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<li><a class="reference internal" href="#">8. Errors and Exceptions</a><ul>
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<li><a class="reference internal" href="#syntax-errors">8.1. Syntax Errors</a></li>
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<li><a class="reference internal" href="#exceptions">8.2. Exceptions</a></li>
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<li><a class="reference internal" href="#handling-exceptions">8.3. Handling Exceptions</a></li>
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<li><a class="reference internal" href="#raising-exceptions">8.4. Raising Exceptions</a></li>
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<li><a class="reference internal" href="#exception-chaining">8.5. Exception Chaining</a></li>
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<li><a class="reference internal" href="#user-defined-exceptions">8.6. User-defined Exceptions</a></li>
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<li><a class="reference internal" href="#defining-clean-up-actions">8.7. Defining Clean-up Actions</a></li>
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<li><a class="reference internal" href="#predefined-clean-up-actions">8.8. Predefined Clean-up Actions</a></li>
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<li><a class="reference internal" href="#raising-and-handling-multiple-unrelated-exceptions">8.9. Raising and Handling Multiple Unrelated Exceptions</a></li>
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<li><a class="reference internal" href="#enriching-exceptions-with-notes">8.10. Enriching Exceptions with Notes</a></li>
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<section id="errors-and-exceptions">
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<span id="tut-errors"></span><h1><span class="section-number">8. </span>Errors and Exceptions<a class="headerlink" href="#errors-and-exceptions" title="Link to this heading">¶</a></h1>
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<p>Until now error messages haven’t been more than mentioned, but if you have tried
|
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out the examples you have probably seen some. There are (at least) two
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distinguishable kinds of errors: <em>syntax errors</em> and <em>exceptions</em>.</p>
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<section id="syntax-errors">
|
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<span id="tut-syntaxerrors"></span><h2><span class="section-number">8.1. </span>Syntax Errors<a class="headerlink" href="#syntax-errors" title="Link to this heading">¶</a></h2>
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<p>Syntax errors, also known as parsing errors, are perhaps the most common kind of
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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>
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||
File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>
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||
<span class="w"> </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="w"> </span><span class="pm">^^^^^</span>
|
||
<span class="gr">SyntaxError</span>: <span class="n">invalid syntax</span>
|
||
</pre></div>
|
||
</div>
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||
<p>The parser repeats the offending line and displays little arrows pointing
|
||
at the place where the error was detected. Note that this is not always the
|
||
place that needs to be fixed. 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 just before it.</p>
|
||
<p>The file name (<code class="docutils literal notranslate"><span class="pre"><stdin></span></code> in our example) and line number are printed so you
|
||
know where to look in case the input came from a file.</p>
|
||
</section>
|
||
<section id="exceptions">
|
||
<span id="tut-exceptions"></span><h2><span class="section-number">8.2. </span>Exceptions<a class="headerlink" href="#exceptions" title="Link to this heading">¶</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
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||
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>
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||
<span class="gt">Traceback (most recent call last):</span>
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||
File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n"><module></span>
|
||
<span class="w"> </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="w"> </span><span class="pm">~^~</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="w"> </span><span class="mi">4</span> <span class="o">+</span> <span class="n">spam</span><span class="o">*</span><span class="mi">3</span>
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<span class="w"> </span><span class="pm">^^^^</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="w"> </span><span class="s1">'2'</span> <span class="o">+</span> <span class="mi">2</span>
|
||
<span class="w"> </span><span class="pm">~~~~^~~</span>
|
||
<span class="gr">TypeError</span>: <span class="n">can only concatenate str (not "int") to str</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
|
||
occurred, 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>
|
||
</section>
|
||
<section id="handling-exceptions">
|
||
<span id="tut-handling"></span><h2><span class="section-number">8.3. </span>Handling Exceptions<a class="headerlink" href="#handling-exceptions" title="Link to this heading">¶</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</kbd>-<kbd class="kbd docutils literal notranslate">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><p>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.</p></li>
|
||
<li><p>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.</p></li>
|
||
<li><p>If an exception occurs during 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> 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 <em>except clause</em> is executed, and then execution
|
||
continues after the try/except block.</p></li>
|
||
<li><p>If an exception occurs which does not match the exception named in the <em>except
|
||
clause</em>, 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 an error message.</p></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 <em>except clause</em>, to specify
|
||
handlers for different exceptions. At most one handler will be executed.
