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<div class="section" id="extending-embedding-faq">
<h1><a class="toc-backref" href="#id2">Extending/Embedding FAQ</a><a class="headerlink" href="#extending-embedding-faq" title="Permalink to this headline">¶</a></h1>
<div class="contents topic" id="contents">
<p class="topic-title first">Contents</p>
<ul class="simple">
<li><a class="reference internal" href="#extending-embedding-faq" id="id2">Extending/Embedding FAQ</a><ul>
<li><a class="reference internal" href="#can-i-create-my-own-functions-in-c" id="id3">Can I create my own functions in C?</a></li>
<li><a class="reference internal" href="#id1" id="id4">Can I create my own functions in C++?</a></li>
<li><a class="reference internal" href="#writing-c-is-hard-are-there-any-alternatives" id="id5">Writing C is hard; are there any alternatives?</a></li>
<li><a class="reference internal" href="#how-can-i-execute-arbitrary-python-statements-from-c" id="id6">How can I execute arbitrary Python statements from C?</a></li>
<li><a class="reference internal" href="#how-can-i-evaluate-an-arbitrary-python-expression-from-c" id="id7">How can I evaluate an arbitrary Python expression from C?</a></li>
<li><a class="reference internal" href="#how-do-i-extract-c-values-from-a-python-object" id="id8">How do I extract C values from a Python object?</a></li>
<li><a class="reference internal" href="#how-do-i-use-py-buildvalue-to-create-a-tuple-of-arbitrary-length" id="id9">How do I use Py_BuildValue() to create a tuple of arbitrary length?</a></li>
<li><a class="reference internal" href="#how-do-i-call-an-object-s-method-from-c" id="id10">How do I call an object’s method from C?</a></li>
<li><a class="reference internal" href="#how-do-i-catch-the-output-from-pyerr-print-or-anything-that-prints-to-stdout-stderr" id="id11">How do I catch the output from PyErr_Print() (or anything that prints to stdout/stderr)?</a></li>
<li><a class="reference internal" href="#how-do-i-access-a-module-written-in-python-from-c" id="id12">How do I access a module written in Python from C?</a></li>
<li><a class="reference internal" href="#how-do-i-interface-to-c-objects-from-python" id="id13">How do I interface to C++ objects from Python?</a></li>
<li><a class="reference internal" href="#i-added-a-module-using-the-setup-file-and-the-make-fails-why" id="id14">I added a module using the Setup file and the make fails; why?</a></li>
<li><a class="reference internal" href="#how-do-i-debug-an-extension" id="id15">How do I debug an extension?</a></li>
<li><a class="reference internal" href="#i-want-to-compile-a-python-module-on-my-linux-system-but-some-files-are-missing-why" id="id16">I want to compile a Python module on my Linux system, but some files are missing. Why?</a></li>
<li><a class="reference internal" href="#what-does-systemerror-pyimport-fixupextension-module-yourmodule-not-loaded-mean" id="id17">What does “SystemError: _PyImport_FixupExtension: module yourmodule not loaded” mean?</a></li>
<li><a class="reference internal" href="#how-do-i-tell-incomplete-input-from-invalid-input" id="id18">How do I tell “incomplete input” from “invalid input”?</a></li>
<li><a class="reference internal" href="#how-do-i-find-undefined-g-symbols-builtin-new-or-pure-virtual" id="id19">How do I find undefined g++ symbols __builtin_new or __pure_virtual?</a></li>
<li><a class="reference internal" href="#can-i-create-an-object-class-with-some-methods-implemented-in-c-and-others-in-python-e-g-through-inheritance" id="id20">Can I create an object class with some methods implemented in C and others in Python (e.g. through inheritance)?</a></li>
<li><a class="reference internal" href="#when-importing-module-x-why-do-i-get-undefined-symbol-pyunicodeucs2" id="id21">When importing module X, why do I get “undefined symbol: PyUnicodeUCS2*”?</a></li>
</ul>
</li>
</ul>
</div>
<div class="section" id="can-i-create-my-own-functions-in-c">
<h2><a class="toc-backref" href="#id3">Can I create my own functions in C?</a><a class="headerlink" href="#can-i-create-my-own-functions-in-c" title="Permalink to this headline">¶</a></h2>
<p>Yes, you can create built-in modules containing functions, variables, exceptions
and even new types in C. This is explained in the document
<a class="reference internal" href="../extending/index.html#extending-index"><em>Extending and Embedding the Python Interpreter</em></a>.</p>
<p>Most intermediate or advanced Python books will also cover this topic.</p>
</div>
<div class="section" id="id1">
<h2><a class="toc-backref" href="#id4">Can I create my own functions in C++?</a><a class="headerlink" href="#id1" title="Permalink to this headline">¶</a></h2>
<p>Yes, using the C compatibility features found in C++. Place <tt class="docutils literal"><span class="pre">extern</span> <span class="pre">"C"</span> <span class="pre">{</span>
<span class="pre">...</span> <span class="pre">}</span></tt> around the Python include files and put <tt class="docutils literal"><span class="pre">extern</span> <span class="pre">"C"</span></tt> before each
function that is going to be called by the Python interpreter. Global or static
C++ objects with constructors are probably not a good idea.</p>
</div>
<div class="section" id="writing-c-is-hard-are-there-any-alternatives">
<span id="c-wrapper-software"></span><h2><a class="toc-backref" href="#id5">Writing C is hard; are there any alternatives?</a><a class="headerlink" href="#writing-c-is-hard-are-there-any-alternatives" title="Permalink to this headline">¶</a></h2>
<p>There are a number of alternatives to writing your own C extensions, depending
on what you’re trying to do.</p>
<p>If you need more speed, <a class="reference external" href="http://psyco.sourceforge.net/">Psyco</a> generates x86
assembly code from Python bytecode. You can use Psyco to compile the most
time-critical functions in your code, and gain a significant improvement with
very little effort, as long as you’re running on a machine with an
x86-compatible processor.</p>
<p><a class="reference external" href="http://www.cosc.canterbury.ac.nz/~greg/python/Pyrex/">Pyrex</a> is a compiler
