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<ul>
<li><a class="reference internal" href="#">Exception Handling</a><ul>
<li><a class="reference internal" href="#unhandled-exceptions">Unhandled Exceptions</a></li>
<li><a class="reference internal" href="#current-exception">Current Exception</a></li>
<li><a class="reference internal" href="#previous-interactive-exception">Previous Interactive Exception</a></li>
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<div class="section" id="exception-handling">
<span id="sys-exceptions"></span><h1>Exception Handling<a class="headerlink" href="#exception-handling" title="Permalink to this headline">¶</a></h1>
<p><a class="reference internal" href="index.html#module-sys" title="sys: System-specific configuration"><tt class="xref py py-mod docutils literal"><span class="pre">sys</span></tt></a> includes features for trapping and working with exceptions.</p>
<div class="section" id="unhandled-exceptions">
<h2>Unhandled Exceptions<a class="headerlink" href="#unhandled-exceptions" title="Permalink to this headline">¶</a></h2>
<p>Many applications are structured with a main loop that wraps execution
in a global exception handler to trap errors not handled at a lower
level. Another way to achieve the same thing is by setting the
<tt class="xref py py-data docutils literal"><span class="pre">sys.excepthook</span></tt> to a function that takes three arguments (error
type, error value, and traceback) and let it deal with unhandled
errors.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">sys</span>
<span class="k">def</span> <span class="nf">my_excepthook</span><span class="p">(</span><span class="nb">type</span><span class="p">,</span> <span class="n">value</span><span class="p">,</span> <span class="n">traceback</span><span class="p">):</span>
<span class="k">print</span> <span class="s">'Unhandled error:'</span><span class="p">,</span> <span class="nb">type</span><span class="p">,</span> <span class="n">value</span>
<span class="n">sys</span><span class="o">.</span><span class="n">excepthook</span> <span class="o">=</span> <span class="n">my_excepthook</span>
<span class="k">print</span> <span class="s">'Before exception'</span>
<span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s">'This is the error message'</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'After exception'</span>
</pre></div>
</div>
<p>Since there is no <strong class="command">try:except</strong> block around the line where
the exception is raised the following <strong class="command">print</strong> statement is
not run, even though the except hook is set.</p>
<div class="highlight-python"><pre>$ python sys_excepthook.py
Before exception
Unhandled error: <type 'exceptions.RuntimeError'> This is the error message</pre>
</div>
</div>
<div class="section" id="current-exception">
<h2>Current Exception<a class="headerlink" href="#current-exception" title="Permalink to this headline">¶</a></h2>
<p>There are times when an explicit exception handler is preferred,
either for code clarity or to avoid conflicts with libraries that try
to install their own excepthook. In these cases you may want to write
a common handler function, but avoid passing the exception object to
it explicitly. You can get the current exception for a thread by
calling <tt class="xref py py-func docutils literal"><span class="pre">exc_info()</span></tt>.</p>
<p>The return value of <tt class="xref py py-func docutils literal"><span class="pre">exc_info()</span></tt> is a three member tuple
containing the exception class, an exception instance, and a
traceback. Using <tt class="xref py py-func docutils literal"><span class="pre">exc_info()</span></tt> is preferred over the old form
(with <tt class="xref py py-const docutils literal"><span class="pre">exc_type</span></tt>, <tt class="xref py py-const docutils literal"><span class="pre">exc_value</span></tt>, and
<tt class="xref py py-const docutils literal"><span class="pre">exc_traceback</span></tt>) because it is thread-safe.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">sys</span>
<span class="kn">import</span> <span class="nn">threading</span>
<span class="kn">import</span> <span class="nn">time</span>
<span class="k">def</span> <span class="nf">do_something_with_exception</span><span class="p">():</span>
<span class="n">exc_type</span><span class="p">,</span> <span class="n">exc_value</span> <span class="o">=</span> <span class="n">sys</span><span class="o">.</span><span class="n">exc_info</span><span class="p">()[:</span><span class="mi">2</span><span class="p">]</span>
<span class="k">print</span> <span class="s">'Handling </span><span class="si">%s</span><span class="s"> exception with message "</span><span class="si">%s</span><span class="s">" in </span><span class="si">%s</span><span class="s">'</span> <span class="o">%</span> \