|
||
Handlers only handle exceptions that occur in the corresponding <em>try clause</em>,
|
||
not in other handlers of the same <code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code> statement. An <em>except clause</em>
|
||
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 matches exceptions which are instances of the
|
||
class itself or one of its derived classes (but not the other way around — an
|
||
<em>except clause</em> listing a derived class does not match instances of its base classes).
|
||
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="w"> </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="w"> </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="w"> </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 <em>except clauses</em> 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 <em>except clause</em> is triggered.</p>
|
||
<p>When an exception occurs, it may have associated values, also known as the
|
||
exception’s <em>arguments</em>. The presence and types of the arguments depend on the
|
||
exception type.</p>
|
||
<p>The <em>except clause</em> may specify a variable after the exception name. The
|
||
variable is bound to the exception instance which typically has an <code class="docutils literal notranslate"><span class="pre">args</span></code>
|
||
attribute that stores the arguments. For convenience, builtin exception
|
||
types define <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> to print all the arguments without explicitly
|
||
accessing <code class="docutils literal notranslate"><span class="pre">.args</span></code>.</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 type</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>The exception’s <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> output is printed as the last part (‘detail’)
|
||
of the message for unhandled exceptions.</p>
|
||
<p><a class="reference internal" href="../library/exceptions.html#BaseException" title="BaseException"><code class="xref py py-exc docutils literal notranslate"><span class="pre">BaseException</span></code></a> is the common base class of all exceptions. One of its
|
||
subclasses, <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>, is the base class of all the non-fatal exceptions.
|
||
Exceptions which are not subclasses of <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> are not typically
|
||
handled, because they are used to indicate that the program should terminate.
|
||
They include <a class="reference internal" href="../library/exceptions.html#SystemExit" title="SystemExit"><code class="xref py py-exc docutils literal notranslate"><span class="pre">SystemExit</span></code></a> which is raised by <a class="reference internal" href="../library/sys.html#sys.exit" title="sys.exit"><code class="xref py py-meth docutils literal notranslate"><span class="pre">sys.exit()</span></code></a> and
|
||
<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> which is raised when a user wishes to interrupt
|
||
the program.</p>
|
||
<p><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> can be used as a wildcard that catches (almost) everything.
|
||
However, it is good practice to be as specific as possible with the types
|
||
of exceptions that we intend to handle, and to allow any unexpected
|
||
exceptions to propagate on.</p>
|
||
<p>The most common pattern for handling <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> is to print or log
|
||
the exception and then re-raise it (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="w"> </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="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="ne">Exception</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="sa">f</span><span class="s2">"Unexpected </span><span class="si">{</span><span class="n">err</span><span class="si">=}</span><span class="s2">, </span><span class="si">{</span><span class="nb">type</span><span class="p">(</span><span class="n">err</span><span class="p">)</span><span class="si">=}</span><span class="s2">"</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 <em>except clauses</em>. It is useful
|
||
for code that must be executed if the <em>try clause</em> 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 <code class="xref std std-keyword docutils literal notranslate"><span class="pre">else</span></code> 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 <code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code> …
|
||
<code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code> statement.</p>