that accepts a slightly modified form of Python and generates the corresponding
C code. Pyrex makes it possible to write an extension without having to learn
Python’s C API.</p>
<p>If you need to interface to some C or C++ library for which no Python extension
currently exists, you can try wrapping the library’s data types and functions
with a tool such as <a class="reference external" href="http://www.swig.org">SWIG</a>. <a class="reference external" href="http://www.riverbankcomputing.co.uk/software/sip/">SIP</a>, <a class="reference external" href="http://cxx.sourceforge.net/">CXX</a> <a class="reference external" href="http://www.boost.org/libs/python/doc/index.html">Boost</a>, or <a class="reference external" href="http://www.scipy.org/Weave">Weave</a> are also alternatives for wrapping
C++ libraries.</p>
</div>
<div class="section" id="how-can-i-execute-arbitrary-python-statements-from-c">
<h2><a class="toc-backref" href="#id6">How can I execute arbitrary Python statements from C?</a><a class="headerlink" href="#how-can-i-execute-arbitrary-python-statements-from-c" title="Permalink to this headline">¶</a></h2>
<p>The highest-level function to do this is <a class="reference internal" href="../c-api/veryhigh.html#PyRun_SimpleString" title="PyRun_SimpleString"><tt class="xref c c-func docutils literal"><span class="pre">PyRun_SimpleString()</span></tt></a> which takes
a single string argument to be executed in the context of the module
<tt class="docutils literal"><span class="pre">__main__</span></tt> and returns 0 for success and -1 when an exception occurred
(including <tt class="docutils literal"><span class="pre">SyntaxError</span></tt>). If you want more control, use
<a class="reference internal" href="../c-api/veryhigh.html#PyRun_String" title="PyRun_String"><tt class="xref c c-func docutils literal"><span class="pre">PyRun_String()</span></tt></a>; see the source for <a class="reference internal" href="../c-api/veryhigh.html#PyRun_SimpleString" title="PyRun_SimpleString"><tt class="xref c c-func docutils literal"><span class="pre">PyRun_SimpleString()</span></tt></a> in
<tt class="docutils literal"><span class="pre">Python/pythonrun.c</span></tt>.</p>
</div>
<div class="section" id="how-can-i-evaluate-an-arbitrary-python-expression-from-c">
<h2><a class="toc-backref" href="#id7">How can I evaluate an arbitrary Python expression from C?</a><a class="headerlink" href="#how-can-i-evaluate-an-arbitrary-python-expression-from-c" title="Permalink to this headline">¶</a></h2>
<p>Call the function <a class="reference internal" href="../c-api/veryhigh.html#PyRun_String" title="PyRun_String"><tt class="xref c c-func docutils literal"><span class="pre">PyRun_String()</span></tt></a> from the previous question with the
start symbol <a class="reference internal" href="../c-api/veryhigh.html#Py_eval_input" title="Py_eval_input"><tt class="xref c c-data docutils literal"><span class="pre">Py_eval_input</span></tt></a>; it parses an expression, evaluates it and
returns its value.</p>
</div>
<div class="section" id="how-do-i-extract-c-values-from-a-python-object">
<h2><a class="toc-backref" href="#id8">How do I extract C values from a Python object?</a><a class="headerlink" href="#how-do-i-extract-c-values-from-a-python-object" title="Permalink to this headline">¶</a></h2>
<p>That depends on the object’s type. If it’s a tuple, <a class="reference internal" href="../c-api/tuple.html#PyTuple_Size" title="PyTuple_Size"><tt class="xref c c-func docutils literal"><span class="pre">PyTuple_Size()</span></tt></a>
returns its length and <a class="reference internal" href="../c-api/tuple.html#PyTuple_GetItem" title="PyTuple_GetItem"><tt class="xref c c-func docutils literal"><span class="pre">PyTuple_GetItem()</span></tt></a> returns the item at a specified
index. Lists have similar functions, <tt class="xref c c-func docutils literal"><span class="pre">PyListSize()</span></tt> and
<a class="reference internal" href="../c-api/list.html#PyList_GetItem" title="PyList_GetItem"><tt class="xref c c-func docutils literal"><span class="pre">PyList_GetItem()</span></tt></a>.</p>
<p>For strings, <a class="reference internal" href="../c-api/string.html#PyString_Size" title="PyString_Size"><tt class="xref c c-func docutils literal"><span class="pre">PyString_Size()</span></tt></a> returns its length and
<a class="reference internal" href="../c-api/string.html#PyString_AsString" title="PyString_AsString"><tt class="xref c c-func docutils literal"><span class="pre">PyString_AsString()</span></tt></a> a pointer to its value. Note that Python strings may
contain null bytes so C’s <tt class="xref c c-func docutils literal"><span class="pre">strlen()</span></tt> should not be used.</p>
<p>To test the type of an object, first make sure it isn’t <em>NULL</em>, and then use
<a class="reference internal" href="../c-api/string.html#PyString_Check" title="PyString_Check"><tt class="xref c c-func docutils literal"><span class="pre">PyString_Check()</span></tt></a>, <a class="reference internal" href="../c-api/tuple.html#PyTuple_Check" title="PyTuple_Check"><tt class="xref c c-func docutils literal"><span class="pre">PyTuple_Check()</span></tt></a>, <a class="reference internal" href="../c-api/list.html#PyList_Check" title="PyList_Check"><tt class="xref c c-func docutils literal"><span class="pre">PyList_Check()</span></tt></a>, etc.</p>
<p>There is also a high-level API to Python objects which is provided by the
so-called ‘abstract’ interface – read <tt class="docutils literal"><span class="pre">Include/abstract.h</span></tt> for further
details. It allows interfacing with any kind of Python sequence using calls
like <a class="reference internal" href="../c-api/sequence.html#PySequence_Length" title="PySequence_Length"><tt class="xref c c-func docutils literal"><span class="pre">PySequence_Length()</span></tt></a>, <a class="reference internal" href="../c-api/sequence.html#PySequence_GetItem" title="PySequence_GetItem"><tt class="xref c c-func docutils literal"><span class="pre">PySequence_GetItem()</span></tt></a>, etc.) as well as
many other useful protocols.</p>
</div>
<div class="section" id="how-do-i-use-py-buildvalue-to-create-a-tuple-of-arbitrary-length">
