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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>8. Errors and Exceptions — 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="9. Classes" href="classes.html" /> <link rel="prev" title="7. Input and Output" href="inputoutput.html" /> <link rel="shortcut icon" type="image/png" href="../_static/py.png" /> <link rel="canonical" href="https://docs.python.org/3/tutorial/errors.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="classes.html" title="9. Classes" accesskey="N">next</a> |</li> <li class="right" > <a href="inputoutput.html" title="7. Input and Output" 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 Tutorial</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="errors-and-exceptions"> <span id="tut-errors"></span><h1>8. Errors and Exceptions<a class="headerlink" href="#errors-and-exceptions" title="Permalink to this headline">¶</a></h1> <p>Until now error messages haven’t been more than mentioned, but if you have tried out the examples you have probably seen some. There are (at least) two distinguishable kinds of errors: <em>syntax errors</em> and <em>exceptions</em>.</p> <div class="section" id="syntax-errors"> <span id="tut-syntaxerrors"></span><h2>8.1. Syntax Errors<a class="headerlink" href="#syntax-errors" title="Permalink to this headline">¶</a></h2> <p>Syntax errors, also known as parsing errors, are perhaps the most common kind of complaint you get while you are still learning Python:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">while</span> <span class="kc">True</span> <span class="nb">print</span><span class="p">(</span><span class="s1">'Hello world'</span><span class="p">)</span> File <span class="nb">"<stdin>"</span>, line <span class="m">1</span> <span class="k">while</span> <span class="kc">True</span> <span class="nb">print</span><span class="p">(</span><span class="s1">'Hello world'</span><span class="p">)</span> <span class="o">^</span> <span class="gr">SyntaxError</span>: <span class="n">invalid syntax</span> </pre></div> </div> <p>The parser repeats the offending line and displays a little ‘arrow’ pointing at the earliest point in the line where the error was detected. The error is caused by (or at least detected at) the token <em>preceding</em> the arrow: in the example, the error is detected at the function <a class="reference internal" href="../library/functions.html#print" title="print"><code class="xref py py-func docutils literal notranslate"><span class="pre">print()</span></code></a>, since a colon (<code class="docutils literal notranslate"><span class="pre">':'</span></code>) is missing before it. File name and line number are printed so you know where to look in case the input came from a script.</p> </div> <div class="section" id="exceptions"> <span id="tut-exceptions"></span><h2>8.2. Exceptions<a class="headerlink" href="#exceptions" title="Permalink to this headline">¶</a></h2> <p>Even if a statement or expression is syntactically correct, it may cause an error when an attempt is made to execute it. Errors detected during execution are called <em>exceptions</em> and are not unconditionally fatal: you will soon learn how to handle them in Python programs. Most exceptions are not handled by programs, however, and result in error messages as shown here:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="mi">10</span> <span class="o">*</span> <span class="p">(</span><span class="mi">1</span><span class="o">/</span><span class="mi">0</span><span class="p">)</span> <span class="gt">Traceback (most recent call last):</span> File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n"><module></span> <span class="gr">ZeroDivisionError</span>: <span class="n">division by zero</span> <span class="gp">>>> </span><span class="mi">4</span> <span class="o">+</span> <span class="n">spam</span><span class="o">*</span><span class="mi">3</span> <span class="gt">Traceback (most recent call last):</span> File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n"><module></span> <span class="gr">NameError</span>: <span class="n">name 'spam' is not defined</span> <span class="gp">>>> </span><span class="s1">'2'</span> <span class="o">+</span> <span class="mi">2</span> <span class="gt">Traceback (most recent call last):</span> File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n"><module></span> <span class="gr">TypeError</span>: <span class="n">Can't convert 'int' object to str implicitly</span> </pre></div> </div> <p>The last line of the error message indicates what happened. Exceptions come in different types, and the type is printed as part of the message: the types in the example are <a class="reference internal" href="../library/exceptions.html#ZeroDivisionError" title="ZeroDivisionError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ZeroDivisionError</span></code></a>, <a class="reference internal" href="../library/exceptions.html#NameError" title="NameError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">NameError</span></code></a> and <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a>. The string printed as the exception type is the name of the built-in exception that occurred. This is true for all built-in exceptions, but need not be true for user-defined exceptions (although it is a useful convention). Standard exception names are built-in identifiers (not reserved keywords).