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<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-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="k">while</span> <span class="bp">True</span> <span class="k">print</span> <span class="s">'Hello world'</span>
<span class="go"> File "<stdin>", line 1, in ?</span>
<span class="go"> while True print 'Hello world'</span>
<span class="go"> ^</span>
<span class="go">SyntaxError: 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 keyword <a class="reference internal" href="../reference/simple_stmts.html#print"><tt class="xref std std-keyword docutils literal"><span class="pre">print</span></tt></a>, since a colon
(<tt class="docutils literal"><span class="pre">':'</span></tt>) 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-python"><div class="highlight"><pre><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">?</span>
<span class="gr">ZeroDivisionError</span>: <span class="n">integer division or modulo 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">?</span>
<span class="gr">NameError</span>: <span class="n">name 'spam' is not defined</span>
<span class="gp">>>> </span><span class="s">'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">?</span>
<span class="gr">TypeError</span>: <span class="n">cannot concatenate 'str' and 'int' objects</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#exceptions.ZeroDivisionError" title="exceptions.ZeroDivisionError"><tt class="xref py py-exc docutils literal"><span class="pre">ZeroDivisionError</span></tt></a>, <a class="reference internal" href="../library/exceptions.html#exceptions.NameError" title="exceptions.NameError"><tt class="xref py py-exc docutils literal"><span class="pre">NameError</span></tt></a> and <a class="reference internal" href="../library/exceptions.html#exceptions.TypeError" title="exceptions.TypeError"><tt class="xref py py-exc docutils literal"><span class="pre">TypeError</span></tt></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"><em>Built-in Exceptions</em></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 <tt class="kbd docutils literal"><span class="pre">Control-C</span></tt> 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#exceptions.KeyboardInterrupt" title="exceptions.KeyboardInterrupt"><tt class="xref py py-exc docutils literal"><span class="pre">KeyboardInterrupt</span></tt></a> exception.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="k">while</span> <span class="bp">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">raw_input</span><span class="p">(</span><span class="s">"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="k">print</span> <span class="s">"Oops! That was no valid number. Try again..."</span>
<span class="gp">...</span>
</pre></div>
</div>
<p>The <a class="reference internal" href="../reference/compound_stmts.html#try"><tt class="xref std std-keyword docutils literal"><span class="pre">try</span></tt></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"><tt class="xref std std-keyword docutils literal"><span class="pre">try</span></tt></a> and
<a class="reference internal" href="../reference/compound_stmts.html#except"><tt class="xref std std-keyword docutils literal"><span class="pre">except</span></tt></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"><tt class="xref std std-keyword docutils literal"><span class="pre">try</span></tt></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"><tt class="xref std std-keyword docutils literal"><span class="pre">except</span></tt></a> keyword, the except clause is executed, and then execution
continues after the <a class="reference internal" href="../reference/compound_stmts.html#try"><tt class="xref std std-keyword docutils literal"><span class="pre">try</span></tt></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"><tt class="xref std std-keyword docutils literal"><span class="pre">try</span></tt></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"><tt class="xref std std-keyword docutils literal"><span class="pre">try</span></tt></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"><tt class="xref std std-keyword docutils literal"><span class="pre">try</span></tt></a> statement. An except clause may
name multiple exceptions as a parenthesized tuple, for example:</p>
<div class="highlight-python"><div class="highlight"><pre><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>Note that the parentheses around this tuple are required, because
<tt class="docutils literal"><span class="pre">except</span> <span class="pre">ValueError,</span> <span class="pre">e:</span></tt> was the syntax used for what is normally
written as <tt class="docutils literal"><span class="pre">except</span> <span class="pre">ValueError</span> <span class="pre">as</span> <span class="pre">e:</span></tt> in modern Python (described
below). The old syntax is still supported for backwards compatibility.