<h2><a class="toc-backref" href="#id9">How do I use Py_BuildValue() to create a tuple of arbitrary length?</a><a class="headerlink" href="#how-do-i-use-py-buildvalue-to-create-a-tuple-of-arbitrary-length" title="Permalink to this headline">¶</a></h2>
<p>You can’t. Use <tt class="docutils literal"><span class="pre">t</span> <span class="pre">=</span> <span class="pre">PyTuple_New(n)</span></tt> instead, and fill it with objects using
<tt class="docutils literal"><span class="pre">PyTuple_SetItem(t,</span> <span class="pre">i,</span> <span class="pre">o)</span></tt> – note that this “eats” a reference count of
<tt class="docutils literal"><span class="pre">o</span></tt>, so you have to <a class="reference internal" href="../c-api/refcounting.html#Py_INCREF" title="Py_INCREF"><tt class="xref c c-func docutils literal"><span class="pre">Py_INCREF()</span></tt></a> it. Lists have similar functions
<tt class="docutils literal"><span class="pre">PyList_New(n)</span></tt> and <tt class="docutils literal"><span class="pre">PyList_SetItem(l,</span> <span class="pre">i,</span> <span class="pre">o)</span></tt>. Note that you <em>must</em> set all
the tuple items to some value before you pass the tuple to Python code –
<tt class="docutils literal"><span class="pre">PyTuple_New(n)</span></tt> initializes them to NULL, which isn’t a valid Python value.</p>
</div>
<div class="section" id="how-do-i-call-an-object-s-method-from-c">
<h2><a class="toc-backref" href="#id10">How do I call an object’s method from C?</a><a class="headerlink" href="#how-do-i-call-an-object-s-method-from-c" title="Permalink to this headline">¶</a></h2>
<p>The <a class="reference internal" href="../c-api/object.html#PyObject_CallMethod" title="PyObject_CallMethod"><tt class="xref c c-func docutils literal"><span class="pre">PyObject_CallMethod()</span></tt></a> function can be used to call an arbitrary
method of an object. The parameters are the object, the name of the method to
call, a format string like that used with <a class="reference internal" href="../c-api/arg.html#Py_BuildValue" title="Py_BuildValue"><tt class="xref c c-func docutils literal"><span class="pre">Py_BuildValue()</span></tt></a>, and the
argument values:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="n">PyObject</span> <span class="o">*</span>
<span class="n">PyObject_CallMethod</span><span class="p">(</span><span class="n">PyObject</span> <span class="o">*</span><span class="n">object</span><span class="p">,</span> <span class="kt">char</span> <span class="o">*</span><span class="n">method_name</span><span class="p">,</span>
<span class="kt">char</span> <span class="o">*</span><span class="n">arg_format</span><span class="p">,</span> <span class="p">...);</span>
</pre></div>
</div>
<p>This works for any object that has methods – whether built-in or user-defined.
You are responsible for eventually <a class="reference internal" href="../c-api/refcounting.html#Py_DECREF" title="Py_DECREF"><tt class="xref c c-func docutils literal"><span class="pre">Py_DECREF()</span></tt></a>‘ing the return value.</p>
<p>To call, e.g., a file object’s “seek” method with arguments 10, 0 (assuming the
file object pointer is “f”):</p>
<div class="highlight-c"><div class="highlight"><pre><span class="n">res</span> <span class="o">=</span> <span class="n">PyObject_CallMethod</span><span class="p">(</span><span class="n">f</span><span class="p">,</span> <span class="s">"seek"</span><span class="p">,</span> <span class="s">"(ii)"</span><span class="p">,</span> <span class="mi">10</span><span class="p">,</span> <span class="mi">0</span><span class="p">);</span>
<span class="k">if</span> <span class="p">(</span><span class="n">res</span> <span class="o">==</span> <span class="nb">NULL</span><span class="p">)</span> <span class="p">{</span>
<span class="p">...</span> <span class="n">an</span> <span class="n">exception</span> <span class="n">occurred</span> <span class="p">...</span>
<span class="p">}</span>
<span class="k">else</span> <span class="p">{</span>
<span class="n">Py_DECREF</span><span class="p">(</span><span class="n">res</span><span class="p">);</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Note that since <a class="reference internal" href="../c-api/object.html#PyObject_CallObject" title="PyObject_CallObject"><tt class="xref c c-func docutils literal"><span class="pre">PyObject_CallObject()</span></tt></a> <em>always</em> wants a tuple for the
argument list, to call a function without arguments, pass “()” for the format,
and to call a function with one argument, surround the argument in parentheses,
e.g. “(i)”.</p>
</div>
<div class="section" id="how-do-i-catch-the-output-from-pyerr-print-or-anything-that-prints-to-stdout-stderr">
<h2><a class="toc-backref" href="#id11">How do I catch the output from PyErr_Print() (or anything that prints to stdout/stderr)?</a><a class="headerlink" href="#how-do-i-catch-the-output-from-pyerr-print-or-anything-that-prints-to-stdout-stderr" title="Permalink to this headline">¶</a></h2>
<p>In Python code, define an object that supports the <tt class="docutils literal"><span class="pre">write()</span></tt> method. Assign
this object to <a class="reference internal" href="../library/sys.html#sys.stdout" title="sys.stdout"><tt class="xref py py-data docutils literal"><span class="pre">sys.stdout</span></tt></a> and <a class="reference internal" href="../library/sys.html#sys.stderr" title="sys.stderr"><tt class="xref py py-data docutils literal"><span class="pre">sys.stderr</span></tt></a>. Call print_error, or
just allow the standard traceback mechanism to work. Then, the output will go
wherever your <tt class="docutils literal"><span class="pre">write()</span></tt> method sends it.</p>
<p>The easiest way to do this is to use the StringIO class in the standard library.</p>
<p>Sample code and use for catching stdout:</p>
<div class="highlight-c"><pre>>>> class StdoutCatcher:
... def __init__(self):
... self.data = ''
... def write(self, stuff):
... self.data = self.data + stuff
...
>>> import sys
>>> sys.stdout = StdoutCatcher()
>>> print 'foo'
>>> print 'hello world!'