|
||
<p>Exception handlers do not handle only exceptions that occur immediately in the
|
||
<em>try clause</em>, but also those that occur inside functions that are called (even
|
||
indirectly) in the <em>try clause</em>. 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="w"> </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>
|
||
</section>
|
||
<section id="raising-exceptions">
|
||
<span id="tut-raising"></span><h2><span class="section-number">8.4. </span>Raising Exceptions<a class="headerlink" href="#raising-exceptions" title="Link to this heading">¶</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="w"> </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="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#BaseException" title="BaseException"><code class="xref py py-class docutils literal notranslate"><span class="pre">BaseException</span></code></a>, such as <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> or one of its
|
||
subclasses). 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="w"> </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="gr">NameError</span>: <span class="n">HiThere</span>
|
||
</pre></div>
|
||
</div>
|
||
</section>
|
||
<section id="exception-chaining">
|
||
<span id="tut-exception-chaining"></span><h2><span class="section-number">8.5. </span>Exception Chaining<a class="headerlink" href="#exception-chaining" title="Link to this heading">¶</a></h2>
|
||
<p>If an unhandled exception occurs inside 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> section, it will
|
||
have the exception being handled attached to it and included in the error
|
||
message:</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="nb">open</span><span class="p">(</span><span class="s2">"database.sqlite"</span><span class="p">)</span>
|
||
<span class="gp">... </span><span class="k">except</span> <span class="ne">OSError</span><span class="p">:</span>
|
||
<span class="gp">... </span> <span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s2">"unable to handle error"</span><span class="p">)</span>
|
||
<span class="gp">...</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="w"> </span><span class="nb">open</span><span class="p">(</span><span class="s2">"database.sqlite"</span><span class="p">)</span>
|
||
<span class="w"> </span><span class="pm">~~~~^^^^^^^^^^^^^^^^^^^</span>
|
||
<span class="gr">FileNotFoundError</span>: <span class="n">[Errno 2] No such file or directory: 'database.sqlite'</span>
|
||
|
||
<span class="gt">During handling of the above exception, another exception occurred:</span>
|
||
|
||
<span class="gt">Traceback (most recent call last):</span>
|
||
File <span class="nb">"<stdin>"</span>, line <span class="m">4</span>, in <span class="n"><module></span>
|
||
<span class="w"> </span><span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s2">"unable to handle error"</span><span class="p">)</span>
|
||
<span class="gr">RuntimeError</span>: <span class="n">unable to handle error</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>To indicate that an exception is a direct consequence of another, 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 an optional <a class="reference internal" href="../reference/simple_stmts.html#raise"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">from</span></code></a> clause:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="c1"># exc must be exception instance or None.</span>
|
||
<span class="k">raise</span> <span class="ne">RuntimeError</span> <span class="kn">from</span><span class="w"> </span><span class="nn">exc</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>This can be useful when you are transforming exceptions. 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="w"> </span><span class="nf">func</span><span class="p">():</span>
|
||
<span class="gp">... </span> <span class="k">raise</span> <span class="ne">ConnectionError</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">func</span><span class="p">()</span>
|
||
<span class="gp">... </span><span class="k">except</span> <span class="ne">ConnectionError</span> <span class="k">as</span> <span class="n">exc</span><span class="p">:</span>
|
||
<span class="gp">... </span> <span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s1">'Failed to open database'</span><span class="p">)</span> <span class="kn">from</span><span class="w"> </span><span class="nn">exc</span>
|
||
<span class="gp">...</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="w"> </span><span class="n">func</span><span class="p">()</span>