This means <tt class="docutils literal"><span class="pre">except</span> <span class="pre">RuntimeError,</span> <span class="pre">TypeError</span></tt> is not equivalent to
<tt class="docutils literal"><span class="pre">except</span> <span class="pre">(RuntimeError,</span> <span class="pre">TypeError):</span></tt> but to <tt class="docutils literal"><span class="pre">except</span> <span class="pre">RuntimeError</span> <span class="pre">as</span>
<span class="pre">TypeError:</span></tt> which is not what you want.</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-python"><div class="highlight"><pre><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="s">'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">IOError</span> <span class="k">as</span> <span class="n">e</span><span class="p">:</span>
<span class="k">print</span> <span class="s">"I/O error({0}): {1}"</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">e</span><span class="o">.</span><span class="n">errno</span><span class="p">,</span> <span class="n">e</span><span class="o">.</span><span class="n">strerror</span><span class="p">)</span>
<span class="k">except</span> <span class="ne">ValueError</span><span class="p">:</span>
<span class="k">print</span> <span class="s">"Could not convert data to an integer."</span>
<span class="k">except</span><span class="p">:</span>
<span class="k">print</span> <span class="s">"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"><tt class="xref std std-keyword docutils literal"><span class="pre">try</span></tt></a> ... <a class="reference internal" href="../reference/compound_stmts.html#except"><tt class="xref std std-keyword docutils literal"><span class="pre">except</span></tt></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-python"><div class="highlight"><pre><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="s">'r'</span><span class="p">)</span>
<span class="k">except</span> <span class="ne">IOError</span><span class="p">:</span>
<span class="k">print</span> <span class="s">'cannot open'</span><span class="p">,</span> <span class="n">arg</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">print</span> <span class="n">arg</span><span class="p">,</span> <span class="s">'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="s">'lines'</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"><tt class="xref std std-keyword docutils literal"><span class="pre">else</span></tt></a> clause is better than adding additional code to
the <a class="reference internal" href="../reference/compound_stmts.html#try"><tt class="xref std std-keyword docutils literal"><span class="pre">try</span></tt></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"><tt class="xref std std-keyword docutils literal"><span class="pre">try</span></tt></a> ...
<a class="reference internal" href="../reference/compound_stmts.html#except"><tt class="xref std std-keyword docutils literal"><span class="pre">except</span></tt></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 (or tuple).
The variable is bound to an exception instance with the arguments stored in
<tt class="docutils literal"><span class="pre">instance.args</span></tt>. For convenience, the exception instance defines
<a class="reference internal" href="../reference/datamodel.html#object.__str__" title="object.__str__"><tt class="xref py py-meth docutils literal"><span class="pre">__str__()</span></tt></a> so the arguments can be printed directly without having to
reference <tt class="docutils literal"><span class="pre">.args</span></tt>.</p>
<p>One may also instantiate an exception first before raising it and add any
attributes to it as desired.</p>
<div class="highlight-python"><div class="highlight"><pre><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="s">'spam'</span><span class="p">,</span> <span class="s">'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="k">print</span> <span class="nb">type</span><span class="p">(</span><span class="n">inst</span><span class="p">)</span> <span class="c"># the exception instance</span>
<span class="gp">... </span> <span class="k">print</span> <span class="n">inst</span><span class="o">.</span><span class="n">args</span> <span class="c"># arguments stored in .args</span>
<span class="gp">... </span> <span class="k">print</span> <span class="n">inst</span> <span class="c"># __str__ allows args to printed directly</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="gp">... </span> <span class="k">print</span> <span class="s">'x ='</span><span class="p">,</span> <span class="n">x</span>
<span class="gp">... </span> <span class="k">print</span> <span class="s">'y ='</span><span class="p">,</span> <span class="n">y</span>
<span class="gp">...</span>
<span class="go"><type 'exceptions.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 an argument, it is 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-python"><div class="highlight"><pre><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">detail</span><span class="p">:</span>
<span class="gp">... </span> <span class="k">print</span> <span class="s">'Handling run-time error:'</span><span class="p">,</span> <span class="n">detail</span>
<span class="gp">...</span>
<span class="go">Handling run-time error: integer division or modulo 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"><tt class="xref std std-keyword docutils literal"><span class="pre">raise</span></tt></a> statement allows the programmer to force a specified
exception to occur. For example:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="k">raise</span> <span class="ne">NameError</span><span class="p">(</span><span class="s">'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">?</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"><tt class="xref std std-keyword docutils literal"><span class="pre">raise</span></tt></a> indicates the exception to be raised.