>>> sys.stderr.write(sys.stdout.data)
foo
hello world!</pre>
</div>
</div>
<div class="section" id="how-do-i-access-a-module-written-in-python-from-c">
<h2><a class="toc-backref" href="#id12">How do I access a module written in Python from C?</a><a class="headerlink" href="#how-do-i-access-a-module-written-in-python-from-c" title="Permalink to this headline">¶</a></h2>
<p>You can get a pointer to the module object as follows:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="n">module</span> <span class="o">=</span> <span class="n">PyImport_ImportModule</span><span class="p">(</span><span class="s">"<modulename>"</span><span class="p">);</span>
</pre></div>
</div>
<p>If the module hasn’t been imported yet (i.e. it is not yet present in
<a class="reference internal" href="../library/sys.html#sys.modules" title="sys.modules"><tt class="xref py py-data docutils literal"><span class="pre">sys.modules</span></tt></a>), this initializes the module; otherwise it simply returns
the value of <tt class="docutils literal"><span class="pre">sys.modules["<modulename>"]</span></tt>. Note that it doesn’t enter the
module into any namespace – it only ensures it has been initialized and is
stored in <a class="reference internal" href="../library/sys.html#sys.modules" title="sys.modules"><tt class="xref py py-data docutils literal"><span class="pre">sys.modules</span></tt></a>.</p>
<p>You can then access the module’s attributes (i.e. any name defined in the
module) as follows:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="n">attr</span> <span class="o">=</span> <span class="n">PyObject_GetAttrString</span><span class="p">(</span><span class="n">module</span><span class="p">,</span> <span class="s">"<attrname>"</span><span class="p">);</span>
</pre></div>
</div>
<p>Calling <a class="reference internal" href="../c-api/object.html#PyObject_SetAttrString" title="PyObject_SetAttrString"><tt class="xref c c-func docutils literal"><span class="pre">PyObject_SetAttrString()</span></tt></a> to assign to variables in the module
also works.</p>
</div>
<div class="section" id="how-do-i-interface-to-c-objects-from-python">
<h2><a class="toc-backref" href="#id13">How do I interface to C++ objects from Python?</a><a class="headerlink" href="#how-do-i-interface-to-c-objects-from-python" title="Permalink to this headline">¶</a></h2>
<p>Depending on your requirements, there are many approaches. To do this manually,
begin by reading <a class="reference internal" href="../extending/index.html#extending-index"><em>the “Extending and Embedding” document</em></a>. Realize that for the Python run-time system, there isn’t a
whole lot of difference between C and C++ – so the strategy of building a new
Python type around a C structure (pointer) type will also work for C++ objects.</p>
<p>For C++ libraries, see <a class="reference internal" href="#c-wrapper-software"><em>Writing C is hard; are there any alternatives?</em></a>.</p>
</div>
<div class="section" id="i-added-a-module-using-the-setup-file-and-the-make-fails-why">
<h2><a class="toc-backref" href="#id14">I added a module using the Setup file and the make fails; why?</a><a class="headerlink" href="#i-added-a-module-using-the-setup-file-and-the-make-fails-why" title="Permalink to this headline">¶</a></h2>
<p>Setup must end in a newline, if there is no newline there, the build process
fails. (Fixing this requires some ugly shell script hackery, and this bug is so
minor that it doesn’t seem worth the effort.)</p>
</div>
<div class="section" id="how-do-i-debug-an-extension">
<h2><a class="toc-backref" href="#id15">How do I debug an extension?</a><a class="headerlink" href="#how-do-i-debug-an-extension" title="Permalink to this headline">¶</a></h2>
<p>When using GDB with dynamically loaded extensions, you can’t set a breakpoint in
your extension until your extension is loaded.</p>
<p>In your <tt class="docutils literal"><span class="pre">.gdbinit</span></tt> file (or interactively), add the command:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="n">br</span> <span class="n">_PyImport_LoadDynamicModule</span>
</pre></div>
</div>
<p>Then, when you run GDB:</p>
<div class="highlight-c"><pre>$ gdb /local/bin/python
gdb) run myscript.py
gdb) continue # repeat until your extension is loaded
gdb) finish # so that your extension is loaded
gdb) br myfunction.c:50
gdb) continue</pre>
</div>
</div>
<div class="section" id="i-want-to-compile-a-python-module-on-my-linux-system-but-some-files-are-missing-why">
<h2><a class="toc-backref" href="#id16">I want to compile a Python module on my Linux system, but some files are missing. Why?</a><a class="headerlink" href="#i-want-to-compile-a-python-module-on-my-linux-system-but-some-files-are-missing-why" title="Permalink to this headline">¶</a></h2>
<p>Most packaged versions of Python don’t include the
<tt class="file docutils literal"><span class="pre">/usr/lib/python2.</span><em><span class="pre">x</span></em><span class="pre">/config/</span></tt> directory, which contains various files
required for compiling Python extensions.</p>
<p>For Red Hat, install the python-devel RPM to get the necessary files.</p>
<p>For Debian, run <tt class="docutils literal"><span class="pre">apt-get</span> <span class="pre">install</span> <span class="pre">python-dev</span></tt>.</p>
</div>
<div class="section" id="what-does-systemerror-pyimport-fixupextension-module-yourmodule-not-loaded-mean">
<h2><a class="toc-backref" href="#id17">What does “SystemError: _PyImport_FixupExtension: module yourmodule not loaded” mean?</a><a class="headerlink" href="#what-does-systemerror-pyimport-fixupextension-module-yourmodule-not-loaded-mean" title="Permalink to this headline">¶</a></h2>
<p>This means that you have created an extension module named “yourmodule”, but
your module init function does not initialize with that name.</p>
<p>Every module init function will have a line similar to:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="n">module</span> <span class="o">=</span> <span class="n">Py_InitModule</span><span class="p">(</span><span class="s">"yourmodule"</span><span class="p">,</span> <span class="n">yourmodule_functions</span><span class="p">);</span>
</pre></div>
</div>
<p>If the string passed to this function is not the same name as your extension