</p> <p>The rest of the line provides detail based on the type of exception and what caused it.</p> <p>The preceding part of the error message shows the context where the exception happened, in the form of a stack traceback. In general it contains a stack traceback listing source lines; however, it will not display lines read from standard input.</p> <p><a class="reference internal" href="../library/exceptions.html#bltin-exceptions"><span class="std std-ref">Built-in Exceptions</span></a> lists the built-in exceptions and their meanings.</p> </div> <div class="section" id="handling-exceptions"> <span id="tut-handling"></span><h2>8.3. Handling Exceptions<a class="headerlink" href="#handling-exceptions" title="Permalink to this headline">¶</a></h2> <p>It is possible to write programs that handle selected exceptions. Look at the following example, which asks the user for input until a valid integer has been entered, but allows the user to interrupt the program (using <kbd class="kbd docutils literal notranslate">Control-C</kbd> or whatever the operating system supports); note that a user-generated interruption is signalled by raising the <a class="reference internal" href="../library/exceptions.html#KeyboardInterrupt" title="KeyboardInterrupt"><code class="xref py py-exc docutils literal notranslate"><span class="pre">KeyboardInterrupt</span></code></a> exception.</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">while</span> <span class="kc">True</span><span class="p">:</span> <span class="gp">... </span> <span class="k">try</span><span class="p">:</span> <span class="gp">... </span> <span class="n">x</span> <span class="o">=</span> <span class="nb">int</span><span class="p">(</span><span class="nb">input</span><span class="p">(</span><span class="s2">"Please enter a number: "</span><span class="p">))</span> <span class="gp">... </span> <span class="k">break</span> <span class="gp">... </span> <span class="k">except</span> <span class="ne">ValueError</span><span class="p">:</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"Oops! That was no valid number. Try again..."</span><span class="p">)</span> <span class="gp">...</span> </pre></div> </div> <p>The <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> statement works as follows.</p> <ul class="simple"> <li>First, the <em>try clause</em> (the statement(s) between the <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> and <a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a> keywords) is executed.</li> <li>If no exception occurs, the <em>except clause</em> is skipped and execution of the <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> statement is finished.</li> <li>If an exception occurs during execution of the try clause, the rest of the clause is skipped. Then if its type matches the exception named after the <a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a> keyword, the except clause is executed, and then execution continues after the <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> statement.</li> <li>If an exception occurs which does not match the exception named in the except clause, it is passed on to outer <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> statements; if no handler is found, it is an <em>unhandled exception</em> and execution stops with a message as shown above.</li> </ul> <p>A <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> statement may have more than one except clause, to specify handlers for different exceptions. At most one handler will be executed. Handlers only handle exceptions that occur in the corresponding try clause, not in other handlers of the same <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> statement. An except clause may name multiple exceptions as a parenthesized tuple, for example:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="o">...</span> <span class="k">except</span> <span class="p">(</span><span class="ne">RuntimeError</span><span class="p">,</span> <span class="ne">TypeError</span><span class="p">,</span> <span class="ne">NameError</span><span class="p">):</span> <span class="o">...