|
||
<span class="w"> </span><span class="pm">~~~~^^</span>
|
||
File <span class="nb">"<stdin>"</span>, line <span class="m">2</span>, in <span class="n">func</span>
|
||
<span class="gr">ConnectionError</span>
|
||
|
||
<span class="gt">The above exception was the direct cause of the following exception:</span>
|
||
|
||
<span class="gt">Traceback (most recent call last):</span>
|
||
File <span class="nb">"<stdin>"</span>, line <span class="m">4</span>, in <span class="n"><module></span>
|
||
<span class="w"> </span><span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s1">'Failed to open database'</span><span class="p">)</span> <span class="kn">from</span><span class="w"> </span><span class="nn">exc</span>
|
||
<span class="gr">RuntimeError</span>: <span class="n">Failed to open database</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>It also allows disabling automatic exception chaining using the <code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">None</span></code>
|
||
idiom:</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="nb">open</span><span class="p">(</span><span class="s1">'database.sqlite'</span><span class="p">)</span>
|
||
<span class="gp">... </span><span class="k">except</span> <span class="ne">OSError</span><span class="p">:</span>
|
||
<span class="gp">... </span> <span class="k">raise</span> <span class="ne">RuntimeError</span> <span class="kn">from</span><span class="w"> </span><span class="kc">None</span>
|
||
<span class="gp">...</span>
|
||
<span class="gt">Traceback (most recent call last):</span>
|
||
File <span class="nb">"<stdin>"</span>, line <span class="m">4</span>, in <span class="n"><module></span>
|
||
<span class="w"> </span><span class="k">raise</span> <span class="ne">RuntimeError</span> <span class="kn">from</span><span class="w"> </span><span class="kc">None</span>
|
||
<span class="gr">RuntimeError</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>For more information about chaining mechanics, see <a class="reference internal" href="../library/exceptions.html#bltin-exceptions"><span class="std std-ref">Built-in Exceptions</span></a>.</p>
|
||
</section>
|
||
<section id="user-defined-exceptions">
|
||
<span id="tut-userexceptions"></span><h2><span class="section-number">8.6. </span>User-defined Exceptions<a class="headerlink" href="#user-defined-exceptions" title="Link to this heading">¶</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.</p>
|
||
<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.</p>
|
||
</section>
|
||
<section id="defining-clean-up-actions">
|
||
<span id="tut-cleanup"></span><h2><span class="section-number">8.7. </span>Defining Clean-up Actions<a class="headerlink" href="#defining-clean-up-actions" title="Link to this heading">¶</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="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="w"> </span><span class="k">raise</span> <span class="ne">KeyboardInterrupt</span>
|
||
<span class="nc">KeyboardInterrupt</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>If a <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 present, the <code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code>
|
||
clause will execute as the last task before 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 completes. The <code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code> clause runs whether or
|
||
not the <code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code> statement produces an exception. The following
|
||
points discuss more complex cases when an exception occurs:</p>
|
||
<ul class="simple">
|
||
<li><p>If an exception occurs during execution of the <code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code>
|
||
clause, the exception may be 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. If the exception is not handled by an <code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code>
|
||
clause, the exception is re-raised after the <code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code>
|
||
clause has been executed.</p></li>
|
||
<li><p>An exception could occur during execution of an <code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code>
|
||
or <code class="xref std std-keyword docutils literal notranslate"><span class="pre">else</span></code> clause. Again, the exception is re-raised after
|
||
the <code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code> clause has been executed.</p></li>
|
||