This must be either an exception instance or an exception class (a class that
derives from <tt class="xref py py-class docutils literal"><span class="pre">Exception</span></tt>).</p>
<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"><tt class="xref std std-keyword docutils literal"><span class="pre">raise</span></tt></a> statement allows you to
re-raise the exception:</p>
<div class="highlight-python"><div class="highlight"><pre><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="s">'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="k">print</span> <span class="s">'An exception flew by!'</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">?</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"><em>Classes</em></a> for more about Python classes). Exceptions should typically
be derived from the <a class="reference internal" href="../library/exceptions.html#exceptions.Exception" title="exceptions.Exception"><tt class="xref py py-exc docutils literal"><span class="pre">Exception</span></tt></a> class, either directly or indirectly. For
example:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="k">class</span> <span class="nc">MyError</span><span class="p">(</span><span class="ne">Exception</span><span class="p">):</span>
<span class="gp">... </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">value</span><span class="p">):</span>
<span class="gp">... </span> <span class="bp">self</span><span class="o">.</span><span class="n">value</span> <span class="o">=</span> <span class="n">value</span>
<span class="gp">... </span> <span class="k">def</span> <span class="nf">__str__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">return</span> <span class="nb">repr</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">value</span><span class="p">)</span>
<span class="gp">...</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="n">MyError</span><span class="p">(</span><span class="mi">2</span><span class="o">*</span><span class="mi">2</span><span class="p">)</span>
<span class="gp">... </span><span class="k">except</span> <span class="n">MyError</span> <span class="k">as</span> <span class="n">e</span><span class="p">:</span>
<span class="gp">... </span> <span class="k">print</span> <span class="s">'My exception occurred, value:'</span><span class="p">,</span> <span class="n">e</span><span class="o">.</span><span class="n">value</span>
<span class="gp">...</span>
<span class="go">My exception occurred, value: 4</span>
<span class="gp">>>> </span><span class="k">raise</span> <span class="n">MyError</span><span class="p">(</span><span class="s">'oops!'</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">?</span>
<span class="gr">__main__.MyError</span>: <span class="n">'oops!'</span>
</pre></div>
</div>
<p>In this example, the default <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><tt class="xref py py-meth docutils literal"><span class="pre">__init__()</span></tt></a> of <tt class="xref py py-class docutils literal"><span class="pre">Exception</span></tt> has been
overridden. The new behavior simply creates the <em>value</em> attribute. This
replaces the default behavior of creating the <em>args</em> attribute.</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-python"><div class="highlight"><pre><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"> expr -- input expression in which the error occurred</span>
<span class="sd"> msg -- 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">expr</span><span class="p">,</span> <span class="n">msg</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">expr</span> <span class="o">=</span> <span class="n">expr</span>
<span class="bp">self</span><span class="o">.</span><span class="n">msg</span> <span class="o">=</span> <span class="n">msg</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"> prev -- state at beginning of transition</span>
<span class="sd"> next -- attempted new state</span>
<span class="sd"> msg -- 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">prev</span><span class="p">,</span> <span class="nb">next</span><span class="p">,</span> <span class="n">msg</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">prev</span> <span class="o">=</span> <span class="n">prev</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">msg</span> <span class="o">=</span> <span class="n">msg</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"><em>Classes</em></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"><tt class="xref std std-keyword docutils literal"><span class="pre">try</span></tt></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-python"><div class="highlight"><pre><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="k">print</span> <span class="s">'Goodbye, world!'</span>
<span class="gp">...</span>
<span class="go">Goodbye, world!</span>
<span class="nc">KeyboardInterrupt</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"><tt class="xref std std-keyword docutils literal"><span class="pre">try</span></tt></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"><tt class="xref std std-keyword docutils literal"><span class="pre">try</span></tt></a> clause and has not been handled by an