</span> <span class="k">pass</span> </pre></div> </div> <p>A class in an <a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a> clause is compatible with an exception if it is the same class or a base class thereof (but not the other way around — an except clause listing a derived class is not compatible with a base class). For example, the following code will print B, C, D in that order:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">B</span><span class="p">(</span><span class="ne">Exception</span><span class="p">):</span> <span class="k">pass</span> <span class="k">class</span> <span class="nc">C</span><span class="p">(</span><span class="n">B</span><span class="p">):</span> <span class="k">pass</span> <span class="k">class</span> <span class="nc">D</span><span class="p">(</span><span class="n">C</span><span class="p">):</span> <span class="k">pass</span> <span class="k">for</span> <span class="bp">cls</span> <span class="ow">in</span> <span class="p">[</span><span class="n">B</span><span class="p">,</span> <span class="n">C</span><span class="p">,</span> <span class="n">D</span><span class="p">]:</span> <span class="k">try</span><span class="p">:</span> <span class="k">raise</span> <span class="bp">cls</span><span class="p">()</span> <span class="k">except</span> <span class="n">D</span><span class="p">:</span> <span class="nb">print</span><span class="p">(</span><span class="s2">"D"</span><span class="p">)</span> <span class="k">except</span> <span class="n">C</span><span class="p">:</span> <span class="nb">print</span><span class="p">(</span><span class="s2">"C"</span><span class="p">)</span> <span class="k">except</span> <span class="n">B</span><span class="p">:</span> <span class="nb">print</span><span class="p">(</span><span class="s2">"B"</span><span class="p">)</span> </pre></div> </div> <p>Note that if the except clauses were reversed (with <code class="docutils literal notranslate"><span class="pre">except</span> <span class="pre">B</span></code> first), it would have printed B, B, B — the first matching except clause is triggered.</p> <p>The last except clause may omit the exception name(s), to serve as a wildcard. Use this with extreme caution, since it is easy to mask a real programming error in this way! It can also be used to print an error message and then re-raise the exception (allowing a caller to handle the exception as well):</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">sys</span> <span class="k">try</span><span class="p">:</span> <span class="n">f</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="s1">'myfile.txt'</span><span class="p">)</span> <span class="n">s</span> <span class="o">=</span> <span class="n">f</span><span class="o">.</span><span class="n">readline</span><span class="p">()</span> <span class="n">i</span> <span class="o">=</span> <span class="nb">int</span><span class="p">(</span><span class="n">s</span><span class="o">.</span><span class="n">strip</span><span class="p">())</span> <span class="k">except</span> <span class="ne">OSError</span> <span class="k">as</span> <span class="n">err</span><span class="p">:</span> <span class="nb">print</span><span class="p">(</span><span class="s2">"OS error: </span><span class="si">{0}</span><span class="s2">"</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">err</span><span class="p">))</span> <span class="k">except</span> <span class="ne">ValueError</span><span class="p">:</span> <span class="nb">print</span><span class="p">(</span><span class="s2">"Could not convert data to an integer."</span><span class="p">)</span> <span class="k">except</span><span class="p">:</span> <span class="nb">print</span><span class="p">(</span><span class="s2">"Unexpected error:"</span><span class="p">,</span> <span class="n">sys</span><span class="o">.</span><span class="n">exc_info</span><span class="p">()[</span><span class="mi">0</span><span class="p">])</span> <span class="k">raise</span> </pre></div> </div> <p>The <a class="reference internal" href="../reference/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="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a> statement has an optional <em>else clause</em>, which, when present, must follow all except clauses. It is useful for code that must be executed if the try clause does not raise an exception. For example:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">for</span> <span class="n">arg</span> <span class="ow">in</span> <span class="n">sys</span><span class="o">.</span><span class="n">argv</span><span class="p">[</span><span class="mi">1</span><span class="p">:]:</span> <span class="k">try</span><span class="p">:</span> <span class="n">f</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="n">arg</span><span class="p">,</span> <span class="s1">'r'</span><span class="p">)</span> <span class="k">except</span> <span class="ne">OSError</span><span class="p">:</span> <span class="nb">print</span><span class="p">(</span><span class="s1">'cannot open'</span><span class="p">,</span> <span class="n">arg</span><span class="p">)</span> <span class="k">else</span><span class="p">:</span> <span class="nb">print</span><span class="p">(</span><span class="n">arg</span><span class="p">,</span> <span class="s1">'has'</span><span class="p">,</span> <span class="nb">len</span><span class="p">(</span><span class="n">f</span><span class="o">.</span><span class="n">readlines</span><span class="p">()),</span> <span class="s1">'lines'</span><span class="p">)</span> <span class="n">f</span><span class="o">.