<li><p>If the <code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code> clause executes 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, exceptions are not
|
||
re-raised.</p></li>
|
||
<li><p>If the <code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code> statement reaches 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, the
|
||
<code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code> clause will execute just prior to the
|
||
<code class="xref std std-keyword docutils literal notranslate"><span class="pre">break</span></code>, <code class="xref std std-keyword docutils literal notranslate"><span class="pre">continue</span></code> or <code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code>
|
||
statement’s execution.</p></li>
|
||
<li><p>If a <code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code> clause includes a <code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code>
|
||
statement, the returned value will be the one from the
|
||
<code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code> clause’s <code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code> statement, not the
|
||
value from the <code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code> clause’s <code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code>
|
||
statement.</p></li>
|
||
</ul>
|
||
<p>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="w"> </span><span class="nf">bool_return</span><span class="p">():</span>
|
||
<span class="gp">... </span> <span class="k">try</span><span class="p">:</span>
|
||
<span class="gp">... </span> <span class="k">return</span> <span class="kc">True</span>
|
||
<span class="gp">... </span> <span class="k">finally</span><span class="p">:</span>
|
||
<span class="gp">... </span> <span class="k">return</span> <span class="kc">False</span>
|
||
<span class="gp">...</span>
|
||
<span class="gp">>>> </span><span class="n">bool_return</span><span class="p">()</span>
|
||
<span class="go">False</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>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="w"> </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>
|
||
<span class="w"> </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="w"> </span><span class="pm">~~~~~~^^^^^^^^^^</span>
|
||
File <span class="nb">"<stdin>"</span>, line <span class="m">3</span>, in <span class="n">divide</span>
|
||
<span class="w"> </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="w"> </span><span class="pm">~~^~~</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 <code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code>
|
||
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>
|
||
</section>
|
||
<section id="predefined-clean-up-actions">
|
||
<span id="tut-cleanup-with"></span><h2><span class="section-number">8.8. </span>Predefined Clean-up Actions<a class="headerlink" href="#predefined-clean-up-actions" title="Link to this heading">¶</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>
|
||
</section>
|
||
<section id="raising-and-handling-multiple-unrelated-exceptions">
|
||
<span id="tut-exception-groups"></span><h2><span class="section-number">8.9. </span>Raising and Handling Multiple Unrelated Exceptions<a class="headerlink" href="#raising-and-handling-multiple-unrelated-exceptions" title="Link to this heading">¶</a></h2>
|
||
<p>There are situations where it is necessary to report several exceptions that
|
||
have occurred. This is often the case in concurrency frameworks, when several
|
||
tasks may have failed in parallel, but there are also other use cases where
|
||
it is desirable to continue execution and collect multiple errors rather than
|
||
raise the first exception.</p>
|
||
<p>The builtin <a class="reference internal" href="../library/exceptions.html#ExceptionGroup" title="ExceptionGroup"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ExceptionGroup</span></code></a> wraps a list of exception instances so
|
||
that they can be raised together. It is an exception itself, so it can be
|
||
caught like any other exception.</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span><span class="w"> </span><span class="nf">f</span><span class="p">():</span>
|
||
<span class="gp">... </span> <span class="n">excs</span> <span class="o">=</span> <span class="p">[</span><span class="ne">OSError</span><span class="p">(</span><span class="s1">'error 1'</span><span class="p">),</span> <span class="ne">SystemError</span><span class="p">(</span><span class="s1">'error 2'</span><span class="p">)]</span>
|
||
