<li><a class="reference internal" href="#can-i-create-my-own-functions-in-c" id="id7">1 Can I create my own functions in C?</a></li>
<li><a class="reference internal" href="#id1" id="id8">2 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="id9">3 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="id10">4 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="id11">5 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="id12">6 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="id13">7 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="id14">8 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="id15">9 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="id16">10 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="id17">11 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="id18">12 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="id19">13 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="id20">14 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="id21">15 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="id22">16 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="id23">17 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="id24">18 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="id25">19 When importing module X, why do I get "undefined symbol: PyUnicodeUCS2*"?</a></li>
<h1><a class="toc-backref" href="#id7">1 Can I create my own functions in C?</a></h1>
<p>Yes, you can create built-in modules containing functions,
variables, exceptions and even new types in C. This is explained in
the document "Extending and Embedding the Python Interpreter" (<a class="reference external" href="http://docs.python.org/ext/ext.html">http://docs.python.org/ext/ext.html</a>).</p>
<p>Most intermediate or advanced Python books will also
cover this topic.</p>
</div>
<div class="section" id="id1">
<h1><a class="toc-backref" href="#id8">2 Can I create my own functions in C++?</a></h1>
<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
<h1><a class="toc-backref" href="#id9">3 Writing C is hard; are there any alternatives?</a></h1>
<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 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>.
For C++ libraries, you can look at <a class="reference external" href="http://www.riverbankcomputing.co.uk/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/site_content/weave">Weave</a>.</p>
<h1><a class="toc-backref" href="#id10">4 How can I execute arbitrary Python statements from C?</a></h1>
<p>The highest-level function to do this is <tt class="docutils literal"><span class="pre">PyRun_SimpleString()</span></tt> 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 <tt class="docutils literal"><span class="pre">PyRun_String()</span></tt>;
see the source for <tt class="docutils literal"><span class="pre">PyRun_SimpleString()</span></tt> in Python/pythonrun.c.</p>
its length and <tt class="docutils literal"><span class="pre">PyString_AsString(o)</span></tt> a pointer to its value.
Note that Python strings may contain null bytes so C's <tt class="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 NULL, and then use <tt class="docutils literal"><span class="pre">PyString_Check(o)</span></tt>, <tt class="docutils literal"><span class="pre">PyTuple_Check(o)</span></tt>,
<h1><a class="toc-backref" href="#id13">7 How do I use Py_BuildValue() to create a tuple of arbitrary length?</a></h1>
<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 <tt class="docutils literal"><span class="pre">Py_INCREF</span></tt> 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
<h1><a class="toc-backref" href="#id14">8 How do I call an object's method from C?</a></h1>
<p>The <tt class="docutils literal"><span class="pre">PyObject_CallMethod()</span></tt> 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 <tt class="docutils literal"><span class="pre">Py_BuildValue()</span></tt>, and the argument values:</p>
<h1><a class="toc-backref" href="#id15">9 How do I catch the output from PyErr_Print() (or anything that prints to stdout/stderr)?</a></h1>
<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 <tt class="docutils literal"><span class="pre">sys.stdout</span></tt> and <tt class="docutils literal"><span class="pre">sys.stderr</span></tt>.
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
<h1><a class="toc-backref" href="#id17">11 How do I interface to C++ objects from Python?</a></h1>
<p>Depending on your requirements, there are many approaches. To do
this manually, begin by reading <a class="reference external" href="http://docs.python.org/ext/ext.html">the "Extending and Embedding" document</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, you can look at <a class="reference external" href="http://www.riverbankcomputing.co.uk/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/site_content/weave">Weave</a>.
<a class="reference external" href="http://www.swig.org">SWIG</a> is a similar automated tool that only supports C libraries.</p>
<h1><a class="toc-backref" href="#id22">16 How do I tell "incomplete input" from "invalid input"?</a></h1>
<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 <tt class="docutils literal"><span class="pre">codeop</span></tt> 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 <tt class="docutils literal"><span class="pre">PyRun_InteractiveLoop()</span></tt>
(perhaps in a separate thread) and let the Python interpreter handle
the input for you. You can also set the <tt class="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
<tt class="docutils literal"><span class="pre">PyRun_InteractiveLoop()</span></tt> to stop while waiting for user input. The
one solution then is to call <tt class="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>
<pre class="literal-block">
#include <Python.h>
#include <node.h>
#include <errcode.h>
#include <grammar.h>
#include <parsetok.h>
#include <compile.h>
int testcomplete(char *code)
/* code should end in \n */
/* return -1 for error, 0 for incomplete, 1 for complete */
{
node *n;
perrdetail e;
n = PyParser_ParseString(code, &_PyParser_Grammar,
Py_file_input, &e);
if (n == NULL) {
if (e.error == E_EOF)
return 0;
return -1;
}
PyNode_Free(n);
return 1;
}
</pre>
<p>Another solution is trying to compile the received string with
<tt class="docutils literal"><span class="pre">Py_CompileString()</span></tt>. If it compiles without errors, try to execute the returned
code object by calling <tt class="docutils literal"><span class="pre">PyEval_EvalCode()</span></tt>. 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 SIGINT while calling readline()):</p>
<h1><a class="toc-backref" href="#id23">17 How do I find undefined g++ symbols __builtin_new or __pure_virtual?</a></h1>
<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., "g++ -shared -o mymodule.so mymodule.o").</p>
<h1><a class="toc-backref" href="#id24">18 Can I create an object class with some methods implemented in C and others in Python (e.g. through inheritance)?</a></h1>
<p>In Python 2.2, you can inherit from builtin classes such as int, list, dict, etc.</p>
