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<title>How do I debug my Python extensions?</title>
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<div class="titlepage"><div><div><h3 class="title">
<a name="faq.how_do_i_debug_my_python_extensi"></a><a class="link" href="how_do_i_debug_my_python_extensi.html" title="How do I debug my Python extensions?">How do I debug
      my Python extensions?</a>
</h3></div></div></div>
<p>
        Greg Burley gives the following answer for Unix GCC users:
      </p>
<div class="blockquote"><blockquote class="blockquote">
<p>
          Once you have created a boost python extension for your c++ library or
          class, you may need to debug the code. Afterall this is one of the reasons
          for wrapping the library in python. An expected side-effect or benefit
          of using BPL is that debugging should be isolated to the c++ library that
          is under test, given that python code is minimal and boost::python either
          works or it doesn't. (ie. While errors can occur when the wrapping method
          is invalid, most errors are caught by the compiler ;-).
        </p>
<p>
          The basic steps required to initiate a gdb session to debug a c++ library
          via python are shown here. Note, however that you should start the gdb
          session in the directory that contains your BPL my_ext.so module.
        </p>
<pre class="programlisting"><span class="special">(</span><span class="identifier">gdb</span><span class="special">)</span> <span class="identifier">target</span> <span class="identifier">exec</span> <span class="identifier">python</span>
<span class="special">(</span><span class="identifier">gdb</span><span class="special">)</span> <span class="identifier">run</span>
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">from</span> <span class="identifier">my_ext</span> <span class="identifier">import</span> <span class="special">*</span>
<span class="special">&gt;&gt;&gt;</span> <span class="special">[</span><span class="identifier">C</span><span class="special">-</span><span class="identifier">c</span><span class="special">]</span>
<span class="special">(</span><span class="identifier">gdb</span><span class="special">)</span> <span class="keyword">break</span> <span class="identifier">MyClass</span><span class="special">::</span><span class="identifier">MyBuggyFunction</span>
<span class="special">(</span><span class="identifier">gdb</span><span class="special">)</span> <span class="identifier">cont</span>
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">pyobj</span> <span class="special">=</span> <span class="identifier">MyClass</span><span class="special">()</span>
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">pyobj</span><span class="special">.</span><span class="identifier">MyBuggyFunction</span><span class="special">()</span>
<span class="identifier">Breakpoint</span> <span class="number">1</span><span class="special">,</span> <span class="identifier">MyClass</span><span class="special">::</span><span class="identifier">MyBuggyFunction</span> <span class="special">...</span>
<span class="identifier">Current</span> <span class="identifier">language</span><span class="special">:</span>  <span class="keyword">auto</span><span class="special">;</span> <span class="identifier">currently</span> <span class="identifier">c</span><span class="special">++</span>
<span class="special">(</span><span class="identifier">gdb</span><span class="special">)</span> <span class="keyword">do</span> <span class="identifier">debugging</span> <span class="identifier">stuff</span>
</pre>
</blockquote></div>
<p>
        Greg's approach works even better using Emacs' "gdb" command, since
        it will show you each line of source as you step through it.
      </p>
<p>
        On <span class="bold"><strong>Windows</strong></span>, my favorite debugging solution
        is the debugger that comes with Microsoft Visual C++ 7. This debugger seems
        to work with code generated by all versions of Microsoft and Metrowerks toolsets;
        it's rock solid and "just works" without requiring any special
        tricks from the user.
      </p>
<p>
        Raoul Gough has provided the following for gdb on Windows:
      </p>
<div class="blockquote"><blockquote class="blockquote">
<p>
          gdb support for Windows DLLs has improved lately, so it is now possible
          to debug Python extensions using a few tricks. Firstly, you will need an
          up-to-date gdb with support for minimal symbol extraction from a DLL. Any
          gdb from version 6 onwards, or Cygwin gdb-20030214-1 and onwards should
          do. A suitable release will have a section in the gdb.info file under Configuration
          - Native - Cygwin Native - Non-debug DLL symbols. Refer to that info section
          for more details of the procedures outlined here.