<a class="reference internal" href="../reference/compound_stmts.html#except"><tt class="xref std std-keyword docutils literal"><span class="pre">except</span></tt></a> clause (or it has occurred in a <a class="reference internal" href="../reference/compound_stmts.html#except"><tt class="xref std std-keyword docutils literal"><span class="pre">except</span></tt></a> or
<a class="reference internal" href="../reference/compound_stmts.html#else"><tt class="xref std std-keyword docutils literal"><span class="pre">else</span></tt></a> clause), it is re-raised after the <a class="reference internal" href="../reference/compound_stmts.html#finally"><tt class="xref std std-keyword docutils literal"><span class="pre">finally</span></tt></a> clause has
been executed. The <a class="reference internal" href="../reference/compound_stmts.html#finally"><tt class="xref std std-keyword docutils literal"><span class="pre">finally</span></tt></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"><tt class="xref std std-keyword docutils literal"><span class="pre">try</span></tt></a> statement is left via a
<a class="reference internal" href="../reference/simple_stmts.html#break"><tt class="xref std std-keyword docutils literal"><span class="pre">break</span></tt></a>, <a class="reference internal" href="../reference/simple_stmts.html#continue"><tt class="xref std std-keyword docutils literal"><span class="pre">continue</span></tt></a> or <a class="reference internal" href="../reference/simple_stmts.html#return"><tt class="xref std std-keyword docutils literal"><span class="pre">return</span></tt></a> statement. A more
complicated example (having <a class="reference internal" href="../reference/compound_stmts.html#except"><tt class="xref std std-keyword docutils literal"><span class="pre">except</span></tt></a> and <a class="reference internal" href="../reference/compound_stmts.html#finally"><tt class="xref std std-keyword docutils literal"><span class="pre">finally</span></tt></a> clauses in
the same <a class="reference internal" href="../reference/compound_stmts.html#try"><tt class="xref std std-keyword docutils literal"><span class="pre">try</span></tt></a> statement works as of Python 2.5):</p>
<div class="highlight-python"><div class="highlight"><pre><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="k">print</span> <span class="s">"division by zero!"</span>
<span class="gp">... </span> <span class="k">else</span><span class="p">:</span>
<span class="gp">... </span> <span class="k">print</span> <span class="s">"result is"</span><span class="p">,</span> <span class="n">result</span>
<span class="gp">... </span> <span class="k">finally</span><span class="p">:</span>
<span class="gp">... </span> <span class="k">print</span> <span class="s">"executing finally clause"</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</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="s">"2"</span><span class="p">,</span> <span class="s">"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">?</span>
File <span class="nb">"<stdin>"</span>, line <span class="m">3</span>, in <span class="n">divide</span>
<span class="gr">TypeError: unsupported operand type(s) for /</span>: <span class="n">'str' and 'str'</span>
</pre></div>
</div>
<p>As you can see, the <a class="reference internal" href="../reference/compound_stmts.html#finally"><tt class="xref std std-keyword docutils literal"><span class="pre">finally</span></tt></a> clause is executed in any event. The
<a class="reference internal" href="../library/exceptions.html#exceptions.TypeError" title="exceptions.TypeError"><tt class="xref py py-exc docutils literal"><span class="pre">TypeError</span></tt></a> raised by dividing two strings is not handled by the
<a class="reference internal" href="../reference/compound_stmts.html#except"><tt class="xref std std-keyword docutils literal"><span class="pre">except</span></tt></a> clause and therefore re-raised after the <a class="reference internal" href="../reference/compound_stmts.html#finally"><tt class="xref std std-keyword docutils literal"><span class="pre">finally</span></tt></a>
clause has been executed.</p>
<p>In real world applications, the <a class="reference internal" href="../reference/compound_stmts.html#finally"><tt class="xref std std-keyword docutils literal"><span class="pre">finally</span></tt></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-python"><div class="highlight"><pre><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="s">"myfile.txt"</span><span class="p">):</span>
<span class="k">print</span> <span class="n">line</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 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"><tt class="xref std std-keyword docutils literal"><span class="pre">with</span></tt></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-python"><div class="highlight"><pre><span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="s">"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="k">print</span> <span class="n">line</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. Other objects which provide
predefined clean-up actions will indicate this in their documentation.</p>
</div>
</div>
</div>
</div>
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<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. Classes</a></p>
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