</span><span class="n">close</span><span class="p">()</span> </pre></div> </div> <p>The use of the <a class="reference internal" href="../reference/compound_stmts.html#else"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">else</span></code></a> clause is better than adding additional code to the <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> clause because it avoids accidentally catching an exception that wasn’t raised by the code being protected by the <a class="reference internal" href="../reference/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="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a> statement.</p> <p>When an exception occurs, it may have an associated value, also known as the exception’s <em>argument</em>. The presence and type of the argument depend on the exception type.</p> <p>The except clause may specify a variable after the exception name. The variable is bound to an exception instance with the arguments stored in <code class="docutils literal notranslate"><span class="pre">instance.args</span></code>. For convenience, the exception instance defines <a class="reference internal" href="../reference/datamodel.html#object.__str__" title="object.__str__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__str__()</span></code></a> so the arguments can be printed directly without having to reference <code class="docutils literal notranslate"><span class="pre">.args</span></code>. One may also instantiate an exception first before raising it and add any attributes to it as desired.</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">try</span><span class="p">:</span> <span class="gp">... </span> <span class="k">raise</span> <span class="ne">Exception</span><span class="p">(</span><span class="s1">'spam'</span><span class="p">,</span> <span class="s1">'eggs'</span><span class="p">)</span> <span class="gp">... </span><span class="k">except</span> <span class="ne">Exception</span> <span class="k">as</span> <span class="n">inst</span><span class="p">:</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="nb">type</span><span class="p">(</span><span class="n">inst</span><span class="p">))</span> <span class="c1"># the exception instance</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="n">inst</span><span class="o">.</span><span class="n">args</span><span class="p">)</span> <span class="c1"># arguments stored in .args</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="n">inst</span><span class="p">)</span> <span class="c1"># __str__ allows args to be printed directly,</span> <span class="gp">... </span> <span class="c1"># but may be overridden in exception subclasses</span> <span class="gp">... </span> <span class="n">x</span><span class="p">,</span> <span class="n">y</span> <span class="o">=</span> <span class="n">inst</span><span class="o">.</span><span class="n">args</span> <span class="c1"># unpack args</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s1">'x ='</span><span class="p">,</span> <span class="n">x</span><span class="p">)</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s1">'y ='</span><span class="p">,</span> <span class="n">y</span><span class="p">)</span> <span class="gp">...</span> <span class="go"><class 'Exception'></span> <span class="go">('spam', 'eggs')</span> <span class="go">('spam', 'eggs')</span> <span class="go">x = spam</span> <span class="go">y = eggs</span> </pre></div> </div> <p>If an exception has arguments, they are printed as the last part (‘detail’) of the message for unhandled exceptions.</p> <p>Exception handlers don’t just handle exceptions if they occur immediately in the try clause, but also if they occur inside functions that are called (even indirectly) in the try clause. For example:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span> <span class="nf">this_fails</span><span class="p">():</span> <span class="gp">... </span> <span class="n">x</span> <span class="o">=</span> <span class="mi">1</span><span class="o">/</span><span class="mi">0</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="k">try</span><span class="p">:</span> <span class="gp">... </span> <span class="n">this_fails</span><span class="p">()</span> <span class="gp">... </span><span class="k">except</span> <span class="ne">ZeroDivisionError</span> <span class="k">as</span> <span class="n">err</span><span class="p">:</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s1">'Handling run-time error:'</span><span class="p">,</span> <span class="n">err</span><span class="p">)</span> <span class="gp">...