module, the <a class="reference internal" href="../library/exceptions.html#exceptions.SystemError" title="exceptions.SystemError"><tt class="xref py py-exc docutils literal"><span class="pre">SystemError</span></tt></a> exception will be raised.</p>
</div>
<div class="section" id="how-do-i-tell-incomplete-input-from-invalid-input">
<h2><a class="toc-backref" href="#id18">How do I tell “incomplete input” from “invalid input”?</a><a class="headerlink" href="#how-do-i-tell-incomplete-input-from-invalid-input" title="Permalink to this headline">¶</a></h2>
<p>Sometimes you want to emulate the Python interactive interpreter’s behavior,
where it gives you a continuation prompt when the input is incomplete (e.g. you
typed the start of an “if” statement or you didn’t close your parentheses or
triple string quotes), but it gives you a syntax error message immediately when
the input is invalid.</p>
<p>In Python you can use the <a class="reference internal" href="../library/codeop.html#module-codeop" title="codeop: Compile (possibly incomplete) Python code."><tt class="xref py py-mod docutils literal"><span class="pre">codeop</span></tt></a> module, which approximates the parser’s
behavior sufficiently. IDLE uses this, for example.</p>
<p>The easiest way to do it in C is to call <a class="reference internal" href="../c-api/veryhigh.html#PyRun_InteractiveLoop" title="PyRun_InteractiveLoop"><tt class="xref c c-func docutils literal"><span class="pre">PyRun_InteractiveLoop()</span></tt></a> (perhaps
in a separate thread) and let the Python interpreter handle the input for
you. You can also set the <tt class="xref c c-func docutils literal"><span class="pre">PyOS_ReadlineFunctionPointer()</span></tt> to point at your
custom input function. See <tt class="docutils literal"><span class="pre">Modules/readline.c</span></tt> and <tt class="docutils literal"><span class="pre">Parser/myreadline.c</span></tt>
for more hints.</p>
<p>However sometimes you have to run the embedded Python interpreter in the same
thread as your rest application and you can’t allow the
<a class="reference internal" href="../c-api/veryhigh.html#PyRun_InteractiveLoop" title="PyRun_InteractiveLoop"><tt class="xref c c-func docutils literal"><span class="pre">PyRun_InteractiveLoop()</span></tt></a> to stop while waiting for user input. The one
solution then is to call <tt class="xref c c-func docutils literal"><span class="pre">PyParser_ParseString()</span></tt> and test for <tt class="docutils literal"><span class="pre">e.error</span></tt>
equal to <tt class="docutils literal"><span class="pre">E_EOF</span></tt>, which means the input is incomplete). Here’s a sample code
fragment, untested, inspired by code from Alex Farber:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="cp">#include <Python.h></span>
<span class="cp">#include <node.h></span>
<span class="cp">#include <errcode.h></span>
<span class="cp">#include <grammar.h></span>
<span class="cp">#include <parsetok.h></span>
<span class="cp">#include <compile.h></span>
<span class="kt">int</span> <span class="nf">testcomplete</span><span class="p">(</span><span class="kt">char</span> <span class="o">*</span><span class="n">code</span><span class="p">)</span>
<span class="cm">/* code should end in \n */</span>
<span class="cm">/* return -1 for error, 0 for incomplete, 1 for complete */</span>
<span class="p">{</span>
<span class="n">node</span> <span class="o">*</span><span class="n">n</span><span class="p">;</span>
<span class="n">perrdetail</span> <span class="n">e</span><span class="p">;</span>
<span class="n">n</span> <span class="o">=</span> <span class="n">PyParser_ParseString</span><span class="p">(</span><span class="n">code</span><span class="p">,</span> <span class="o">&</span><span class="n">_PyParser_Grammar</span><span class="p">,</span>
<span class="n">Py_file_input</span><span class="p">,</span> <span class="o">&</span><span class="n">e</span><span class="p">);</span>
<span class="k">if</span> <span class="p">(</span><span class="n">n</span> <span class="o">==</span> <span class="nb">NULL</span><span class="p">)</span> <span class="p">{</span>
<span class="k">if</span> <span class="p">(</span><span class="n">e</span><span class="p">.</span><span class="n">error</span> <span class="o">==</span> <span class="n">E_EOF</span><span class="p">)</span>
<span class="k">return</span> <span class="mi">0</span><span class="p">;</span>
<span class="k">return</span> <span class="o">-</span><span class="mi">1</span><span class="p">;</span>
<span class="p">}</span>
<span class="n">PyNode_Free</span><span class="p">(</span><span class="n">n</span><span class="p">);</span>
<span class="k">return</span> <span class="mi">1</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Another solution is trying to compile the received string with
<a class="reference internal" href="../c-api/veryhigh.html#Py_CompileString" title="Py_CompileString"><tt class="xref c c-func docutils literal"><span class="pre">Py_CompileString()</span></tt></a>. If it compiles without errors, try to execute the
returned code object by calling <a class="reference internal" href="../c-api/veryhigh.html#PyEval_EvalCode" title="PyEval_EvalCode"><tt class="xref c c-func docutils literal"><span class="pre">PyEval_EvalCode()</span></tt></a>. Otherwise save the
input for later. If the compilation fails, find out if it’s an error or just
more input is required - by extracting the message string from the exception
tuple and comparing it to the string “unexpected EOF while parsing”. Here is a
complete example using the GNU readline library (you may want to ignore
<strong>SIGINT</strong> while calling readline()):</p>
<div class="highlight-c"><div class="highlight"><pre><span class="cp">#include <stdio.h></span>
<span class="cp">#include <readline.h></span>
<span class="cp">#include <Python.h></span>
<span class="cp">#include <object.h></span>
<span class="cp">#include <compile.h></span>
<span class="cp">#include <eval.h></span>
<span class="kt">int</span> <span class="nf">main</span> <span class="p">(</span><span class="kt">int</span> <span class="n">argc</span><span class="p">,</span> <span class="kt">char</span><span class="o">*</span> <span class="n">argv</span><span class="p">[])</span>
<span class="p">{</span>