        </p>
<p>
          Secondly, it seems necessary to set a breakpoint in the Python interpreter,
          rather than using ^C to break execution. A good place to set this breakpoint
          is PyOS_Readline, which will stop execution immediately before reading
          each interactive Python command. You have to let Python start once under
          the debugger, so that it loads its own DLL, before you can set the breakpoint:
        </p>
<pre class="programlisting"><span class="error">$</span> <span class="identifier">gdb</span> <span class="identifier">python</span>
<span class="identifier">GNU</span> <span class="identifier">gdb</span> <span class="number">2003</span><span class="special">-</span><span class="number">09</span><span class="special">-</span><span class="number">02</span><span class="special">-</span><span class="identifier">cvs</span> <span class="special">(</span><span class="identifier">cygwin</span><span class="special">-</span><span class="identifier">special</span><span class="special">)</span>
<span class="special">[...]</span>

<span class="special">(</span><span class="identifier">gdb</span><span class="special">)</span> <span class="identifier">run</span>
<span class="identifier">Starting</span> <span class="identifier">program</span><span class="special">:</span> <span class="special">/</span><span class="identifier">cygdrive</span><span class="special">/</span><span class="identifier">c</span><span class="special">/</span><span class="identifier">Python22</span><span class="special">/</span><span class="identifier">python</span><span class="special">.</span><span class="identifier">exe</span>
<span class="identifier">Python</span> <span class="number">2.2</span><span class="special">.</span><span class="number">2</span> <span class="special">(#</span><span class="number">37</span><span class="special">,</span> <span class="identifier">Oct</span> <span class="number">14</span> <span class="number">2002</span><span class="special">,</span> <span class="number">17</span><span class="special">:</span><span class="number">02</span><span class="special">:</span><span class="number">34</span><span class="special">)</span> <span class="special">[</span><span class="identifier">MSC</span> <span class="number">32</span> <span class="identifier">bit</span> <span class="special">(</span><span class="identifier">Intel</span><span class="special">)]</span> <span class="identifier">on</span> <span class="identifier">win32</span>
<span class="identifier">Type</span> <span class="string">"help"</span><span class="special">,</span> <span class="string">"copyright"</span><span class="special">,</span> <span class="string">"credits"</span> <span class="keyword">or</span> <span class="string">"license"</span> <span class="keyword">for</span> <span class="identifier">more</span> <span class="identifier">information</span><span class="special">.</span>
<span class="special">&gt;&gt;&gt;</span> <span class="special">^</span><span class="identifier">Z</span>


<span class="identifier">Program</span> <span class="identifier">exited</span> <span class="identifier">normally</span><span class="special">.</span>
<span class="special">(</span><span class="identifier">gdb</span><span class="special">)</span> <span class="keyword">break</span> <span class="special">*&amp;</span><span class="identifier">PyOS_Readline</span>
<span class="identifier">Breakpoint</span> <span class="number">1</span> <span class="identifier">at</span> <span class="number">0x1e04eff0</span>
<span class="special">(</span><span class="identifier">gdb</span><span class="special">)</span> <span class="identifier">run</span>
<span class="identifier">Starting</span> <span class="identifier">program</span><span class="special">:</span> <span class="special">/</span><span class="identifier">cygdrive</span><span class="special">/</span><span class="identifier">c</span><span class="special">/</span><span class="identifier">Python22</span><span class="special">/</span><span class="identifier">python</span><span class="special">.</span><span class="identifier">exe</span>
<span class="identifier">Python</span> <span class="number">2.2</span><span class="special">.</span><span class="number">2</span> <span class="special">(#</span><span class="number">37</span><span class="special">,</span> <span class="identifier">Oct</span> <span class="number">14</span> <span class="number">2002</span><span class="special">,</span> <span class="number">17</span><span class="special">:</span><span class="number">02</span><span class="special">:</span><span class="number">34</span><span class="special">)</span> <span class="special">[</span><span class="identifier">MSC</span> <span class="number">32</span> <span class="identifier">bit</span> <span class="special">(</span><span class="identifier">Intel</span><span class="special">)]</span> <span class="identifier">on</span> <span class="identifier">win32</span>