</span> <span class="go">Handling run-time error: division by zero</span> </pre></div> </div> </div> <div class="section" id="raising-exceptions"> <span id="tut-raising"></span><h2>8.4. Raising Exceptions<a class="headerlink" href="#raising-exceptions" title="Permalink to this headline">¶</a></h2> <p>The <a class="reference internal" href="../reference/simple_stmts.html#raise"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">raise</span></code></a> statement allows the programmer to force a specified exception to occur. For example:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">raise</span> <span class="ne">NameError</span><span class="p">(</span><span class="s1">'HiThere'</span><span class="p">)</span> <span class="gt">Traceback (most recent call last):</span> File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n"><module></span> <span class="gr">NameError</span>: <span class="n">HiThere</span> </pre></div> </div> <p>The sole argument to <a class="reference internal" href="../reference/simple_stmts.html#raise"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">raise</span></code></a> indicates the exception to be raised. This must be either an exception instance or an exception class (a class that derives from <a class="reference internal" href="../library/exceptions.html#Exception" title="Exception"><code class="xref py py-class docutils literal notranslate"><span class="pre">Exception</span></code></a>). If an exception class is passed, it will be implicitly instantiated by calling its constructor with no arguments:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">raise</span> <span class="ne">ValueError</span> <span class="c1"># shorthand for 'raise ValueError()'</span> </pre></div> </div> <p>If you need to determine whether an exception was raised but don’t intend to handle it, a simpler form of the <a class="reference internal" href="../reference/simple_stmts.html#raise"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">raise</span></code></a> statement allows you to re-raise the exception:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">try</span><span class="p">:</span> <span class="gp">... </span> <span class="k">raise</span> <span class="ne">NameError</span><span class="p">(</span><span class="s1">'HiThere'</span><span class="p">)</span> <span class="gp">... </span><span class="k">except</span> <span class="ne">NameError</span><span class="p">:</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s1">'An exception flew by!'</span><span class="p">)</span> <span class="gp">... </span> <span class="k">raise</span> <span class="gp">...</span> <span class="go">An exception flew by!</span> <span class="gt">Traceback (most recent call last):</span> File <span class="nb">"<stdin>"</span>, line <span class="m">2</span>, in <span class="n"><module></span> <span class="gr">NameError</span>: <span class="n">HiThere</span> </pre></div> </div> </div> <div class="section" id="user-defined-exceptions"> <span id="tut-userexceptions"></span><h2>8.5. User-defined Exceptions<a class="headerlink" href="#user-defined-exceptions" title="Permalink to this headline">¶</a></h2> <p>Programs may name their own exceptions by creating a new exception class (see <a class="reference internal" href="classes.html#tut-classes"><span class="std std-ref">Classes</span></a> for more about Python classes). Exceptions should typically be derived from the <a class="reference internal" href="../library/exceptions.html#Exception" title="Exception"><code class="xref py py-exc docutils literal notranslate"><span class="pre">Exception</span></code></a> class, either directly or indirectly.</p> <p>Exception classes can be defined which do anything any other class can do, but are usually kept simple, often only offering a number of attributes that allow information about the error to be extracted by handlers for the exception. When creating a module that can raise several distinct errors, a common practice is to create a base class for exceptions defined by that module, and subclass that to create specific exception classes for different error conditions:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Error</span><span class="p">(</span><span class="ne">Exception</span><span class="p">):</span> <span class="sd">"""Base class for exceptions in this module."""</span> <span class="k">pass</span> <span class="k">class</span> <span class="nc">InputError</span><span class="p">(</span><span class="n">Error</span><span class="p">):</span> <span class="sd">"""Exception raised for errors in the input.</span> <span class="sd"> Attributes:</span> <span class="sd"> expression -- input expression in which the error occurred</span> <span class="sd"> message -- explanation of the error</span> <span class="sd"> """</span> <span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">expression</span><span class="p">,</span> <span class="n">message</span><span class="p">):</span> <span class="bp">self</span><span class="o">.</span><span class="n">expression</span> <span class="o">=</span> <span class="n">expression</span> <span class="bp">self</span><span class="o">.</span><span class="n">message</span> <span class="o">=</span> <span class="n">message</span> <span class="k">class</span> <span class="nc">TransitionError</span><span class="p">(</span><span class="n">Error</span><span class="p">):</span> <span class="sd">"""Raised when an operation attempts a state transition that's not</span> <span class="sd"> allowed.