<span class="p">(</span><span class="n">exc_type</span><span class="o">.</span><span class="n">__name__</span><span class="p">,</span> <span class="n">exc_value</span><span class="p">,</span> <span class="n">threading</span><span class="o">.</span><span class="n">current_thread</span><span class="p">()</span><span class="o">.</span><span class="n">name</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">cause_exception</span><span class="p">(</span><span class="n">delay</span><span class="p">):</span>
<span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="n">delay</span><span class="p">)</span>
<span class="k">raise</span> <span class="ne">RuntimeError</span><span class="p">(</span><span class="s">'This is the error message'</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">thread_target</span><span class="p">(</span><span class="n">delay</span><span class="p">):</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">cause_exception</span><span class="p">(</span><span class="n">delay</span><span class="p">)</span>
<span class="k">except</span><span class="p">:</span>
<span class="n">do_something_with_exception</span><span class="p">()</span>
<span class="n">threads</span> <span class="o">=</span> <span class="p">[</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">thread_target</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="mf">0.3</span><span class="p">,)),</span>
<span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">thread_target</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="mf">0.1</span><span class="p">,)),</span>
<span class="p">]</span>
<span class="k">for</span> <span class="n">t</span> <span class="ow">in</span> <span class="n">threads</span><span class="p">:</span>
<span class="n">t</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="k">for</span> <span class="n">t</span> <span class="ow">in</span> <span class="n">threads</span><span class="p">:</span>
<span class="n">t</span><span class="o">.</span><span class="n">join</span><span class="p">()</span>
</pre></div>
</div>
<p>This example avoids introducing a circular reference between the
traceback object and a local variable in the current frame by ignoring
that part of the return value from <tt class="xref py py-func docutils literal"><span class="pre">exc_info()</span></tt>. If the traceback
is needed (e.g., so it can be logged), explicitly delete the local
variable (using <strong class="command">del</strong>) to avoid cycles.</p>
<div class="highlight-python"><pre>$ python sys_exc_info.py
Handling RuntimeError exception with message "This is the error message" in Thread-2
Handling RuntimeError exception with message "This is the error message" in Thread-1</pre>
</div>
</div>
<div class="section" id="previous-interactive-exception">
<h2>Previous Interactive Exception<a class="headerlink" href="#previous-interactive-exception" title="Permalink to this headline">¶</a></h2>
<p>In the interactive interpreter, there is only one thread of
interaction. Unhandled exceptions in that thread are saved to three
variables in <a class="reference internal" href="index.html#module-sys" title="sys: System-specific configuration"><tt class="xref py py-mod docutils literal"><span class="pre">sys</span></tt></a> (<tt class="xref py py-const docutils literal"><span class="pre">last_type</span></tt>, <tt class="xref py py-const docutils literal"><span class="pre">last_value</span></tt>, and
<tt class="xref py py-const docutils literal"><span class="pre">last_traceback</span></tt>) to make it easy to retrieve them for
debugging. Using the post-mortem debugger in <a class="reference internal" href="../pdb/index.html#module-pdb" title="pdb: Interactive Debugger"><tt class="xref py py-mod docutils literal"><span class="pre">pdb</span></tt></a> avoids any
need to use the values directly.</p>
<div class="highlight-python"><pre>$ python
Python 2.6.2 (r262:71600, Apr 16 2009, 09:17:39)
[GCC 4.0.1 (Apple Computer, Inc. build 5250)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> def cause_exception():
... raise RuntimeError('This is the error message')
...
>>> cause_exception()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 2, in cause_exception
RuntimeError: This is the error message
>>> import pdb
>>> pdb.pm()
> <stdin>(2)cause_exception()
(Pdb) where
<stdin>(1)<module>()
> <stdin>(2)cause_exception()
(Pdb)</pre>
</div>
<div class="admonition-see-also admonition seealso">
<p class="first admonition-title">See also</p>
<dl class="last docutils">
<dt><a class="reference internal" href="../exceptions/index.html#module-exceptions" title="exceptions: Built-in error classes"><tt class="xref py py-mod docutils literal"><span class="pre">exceptions</span></tt></a></dt>
<dd>Built-in errors</dd>
<dt><a class="reference internal" href="../pdb/index.html#module-pdb" title="pdb: Interactive Debugger"><tt class="xref py py-mod docutils literal"><span class="pre">pdb</span></tt></a></dt>
<dd>Python debugger</dd>
<dt><a class="reference internal" href="../traceback/index.html#module-traceback" title="traceback: Extract, format, and print exceptions and stack traces."><tt class="xref py py-mod docutils literal"><span class="pre">traceback</span></tt></a></dt>
<dd>Module for working with tracebacks.</dd>
</dl>
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