<span class="identifier">Type</span> <span class="string">"help"</span><span class="special">,</span> <span class="string">"copyright"</span><span class="special">,</span> <span class="string">"credits"</span> <span class="keyword">or</span> <span class="string">"license"</span> <span class="keyword">for</span> <span class="identifier">more</span> <span class="identifier">information</span><span class="special">.</span>

<span class="identifier">Breakpoint</span> <span class="number">1</span><span class="special">,</span> <span class="number">0x1e04eff0</span> <span class="identifier">in</span> <span class="identifier">python22</span><span class="special">!</span><span class="identifier">PyOS_Readline</span> <span class="special">()</span>
   <span class="identifier">from</span> <span class="special">/</span><span class="identifier">cygdrive</span><span class="special">/</span><span class="identifier">c</span><span class="special">/</span><span class="identifier">WINNT</span><span class="special">/</span><span class="identifier">system32</span><span class="special">/</span><span class="identifier">python22</span><span class="special">.</span><span class="identifier">dll</span>
<span class="special">(</span><span class="identifier">gdb</span><span class="special">)</span> <span class="identifier">cont</span>
<span class="identifier">Continuing</span><span class="special">.</span>
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">from</span> <span class="identifier">my_ext</span> <span class="identifier">import</span> <span class="special">*</span>

<span class="identifier">Breakpoint</span> <span class="number">1</span><span class="special">,</span> <span class="number">0x1e04eff0</span> <span class="identifier">in</span> <span class="identifier">python22</span><span class="special">!</span><span class="identifier">PyOS_Readline</span> <span class="special">()</span>
   <span class="identifier">from</span> <span class="special">/</span><span class="identifier">cygdrive</span><span class="special">/</span><span class="identifier">c</span><span class="special">/</span><span class="identifier">WINNT</span><span class="special">/</span><span class="identifier">system32</span><span class="special">/</span><span class="identifier">python22</span><span class="special">.</span><span class="identifier">dll</span>
<span class="special">(</span><span class="identifier">gdb</span><span class="special">)</span> <span class="special">#</span> <span class="identifier">my_ext</span> <span class="identifier">now</span> <span class="identifier">loaded</span> <span class="special">(</span><span class="identifier">with</span> <span class="identifier">any</span> <span class="identifier">debugging</span> <span class="identifier">symbols</span> <span class="identifier">it</span> <span class="identifier">contains</span><span class="special">)</span>
</pre>
</blockquote></div>
<h3>
<a name="faq.how_do_i_debug_my_python_extensi.h0"></a>
        <span class="phrase"><a name="faq.how_do_i_debug_my_python_extensi.debugging_extensions_through_boo"></a></span><a class="link" href="how_do_i_debug_my_python_extensi.html#faq.how_do_i_debug_my_python_extensi.debugging_extensions_through_boo">Debugging
        extensions through Boost.Build</a>
      </h3>
<p>
        If you are launching your extension module tests with <a href="http://www.boost.org/build" target="_top">Boost.Build</a>
        using the <code class="computeroutput"><span class="identifier">boost</span><span class="special">-</span><span class="identifier">python</span><span class="special">-</span><span class="identifier">runtest</span></code> rule, you can ask it to launch
        your debugger for you by adding "--debugger=<span class="emphasis"><em>debugger</em></span>"
        to your bjam command-line:
      </p>
<pre class="programlisting"><span class="identifier">bjam</span> <span class="special">-</span><span class="identifier">sTOOLS</span><span class="special">=</span><span class="identifier">vc7</span><span class="special">.</span><span class="number">1</span> <span class="string">"--debugger=devenv /debugexe"</span> <span class="identifier">test</span>
<span class="identifier">bjam</span> <span class="special">-</span><span class="identifier">sTOOLS</span><span class="special">=</span><span class="identifier">gcc</span> <span class="special">-</span><span class="identifier">sPYTHON_LAUNCH</span><span class="special">=</span><span class="identifier">gdb</span> <span class="identifier">test</span>
</pre>
<p>
        It can also be extremely useful to add the <code class="computeroutput"><span class="special">-</span><span class="identifier">d</span><span class="special">+</span><span class="number">2</span></code>
        option when you run your test, because Boost.Build will then show you the
        exact commands it uses to invoke it. This will invariably involve setting
        up PYTHONPATH and other important environment variables such as LD_LIBRARY_PATH
        which may be needed by your debugger in order to get things to work right.
      </p>
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        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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