</span> <span class="sd"> Attributes:</span> <span class="sd"> previous -- state at beginning of transition</span> <span class="sd"> next -- attempted new state</span> <span class="sd"> message -- explanation of why the specific transition is not allowed</span> <span class="sd"> """</span> <span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">previous</span><span class="p">,</span> <span class="nb">next</span><span class="p">,</span> <span class="n">message</span><span class="p">):</span> <span class="bp">self</span><span class="o">.</span><span class="n">previous</span> <span class="o">=</span> <span class="n">previous</span> <span class="bp">self</span><span class="o">.</span><span class="n">next</span> <span class="o">=</span> <span class="nb">next</span> <span class="bp">self</span><span class="o">.</span><span class="n">message</span> <span class="o">=</span> <span class="n">message</span> </pre></div> </div> <p>Most exceptions are defined with names that end in “Error,” similar to the naming of the standard exceptions.</p> <p>Many standard modules define their own exceptions to report errors that may occur in functions they define. More information on classes is presented in chapter <a class="reference internal" href="classes.html#tut-classes"><span class="std std-ref">Classes</span></a>.</p> </div> <div class="section" id="defining-clean-up-actions"> <span id="tut-cleanup"></span><h2>8.6. Defining Clean-up Actions<a class="headerlink" href="#defining-clean-up-actions" title="Permalink to this headline">¶</a></h2> <p>The <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> statement has another optional clause which is intended to define clean-up actions that must be executed under all circumstances. For example:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">try</span><span class="p">:</span> <span class="gp">... </span> <span class="k">raise</span> <span class="ne">KeyboardInterrupt</span> <span class="gp">... </span><span class="k">finally</span><span class="p">:</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s1">'Goodbye, world!'</span><span class="p">)</span> <span class="gp">...</span> <span class="go">Goodbye, world!</span> <span class="nc">KeyboardInterrupt</span> <span class="gt">Traceback (most recent call last):</span> File <span class="nb">"<stdin>"</span>, line <span class="m">2</span>, in <span class="n"><module></span> </pre></div> </div> <p>A <em>finally clause</em> is always executed before leaving the <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> statement, whether an exception has occurred or not. When an exception has occurred in the <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> clause and has not been handled by an <a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a> clause (or it has occurred in an <a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a> or <a class="reference internal" href="../reference/compound_stmts.html#else"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">else</span></code></a> clause), it is re-raised after the <a class="reference internal" href="../reference/compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> clause has been executed. The <a class="reference internal" href="../reference/compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> clause is also executed “on the way out” when any other clause of the <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> statement is left via a <a class="reference internal" href="../reference/simple_stmts.html#break"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">break</span></code></a>, <a class="reference internal" href="../reference/simple_stmts.html#continue"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">continue</span></code></a> or <a class="reference internal" href="../reference/simple_stmts.html#return"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code></a> statement. A more complicated example:</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span> <span class="nf">divide</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">):</span> <span class="gp">... </span> <span class="k">try</span><span class="p">:</span> <span class="gp">... </span> <span class="n">result</span> <span class="o">=</span> <span class="n">x</span> <span class="o">/</span> <span class="n">y</span> <span class="gp">... </span> <span class="k">except</span> <span class="ne">ZeroDivisionError</span><span class="p">:</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"division by zero!"</span><span class="p">)</span> <span class="gp">... </span> <span class="k">else</span><span class="p">:</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"result is"</span><span class="p">,</span> <span class="n">result</span><span class="p">)</span> <span class="gp">... </span> <span class="k">finally</span><span class="p">:</span> <span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"executing finally clause"</span><span class="p">)</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="n">divide</span><span class="p">(</span><span class="mi">2</span><span class="p">,</span> <span class="mi">1</span><span class="p">)</span> <span class="go">result is 2.0</span> <span class="go">executing finally clause</span> <span class="gp">>>> </span><span class="n">divide</span><span class="p">(</span><span class="mi">2</span><span class="p">,</span> <span class="mi">0</span><span class="p">)</span> <span class="go">division by zero!