<span class="kt">int</span> <span class="n">i</span><span class="p">,</span> <span class="n">j</span><span class="p">,</span> <span class="n">done</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span> <span class="cm">/* lengths of line, code */</span>
<span class="kt">char</span> <span class="n">ps1</span><span class="p">[]</span> <span class="o">=</span> <span class="s">">>> "</span><span class="p">;</span>
<span class="kt">char</span> <span class="n">ps2</span><span class="p">[]</span> <span class="o">=</span> <span class="s">"... "</span><span class="p">;</span>
<span class="kt">char</span> <span class="o">*</span><span class="n">prompt</span> <span class="o">=</span> <span class="n">ps1</span><span class="p">;</span>
<span class="kt">char</span> <span class="o">*</span><span class="n">msg</span><span class="p">,</span> <span class="o">*</span><span class="n">line</span><span class="p">,</span> <span class="o">*</span><span class="n">code</span> <span class="o">=</span> <span class="nb">NULL</span><span class="p">;</span>
<span class="n">PyObject</span> <span class="o">*</span><span class="n">src</span><span class="p">,</span> <span class="o">*</span><span class="n">glb</span><span class="p">,</span> <span class="o">*</span><span class="n">loc</span><span class="p">;</span>
<span class="n">PyObject</span> <span class="o">*</span><span class="n">exc</span><span class="p">,</span> <span class="o">*</span><span class="n">val</span><span class="p">,</span> <span class="o">*</span><span class="n">trb</span><span class="p">,</span> <span class="o">*</span><span class="n">obj</span><span class="p">,</span> <span class="o">*</span><span class="n">dum</span><span class="p">;</span>
<span class="n">Py_Initialize</span> <span class="p">();</span>
<span class="n">loc</span> <span class="o">=</span> <span class="n">PyDict_New</span> <span class="p">();</span>
<span class="n">glb</span> <span class="o">=</span> <span class="n">PyDict_New</span> <span class="p">();</span>
<span class="n">PyDict_SetItemString</span> <span class="p">(</span><span class="n">glb</span><span class="p">,</span> <span class="s">"__builtins__"</span><span class="p">,</span> <span class="n">PyEval_GetBuiltins</span> <span class="p">());</span>
<span class="k">while</span> <span class="p">(</span><span class="o">!</span><span class="n">done</span><span class="p">)</span>
<span class="p">{</span>
<span class="n">line</span> <span class="o">=</span> <span class="n">readline</span> <span class="p">(</span><span class="n">prompt</span><span class="p">);</span>
<span class="k">if</span> <span class="p">(</span><span class="nb">NULL</span> <span class="o">==</span> <span class="n">line</span><span class="p">)</span> <span class="cm">/* CTRL-D pressed */</span>
<span class="p">{</span>
<span class="n">done</span> <span class="o">=</span> <span class="mi">1</span><span class="p">;</span>
<span class="p">}</span>
<span class="k">else</span>
<span class="p">{</span>
<span class="n">i</span> <span class="o">=</span> <span class="n">strlen</span> <span class="p">(</span><span class="n">line</span><span class="p">);</span>
<span class="k">if</span> <span class="p">(</span><span class="n">i</span> <span class="o">></span> <span class="mi">0</span><span class="p">)</span>
<span class="n">add_history</span> <span class="p">(</span><span class="n">line</span><span class="p">);</span> <span class="cm">/* save non-empty lines */</span>
<span class="k">if</span> <span class="p">(</span><span class="nb">NULL</span> <span class="o">==</span> <span class="n">code</span><span class="p">)</span> <span class="cm">/* nothing in code yet */</span>
<span class="n">j</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>
<span class="k">else</span>
<span class="n">j</span> <span class="o">=</span> <span class="n">strlen</span> <span class="p">(</span><span class="n">code</span><span class="p">);</span>
<span class="n">code</span> <span class="o">=</span> <span class="n">realloc</span> <span class="p">(</span><span class="n">code</span><span class="p">,</span> <span class="n">i</span> <span class="o">+</span> <span class="n">j</span> <span class="o">+</span> <span class="mi">2</span><span class="p">);</span>
<span class="k">if</span> <span class="p">(</span><span class="nb">NULL</span> <span class="o">==</span> <span class="n">code</span><span class="p">)</span> <span class="cm">/* out of memory */</span>
<span class="n">exit</span> <span class="p">(</span><span class="mi">1</span><span class="p">);</span>
<span class="k">if</span> <span class="p">(</span><span class="mi">0</span> <span class="o">==</span> <span class="n">j</span><span class="p">)</span> <span class="cm">/* code was empty, so */</span>
<span class="n">code</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="o">=</span> <span class="sc">'\0'</span><span class="p">;</span> <span class="cm">/* keep strncat happy */</span>
<span class="n">strncat</span> <span class="p">(</span><span class="n">code</span><span class="p">,</span> <span class="n">line</span><span class="p">,</span> <span class="n">i</span><span class="p">);</span> <span class="cm">/* append line to code */</span>
<span class="n">code</span><span class="p">[</span><span class="n">i</span> <span class="o">+</span> <span class="n">j</span><span class="p">]</span> <span class="o">=</span> <span class="sc">'\n'</span><span class="p">;</span> <span class="cm">/* append '\n' to code */</span>
<span class="n">code</span><span class="p">[</span><span class="n">i</span> <span class="o">+</span> <span class="n">j</span> <span class="o">+</span> <span class="mi">1</span><span class="p">]</span> <span class="o">=</span> <span class="sc">'\0'</span><span class="p">;</span>
<span class="n">src</span> <span class="o">=</span> <span class="n">Py_CompileString</span> <span class="p">(</span><span class="n">code</span><span class="p">,</span> <span class="s">"<stdin>"</span><span class="p">,</span> <span class="n">Py_single_input</span><span class="p">);</span>
<span class="k">if</span> <span class="p">(</span><span class="nb">NULL</span> <span class="o">!=</span> <span class="n">src</span><span class="p">)</span> <span class="cm">/* compiled just fine - */</span>
<span class="p">{</span>
<span class="k">if</span> <span class="p">(</span><span class="n">ps1</span> <span class="o">==</span> <span class="n">prompt</span> <span class="o">||</span> <span class="cm">/* ">>> " or */</span>
