2.1. Line structure</a><ul> <li><a class="reference internal" href="#logical-lines">2.1.1. Logical lines</a></li> <li><a class="reference internal" href="#physical-lines">2.1.2. Physical lines</a></li> <li><a class="reference internal" href="#comments">2.1.3. Comments</a></li> <li><a class="reference internal" href="#encoding-declarations">2.1.4. Encoding declarations</a></li> <li><a class="reference internal" href="#explicit-line-joining">2.1.5. Explicit line joining</a></li> <li><a class="reference internal" href="#implicit-line-joining">2.1.6. Implicit line joining</a></li> <li><a class="reference internal" href="#blank-lines">2.1.7. Blank lines</a></li> <li><a class="reference internal" href="#indentation">2.1.8. Indentation</a></li> <li><a class="reference internal" href="#whitespace-between-tokens">2.1.9. Whitespace between tokens</a></li> </ul> </li> <li><a class="reference internal" href="#other-tokens">2.2. Other tokens</a></li> <li><a class="reference internal" href="#identifiers">2.3. Identifiers and keywords</a><ul> <li><a class="reference internal" href="#keywords">2.3.1. Keywords</a></li> <li><a class="reference internal" href="#reserved-classes-of-identifiers">2.3.2. Reserved classes of identifiers</a></li> </ul> </li> <li><a class="reference internal" href="#literals">2.4. Literals</a><ul> <li><a class="reference internal" href="#string-and-bytes-literals">2.4.1. String and Bytes literals</a></li> <li><a class="reference internal" href="#string-literal-concatenation">2.4.2. String literal concatenation</a></li> <li><a class="reference internal" href="#formatted-string-literals">2.4.3. Formatted string literals</a></li> <li><a class="reference internal" href="#numeric-literals">2.4.4. Numeric literals</a></li> <li><a class="reference internal" href="#integer-literals">2.4.5. Integer literals</a></li> <li><a class="reference internal" href="#floating-point-literals">2.4.6. Floating point literals</a></li> <li><a class="reference internal" href="#imaginary-literals">2.4.7. Imaginary literals</a></li> </ul> </li> <li><a class="reference internal" href="#operators">2.5. Operators</a></li> <li><a class="reference internal" href="#delimiters">2.6. Delimiters</a></li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="introduction.html" title="previous chapter">1. Introduction</a></p> <h4>Next topic</h4> <p class="topless"><a href="datamodel.html" title="next chapter">3. 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