<span class="gp">... </span> <span class="k">raise</span> <span class="n">ExceptionGroup</span><span class="p">(</span><span class="s1">'there were problems'</span><span class="p">,</span> <span class="n">excs</span><span class="p">)</span>
|
||
<span class="gp">...</span>
|
||
<span class="gp">>>> </span><span class="n">f</span><span class="p">()</span>
|
||
<span class="go"> + Exception Group Traceback (most recent call last):</span>
|
||
<span class="go"> | File "<stdin>", line 1, in <module></span>
|
||
<span class="go"> | f()</span>
|
||
<span class="go"> | ~^^</span>
|
||
<span class="go"> | File "<stdin>", line 3, in f</span>
|
||
<span class="go"> | raise ExceptionGroup('there were problems', excs)</span>
|
||
<span class="go"> | ExceptionGroup: there were problems (2 sub-exceptions)</span>
|
||
<span class="go"> +-+---------------- 1 ----------------</span>
|
||
<span class="go"> | OSError: error 1</span>
|
||
<span class="go"> +---------------- 2 ----------------</span>
|
||
<span class="go"> | SystemError: error 2</span>
|
||
<span class="go"> +------------------------------------</span>
|
||
<span class="gp">>>> </span><span class="k">try</span><span class="p">:</span>
|
||
<span class="gp">... </span> <span class="n">f</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">e</span><span class="p">:</span>
|
||
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s1">'caught </span><span class="si">{</span><span class="nb">type</span><span class="p">(</span><span class="n">e</span><span class="p">)</span><span class="si">}</span><span class="s1">: e'</span><span class="p">)</span>
|
||
<span class="gp">...</span>
|
||
<span class="go">caught <class 'ExceptionGroup'>: e</span>
|
||
<span class="gp">>>></span>
|
||
</pre></div>
|
||
</div>
|
||
<p>By using <code class="docutils literal notranslate"><span class="pre">except*</span></code> instead of <code class="docutils literal notranslate"><span class="pre">except</span></code>, we can selectively
|
||
handle only the exceptions in the group that match a certain
|
||
type. In the following example, which shows a nested exception
|
||
group, each <code class="docutils literal notranslate"><span class="pre">except*</span></code> clause extracts from the group exceptions
|
||
of a certain type while letting all other exceptions propagate to
|
||
other clauses and eventually to be reraised.</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span><span class="w"> </span><span class="nf">f</span><span class="p">():</span>
|
||
<span class="gp">... </span> <span class="k">raise</span> <span class="n">ExceptionGroup</span><span class="p">(</span>
|
||
<span class="gp">... </span> <span class="s2">"group1"</span><span class="p">,</span>
|
||
<span class="gp">... </span> <span class="p">[</span>
|
||
<span class="gp">... </span> <span class="ne">OSError</span><span class="p">(</span><span class="mi">1</span><span class="p">),</span>
|
||
<span class="gp">... </span> <span class="ne">SystemError</span><span class="p">(</span><span class="mi">2</span><span class="p">),</span>
|
||
<span class="gp">... </span> <span class="n">ExceptionGroup</span><span class="p">(</span>
|
||
<span class="gp">... </span> <span class="s2">"group2"</span><span class="p">,</span>
|
||
<span class="gp">... </span> <span class="p">[</span>
|
||
<span class="gp">... </span> <span class="ne">OSError</span><span class="p">(</span><span class="mi">3</span><span class="p">),</span>
|
||
<span class="gp">... </span> <span class="ne">RecursionError</span><span class="p">(</span><span class="mi">4</span><span class="p">)</span>
|
||
<span class="gp">... </span> <span class="p">]</span>
|
||
<span class="gp">... </span> <span class="p">)</span>
|
||
<span class="gp">... </span> <span class="p">]</span>
|
||
<span class="gp">... </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="n">f</span><span class="p">()</span>
|
||
<span class="gp">... </span><span class="k">except</span><span class="o">*</span> <span class="ne">OSError</span> <span class="k">as</span> <span class="n">e</span><span class="p">:</span>
|
||
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"There were OSErrors"</span><span class="p">)</span>
|
||
<span class="gp">... </span><span class="k">except</span><span class="o">*</span> <span class="ne">SystemError</span> <span class="k">as</span> <span class="n">e</span><span class="p">:</span>
|
||
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s2">"There were SystemErrors"</span><span class="p">)</span>
|
||
<span class="gp">...</span>
|
||
<span class="go">There were OSErrors</span>
|
||