<span class="sc">'\n'</span> <span class="o">==</span> <span class="n">code</span><span class="p">[</span><span class="n">i</span> <span class="o">+</span> <span class="n">j</span> <span class="o">-</span> <span class="mi">1</span><span class="p">])</span> <span class="cm">/* "... " and double '\n' */</span>
<span class="p">{</span> <span class="cm">/* so execute it */</span>
<span class="n">dum</span> <span class="o">=</span> <span class="n">PyEval_EvalCode</span> <span class="p">((</span><span class="n">PyCodeObject</span> <span class="o">*</span><span class="p">)</span><span class="n">src</span><span class="p">,</span> <span class="n">glb</span><span class="p">,</span> <span class="n">loc</span><span class="p">);</span>
<span class="n">Py_XDECREF</span> <span class="p">(</span><span class="n">dum</span><span class="p">);</span>
<span class="n">Py_XDECREF</span> <span class="p">(</span><span class="n">src</span><span class="p">);</span>
<span class="n">free</span> <span class="p">(</span><span class="n">code</span><span class="p">);</span>
<span class="n">code</span> <span class="o">=</span> <span class="nb">NULL</span><span class="p">;</span>
<span class="k">if</span> <span class="p">(</span><span class="n">PyErr_Occurred</span> <span class="p">())</span>
<span class="n">PyErr_Print</span> <span class="p">();</span>
<span class="n">prompt</span> <span class="o">=</span> <span class="n">ps1</span><span class="p">;</span>
<span class="p">}</span>
<span class="p">}</span> <span class="cm">/* syntax error or E_EOF? */</span>
<span class="k">else</span> <span class="k">if</span> <span class="p">(</span><span class="n">PyErr_ExceptionMatches</span> <span class="p">(</span><span class="n">PyExc_SyntaxError</span><span class="p">))</span>
<span class="p">{</span>
<span class="n">PyErr_Fetch</span> <span class="p">(</span><span class="o">&</span><span class="n">exc</span><span class="p">,</span> <span class="o">&</span><span class="n">val</span><span class="p">,</span> <span class="o">&</span><span class="n">trb</span><span class="p">);</span> <span class="cm">/* clears exception! */</span>
<span class="k">if</span> <span class="p">(</span><span class="n">PyArg_ParseTuple</span> <span class="p">(</span><span class="n">val</span><span class="p">,</span> <span class="s">"sO"</span><span class="p">,</span> <span class="o">&</span><span class="n">msg</span><span class="p">,</span> <span class="o">&</span><span class="n">obj</span><span class="p">)</span> <span class="o">&&</span>
<span class="o">!</span><span class="n">strcmp</span> <span class="p">(</span><span class="n">msg</span><span class="p">,</span> <span class="s">"unexpected EOF while parsing"</span><span class="p">))</span> <span class="cm">/* E_EOF */</span>
<span class="p">{</span>
<span class="n">Py_XDECREF</span> <span class="p">(</span><span class="n">exc</span><span class="p">);</span>
<span class="n">Py_XDECREF</span> <span class="p">(</span><span class="n">val</span><span class="p">);</span>
<span class="n">Py_XDECREF</span> <span class="p">(</span><span class="n">trb</span><span class="p">);</span>
<span class="n">prompt</span> <span class="o">=</span> <span class="n">ps2</span><span class="p">;</span>
<span class="p">}</span>
<span class="k">else</span> <span class="cm">/* some other syntax error */</span>
<span class="p">{</span>
<span class="n">PyErr_Restore</span> <span class="p">(</span><span class="n">exc</span><span class="p">,</span> <span class="n">val</span><span class="p">,</span> <span class="n">trb</span><span class="p">);</span>
<span class="n">PyErr_Print</span> <span class="p">();</span>
<span class="n">free</span> <span class="p">(</span><span class="n">code</span><span class="p">);</span>
<span class="n">code</span> <span class="o">=</span> <span class="nb">NULL</span><span class="p">;</span>
<span class="n">prompt</span> <span class="o">=</span> <span class="n">ps1</span><span class="p">;</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="k">else</span> <span class="cm">/* some non-syntax error */</span>
<span class="p">{</span>
<span class="n">PyErr_Print</span> <span class="p">();</span>
<span class="n">free</span> <span class="p">(</span><span class="n">code</span><span class="p">);</span>
<span class="n">code</span> <span class="o">=</span> <span class="nb">NULL</span><span class="p">;</span>
<span class="n">prompt</span> <span class="o">=</span> <span class="n">ps1</span><span class="p">;</span>
<span class="p">}</span>
<span class="n">free</span> <span class="p">(</span><span class="n">line</span><span class="p">);</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="n">Py_XDECREF</span><span class="p">(</span><span class="n">glb</span><span class="p">);</span>
<span class="n">Py_XDECREF</span><span class="p">(</span><span class="n">loc</span><span class="p">);</span>
<span class="n">Py_Finalize</span><span class="p">();</span>
<span class="n">exit</span><span class="p">(</span><span class="mi">0</span><span class="p">);</span>
<span class="p">}</span>
</pre></div>
</div>
</div>
<div class="section" id="how-do-i-find-undefined-g-symbols-builtin-new-or-pure-virtual">
<h2><a class="toc-backref" href="#id19">How do I find undefined g++ symbols __builtin_new or __pure_virtual?</a><a class="headerlink" href="#how-do-i-find-undefined-g-symbols-builtin-new-or-pure-virtual" title="Permalink to this headline">¶</a></h2>
<p>To dynamically load g++ extension modules, you must recompile Python, relink it
using g++ (change LINKCC in the Python Modules Makefile), and link your
extension module using g++ (e.g., <tt class="docutils literal"><span class="pre">g++</span> <span class="pre">-shared</span> <span class="pre">-o</span> <span class="pre">mymodule.so</span> <span class="pre">mymodule.o</span></tt>).</p>
</div>
<div class="section" id="can-i-create-an-object-class-with-some-methods-implemented-in-c-and-others-in-python-e-g-through-inheritance">
<h2><a class="toc-backref" href="#id20">Can I create an object class with some methods implemented in C and others in Python (e.g. through inheritance)?</a><a class="headerlink" href="#can-i-create-an-object-class-with-some-methods-implemented-in-c-and-others-in-python-e-g-through-inheritance" title="Permalink to this headline">¶</a></h2>