</span> <span class="go">executing finally clause</span> <span class="gp">>>> </span><span class="n">divide</span><span class="p">(</span><span class="s2">"2"</span><span class="p">,</span> <span class="s2">"1"</span><span class="p">)</span> <span class="go">executing finally clause</span> <span class="gt">Traceback (most recent call last):</span> File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n"><module></span> File <span class="nb">"<stdin>"</span>, line <span class="m">3</span>, in <span class="n">divide</span> <span class="gr">TypeError</span>: <span class="n">unsupported operand type(s) for /: 'str' and 'str'</span> </pre></div> </div> <p>As you can see, the <a class="reference internal" href="../reference/compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> clause is executed in any event. The <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> raised by dividing two strings is not handled by the <a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a> clause and therefore re-raised after the <a class="reference internal" href="../reference/compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> clause has been executed.</p> <p>In real world applications, the <a class="reference internal" href="../reference/compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> clause is useful for releasing external resources (such as files or network connections), regardless of whether the use of the resource was successful.</p> </div> <div class="section" id="predefined-clean-up-actions"> <span id="tut-cleanup-with"></span><h2>8.7. Predefined Clean-up Actions<a class="headerlink" href="#predefined-clean-up-actions" title="Permalink to this headline">¶</a></h2> <p>Some objects define standard clean-up actions to be undertaken when the object is no longer needed, regardless of whether or not the operation using the object succeeded or failed. Look at the following example, which tries to open a file and print its contents to the screen.</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">for</span> <span class="n">line</span> <span class="ow">in</span> <span class="nb">open</span><span class="p">(</span><span class="s2">"myfile.txt"</span><span class="p">):</span> <span class="nb">print</span><span class="p">(</span><span class="n">line</span><span class="p">,</span> <span class="n">end</span><span class="o">=</span><span class="s2">""</span><span class="p">)</span> </pre></div> </div> <p>The problem with this code is that it leaves the file open for an indeterminate amount of time after this part of the code has finished executing. This is not an issue in simple scripts, but can be a problem for larger applications. The <a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement allows objects like files to be used in a way that ensures they are always cleaned up promptly and correctly.</p> <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="s2">"myfile.txt"</span><span class="p">)</span> <span class="k">as</span> <span class="n">f</span><span class="p">:</span> <span class="k">for</span> <span class="n">line</span> <span class="ow">in</span> <span class="n">f</span><span class="p">:</span> <span class="nb">print</span><span class="p">(</span><span class="n">line</span><span class="p">,</span> <span class="n">end</span><span class="o">=</span><span class="s2">""</span><span class="p">)</span> </pre></div> </div> <p>After the statement is executed, the file <em>f</em> is always closed, even if a problem was encountered while processing the lines. Objects which, like files, provide predefined clean-up actions will indicate this in their documentation.</p> </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="#">8. Errors and Exceptions</a><ul> <li><a class="reference internal" href="#syntax-errors">8.1. Syntax Errors</a></li> <li><a class="reference internal" href="#exceptions">8.2. Exceptions</a></li> <li><a class="reference internal" href="#handling-exceptions">8.3. Handling Exceptions</a></li> <li><a class="reference internal" href="#raising-exceptions">8.4. Raising Exceptions</a></li> <li><a class="reference internal" href="#user-defined-exceptions">8.5. User-defined Exceptions</a></li> <li><a class="reference internal" href="#defining-clean-up-actions">8.6. Defining Clean-up Actions</a></li> <li><a class="reference internal" href="#predefined-clean-up-actions">8.7. Predefined Clean-up Actions</a></li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="inputoutput.html" title="previous chapter">7. Input and Output</a></p> <h4>Next topic</h4> <p class="topless"><a href="classes.html" title="next chapter">9. 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