<span class="go">There were SystemErrors</span>
|
||
<span class="go"> + Exception Group Traceback (most recent call last):</span>
|
||
<span class="go"> | File "<stdin>", line 2, in <module></span>
|
||
<span class="go"> | f()</span>
|
||
<span class="go"> | ~^^</span>
|
||
<span class="go"> | File "<stdin>", line 2, in f</span>
|
||
<span class="go"> | raise ExceptionGroup(</span>
|
||
<span class="go"> | ...<12 lines>...</span>
|
||
<span class="go"> | )</span>
|
||
<span class="go"> | ExceptionGroup: group1 (1 sub-exception)</span>
|
||
<span class="go"> +-+---------------- 1 ----------------</span>
|
||
<span class="go"> | ExceptionGroup: group2 (1 sub-exception)</span>
|
||
<span class="go"> +-+---------------- 1 ----------------</span>
|
||
<span class="go"> | RecursionError: 4</span>
|
||
<span class="go"> +------------------------------------</span>
|
||
<span class="gp">>>></span>
|
||
</pre></div>
|
||
</div>
|
||
<p>Note that the exceptions nested in an exception group must be instances,
|
||
not types. This is because in practice the exceptions would typically
|
||
be ones that have already been raised and caught by the program, along
|
||
the following pattern:</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">excs</span> <span class="o">=</span> <span class="p">[]</span>
|
||
<span class="gp">... </span><span class="k">for</span> <span class="n">test</span> <span class="ow">in</span> <span class="n">tests</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">test</span><span class="o">.</span><span class="n">run</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">e</span><span class="p">:</span>
|
||
<span class="gp">... </span> <span class="n">excs</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">e</span><span class="p">)</span>
|
||
<span class="gp">...</span>
|
||
<span class="gp">>>> </span><span class="k">if</span> <span class="n">excs</span><span class="p">:</span>
|
||
<span class="gp">... </span> <span class="k">raise</span> <span class="n">ExceptionGroup</span><span class="p">(</span><span class="s2">"Test Failures"</span><span class="p">,</span> <span class="n">excs</span><span class="p">)</span>
|
||
<span class="gp">...</span>
|
||
</pre></div>
|
||
</div>
|
||
</section>
|
||
<section id="enriching-exceptions-with-notes">
|
||
<span id="tut-exception-notes"></span><h2><span class="section-number">8.10. </span>Enriching Exceptions with Notes<a class="headerlink" href="#enriching-exceptions-with-notes" title="Link to this heading">¶</a></h2>
|
||
<p>When an exception is created in order to be raised, it is usually initialized
|
||
with information that describes the error that has occurred. There are cases
|
||
where it is useful to add information after the exception was caught. For this
|
||
purpose, exceptions have a method <code class="docutils literal notranslate"><span class="pre">add_note(note)</span></code> that accepts a string and
|
||
adds it to the exception’s notes list. The standard traceback rendering
|
||
includes all notes, in the order they were added, after 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">TypeError</span><span class="p">(</span><span class="s1">'bad type'</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">e</span><span class="p">:</span>
|
||
<span class="gp">... </span> <span class="n">e</span><span class="o">.</span><span class="n">add_note</span><span class="p">(</span><span class="s1">'Add some information'</span><span class="p">)</span>
|
||
<span class="gp">... </span> <span class="n">e</span><span class="o">.</span><span class="n">add_note</span><span class="p">(</span><span class="s1">'Add some more information'</span><span class="p">)</span>
|
||
<span class="gp">... </span> <span class="k">raise</span>
|
||
<span class="gp">...</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="w"> </span><span class="k">raise</span> <span class="ne">TypeError</span><span class="p">(</span><span class="s1">'bad type'</span><span class="p">)</span>
|
||
<span class="gr">TypeError</span>: <span class="n">bad type</span>
|
||
<span class="x">Add some information</span>
|
||
<span class="x">Add some more information</span>
|
||
<span class="gp">>>></span>
|
||
</pre></div>
|
||
</div>
|
||
<p>For example, when collecting exceptions into an exception group, we may want
|
||
to add context information for the individual errors. In the following each
|
||
exception in the group has a note indicating when this error has occurred.</p>
|
||
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">def</span><span class="w"> </span><span class="nf">f</span><span class="p">():</span>