<p>In Python 2.2, you can inherit from built-in classes such as <a class="reference internal" href="../library/functions.html#int" title="int"><tt class="xref py py-class docutils literal"><span class="pre">int</span></tt></a>,
<a class="reference internal" href="../library/functions.html#list" title="list"><tt class="xref py py-class docutils literal"><span class="pre">list</span></tt></a>, <a class="reference internal" href="../library/stdtypes.html#dict" title="dict"><tt class="xref py py-class docutils literal"><span class="pre">dict</span></tt></a>, etc.</p>
<p>The Boost Python Library (BPL, <a class="reference external" href="http://www.boost.org/libs/python/doc/index.html">http://www.boost.org/libs/python/doc/index.html</a>)
provides a way of doing this from C++ (i.e. you can inherit from an extension
class written in C++ using the BPL).</p>
</div>
<div class="section" id="when-importing-module-x-why-do-i-get-undefined-symbol-pyunicodeucs2">
<h2><a class="toc-backref" href="#id21">When importing module X, why do I get “undefined symbol: PyUnicodeUCS2*”?</a><a class="headerlink" href="#when-importing-module-x-why-do-i-get-undefined-symbol-pyunicodeucs2" title="Permalink to this headline">¶</a></h2>
<p>You are using a version of Python that uses a 4-byte representation for Unicode
characters, but some C extension module you are importing was compiled using a
Python that uses a 2-byte representation for Unicode characters (the default).</p>
<p>If instead the name of the undefined symbol starts with <tt class="docutils literal"><span class="pre">PyUnicodeUCS4</span></tt>, the
problem is the reverse: Python was built using 2-byte Unicode characters, and
the extension module was compiled using a Python with 4-byte Unicode characters.</p>
<p>This can easily occur when using pre-built extension packages. RedHat Linux
7.x, in particular, provided a “python2” binary that is compiled with 4-byte
Unicode. This only causes the link failure if the extension uses any of the
<tt class="docutils literal"><span class="pre">PyUnicode_*()</span></tt> functions. It is also a problem if an extension uses any of
the Unicode-related format specifiers for <a class="reference internal" href="../c-api/arg.html#Py_BuildValue" title="Py_BuildValue"><tt class="xref c c-func docutils literal"><span class="pre">Py_BuildValue()</span></tt></a> (or similar) or
parameter specifications for <a class="reference internal" href="../c-api/arg.html#PyArg_ParseTuple" title="PyArg_ParseTuple"><tt class="xref c c-func docutils literal"><span class="pre">PyArg_ParseTuple()</span></tt></a>.</p>
<p>You can check the size of the Unicode character a Python interpreter is using by
checking the value of sys.maxunicode:</p>
<div class="highlight-c"><pre>>>> import sys
>>> if sys.maxunicode > 65535:
... print 'UCS4 build'
... else:
... print 'UCS2 build'</pre>
</div>
<p>The only way to solve this problem is to use extension modules compiled with a
Python binary built using the same size for Unicode characters.</p>
</div>
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<ul>
<li><a class="reference internal" href="#">Extending/Embedding FAQ</a><ul>
<li><a class="reference internal" href="#can-i-create-my-own-functions-in-c">Can I create my own functions in C?</a></li>
<li><a class="reference internal" href="#id1">Can I create my own functions in C++?</a></li>
<li><a class="reference internal" href="#writing-c-is-hard-are-there-any-alternatives">Writing C is hard; are there any alternatives?</a></li>
<li><a class="reference internal" href="#how-can-i-execute-arbitrary-python-statements-from-c">How can I execute arbitrary Python statements from C?</a></li>
<li><a class="reference internal" href="#how-can-i-evaluate-an-arbitrary-python-expression-from-c">How can I evaluate an arbitrary Python expression from C?</a></li>
<li><a class="reference internal" href="#how-do-i-extract-c-values-from-a-python-object">How do I extract C values from a Python object?</a></li>
<li><a class="reference internal" href="#how-do-i-use-py-buildvalue-to-create-a-tuple-of-arbitrary-length">How do I use Py_BuildValue() to create a tuple of arbitrary length?</a></li>
<li><a class="reference internal" href="#how-do-i-call-an-object-s-method-from-c">How do I call an object’s method from C?</a></li>
<li><a class="reference internal" href="#how-do-i-catch-the-output-from-pyerr-print-or-anything-that-prints-to-stdout-stderr">How do I catch the output from PyErr_Print() (or anything that prints to stdout/stderr)?</a></li>
<li><a class="reference internal" href="#how-do-i-access-a-module-written-in-python-from-c">How do I access a module written in Python from C?</a></li>
<li><a class="reference internal" href="#how-do-i-interface-to-c-objects-from-python">How do I interface to C++ objects from Python?</a></li>
<li><a class="reference internal" href="#i-added-a-module-using-the-setup-file-and-the-make-fails-why">I added a module using the Setup file and the make fails; why?</a></li>
<li><a class="reference internal" href="#how-do-i-debug-an-extension">How do I debug an extension?</a></li>
<li><a class="reference internal" href="#i-want-to-compile-a-python-module-on-my-linux-system-but-some-files-are-missing-why">I want to compile a Python module on my Linux system, but some files are missing. Why?</a></li>
<li><a class="reference internal" href="#what-does-systemerror-pyimport-fixupextension-module-yourmodule-not-loaded-mean">What does “SystemError: _PyImport_FixupExtension: module yourmodule not loaded” mean?</a></li>
<li><a class="reference internal" href="#how-do-i-tell-incomplete-input-from-invalid-input">How do I tell “incomplete input” from “invalid input”?</a></li>
<li><a class="reference internal" href="#how-do-i-find-undefined-g-symbols-builtin-new-or-pure-virtual">How do I find undefined g++ symbols __builtin_new or __pure_virtual?</a></li>
<li><a class="reference internal" href="#can-i-create-an-object-class-with-some-methods-implemented-in-c-and-others-in-python-e-g-through-inheritance">Can I create an object class with some methods implemented in C and others in Python (e.g. through inheritance)?</a></li>
<li><a class="reference internal" href="#when-importing-module-x-why-do-i-get-undefined-symbol-pyunicodeucs2">When importing module X, why do I get “undefined symbol: PyUnicodeUCS2*”?</a></li>
</ul>
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