|
||
<span class="gp">... </span> <span class="k">raise</span> <span class="ne">OSError</span><span class="p">(</span><span class="s1">'operation failed'</span><span class="p">)</span>
|
||
<span class="gp">...</span>
|
||
<span class="gp">>>> </span><span class="n">excs</span> <span class="o">=</span> <span class="p">[]</span>
|
||
<span class="gp">>>> </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="mi">3</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">f</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">e</span><span class="p">:</span>
|
||
<span class="gp">... </span> <span class="n">e</span><span class="o">.</span><span class="n">add_note</span><span class="p">(</span><span class="sa">f</span><span class="s1">'Happened in Iteration </span><span class="si">{</span><span class="n">i</span><span class="o">+</span><span class="mi">1</span><span class="si">}</span><span class="s1">'</span><span class="p">)</span>
|
||
<span class="gp">... </span> <span class="n">excs</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">e</span><span class="p">)</span>
|
||
<span class="gp">...</span>
|
||
<span class="gp">>>> </span><span class="k">raise</span> <span class="n">ExceptionGroup</span><span class="p">(</span><span class="s1">'We have some problems'</span><span class="p">,</span> <span class="n">excs</span><span class="p">)</span>
|
||
<span class="go"> + Exception Group Traceback (most recent call last):</span>
|
||
<span class="go"> | File "<stdin>", line 1, in <module></span>
|
||
<span class="go"> | raise ExceptionGroup('We have some problems', excs)</span>
|
||
<span class="go"> | ExceptionGroup: We have some problems (3 sub-exceptions)</span>
|
||
<span class="go"> +-+---------------- 1 ----------------</span>
|
||
<span class="go"> | Traceback (most recent call last):</span>
|
||
<span class="go"> | File "<stdin>", line 3, in <module></span>
|
||
<span class="go"> | f()</span>
|
||
<span class="go"> | ~^^</span>
|
||
<span class="go"> | File "<stdin>", line 2, in f</span>
|
||
<span class="go"> | raise OSError('operation failed')</span>
|
||
<span class="go"> | OSError: operation failed</span>
|
||
<span class="go"> | Happened in Iteration 1</span>
|
||
<span class="go"> +---------------- 2 ----------------</span>
|
||
<span class="go"> | Traceback (most recent call last):</span>
|
||
<span class="go"> | File "<stdin>", line 3, in <module></span>
|
||
<span class="go"> | f()</span>
|
||
<span class="go"> | ~^^</span>
|
||
<span class="go"> | File "<stdin>", line 2, in f</span>
|
||
<span class="go"> | raise OSError('operation failed')</span>
|
||
<span class="go"> | OSError: operation failed</span>
|
||
<span class="go"> | Happened in Iteration 2</span>
|
||
<span class="go"> +---------------- 3 ----------------</span>
|
||
<span class="go"> | Traceback (most recent call last):</span>
|
||
<span class="go"> | File "<stdin>", line 3, in <module></span>
|
||
<span class="go"> | f()</span>
|
||
<span class="go"> | ~^^</span>
|
||
<span class="go"> | File "<stdin>", line 2, in f</span>
|
||
<span class="go"> | raise OSError('operation failed')</span>
|
||
<span class="go"> | OSError: operation failed</span>
|
||
<span class="go"> | Happened in Iteration 3</span>
|
||
<span class="go"> +------------------------------------</span>
|
||
<span class="gp">>>></span>
|
||
</pre></div>
|
||
</div>
|
||
</section>
|
||
</section>
|
||
|
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|
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<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>
|
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<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>
|
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<li><a class="reference internal" href="#exception-chaining">8.5. Exception Chaining</a></li>
|
||
<li><a class="reference internal" href="#user-defined-exceptions">8.6. User-defined Exceptions</a></li>
|
||
<li><a class="reference internal" href="#defining-clean-up-actions">8.7. Defining Clean-up Actions</a></li>
|
||
<li><a class="reference internal" href="#predefined-clean-up-actions">8.8. Predefined Clean-up Actions</a></li>
|
||
<li><a class="reference internal" href="#raising-and-handling-multiple-unrelated-exceptions">8.9. Raising and Handling Multiple Unrelated Exceptions</a></li>
|
||
<li><a class="reference internal" href="#enriching-exceptions-with-notes">8.10. Enriching Exceptions with Notes</a></li>
|
||
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