| Server IP : 85.214.239.14 / Your IP : 3.142.212.225 Web Server : Apache/2.4.62 (Debian) System : Linux h2886529.stratoserver.net 4.9.0 #1 SMP Tue Jan 9 19:45:01 MSK 2024 x86_64 User : www-data ( 33) PHP Version : 7.4.18 Disable Function : pcntl_alarm,pcntl_fork,pcntl_waitpid,pcntl_wait,pcntl_wifexited,pcntl_wifstopped,pcntl_wifsignaled,pcntl_wifcontinued,pcntl_wexitstatus,pcntl_wtermsig,pcntl_wstopsig,pcntl_signal,pcntl_signal_get_handler,pcntl_signal_dispatch,pcntl_get_last_error,pcntl_strerror,pcntl_sigprocmask,pcntl_sigwaitinfo,pcntl_sigtimedwait,pcntl_exec,pcntl_getpriority,pcntl_setpriority,pcntl_async_signals,pcntl_unshare, MySQL : OFF | cURL : OFF | WGET : ON | Perl : ON | Python : ON | Sudo : ON | Pkexec : OFF Directory : /proc/2/root/proc/3/root/proc/3/cwd/proc/2/cwd/usr/share/doc/python/faq/ |
Upload File : |
<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<meta name="generator" content="Docutils 0.12: http://docutils.sourceforge.net/" />
<title>Extending/Embedding FAQ</title>
<meta name="date" content="2004-04-08" />
<style type="text/css">
/*
:Author: David Goodger (goodger@python.org)
:Id: $Id: html4css1.css 7614 2013-02-21 15:55:51Z milde $
:Copyright: This stylesheet has been placed in the public domain.
Default cascading style sheet for the HTML output of Docutils.
See http://docutils.sf.net/docs/howto/html-stylesheets.html for how to
customize this style sheet.
*/
/* used to remove borders from tables and images */
.borderless, table.borderless td, table.borderless th {
border: 0 }
table.borderless td, table.borderless th {
/* Override padding for "table.docutils td" with "! important".
The right padding separates the table cells. */
padding: 0 0.5em 0 0 ! important }
.first {
/* Override more specific margin styles with "! important". */
margin-top: 0 ! important }
.last, .with-subtitle {
margin-bottom: 0 ! important }
.hidden {
display: none }
a.toc-backref {
text-decoration: none ;
color: black }
blockquote.epigraph {
margin: 2em 5em ; }
dl.docutils dd {
margin-bottom: 0.5em }
object[type="image/svg+xml"], object[type="application/x-shockwave-flash"] {
overflow: hidden;
}
/* Uncomment (and remove this text!) to get bold-faced definition list terms
dl.docutils dt {
font-weight: bold }
*/
div.abstract {
margin: 2em 5em }
div.abstract p.topic-title {
font-weight: bold ;
text-align: center }
div.admonition, div.attention, div.caution, div.danger, div.error,
div.hint, div.important, div.note, div.tip, div.warning {
margin: 2em ;
border: medium outset ;
padding: 1em }
div.admonition p.admonition-title, div.hint p.admonition-title,
div.important p.admonition-title, div.note p.admonition-title,
div.tip p.admonition-title {
font-weight: bold ;
font-family: sans-serif }
div.attention p.admonition-title, div.caution p.admonition-title,
div.danger p.admonition-title, div.error p.admonition-title,
div.warning p.admonition-title, .code .error {
color: red ;
font-weight: bold ;
font-family: sans-serif }
/* Uncomment (and remove this text!) to get reduced vertical space in
compound paragraphs.
div.compound .compound-first, div.compound .compound-middle {
margin-bottom: 0.5em }
div.compound .compound-last, div.compound .compound-middle {
margin-top: 0.5em }
*/
div.dedication {
margin: 2em 5em ;
text-align: center ;
font-style: italic }
div.dedication p.topic-title {
font-weight: bold ;
font-style: normal }
div.figure {
margin-left: 2em ;
margin-right: 2em }
div.footer, div.header {
clear: both;
font-size: smaller }
div.line-block {
display: block ;
margin-top: 1em ;
margin-bottom: 1em }
div.line-block div.line-block {
margin-top: 0 ;
margin-bottom: 0 ;
margin-left: 1.5em }
div.sidebar {
margin: 0 0 0.5em 1em ;
border: medium outset ;
padding: 1em ;
background-color: #ffffee ;
width: 40% ;
float: right ;
clear: right }
div.sidebar p.rubric {
font-family: sans-serif ;
font-size: medium }
div.system-messages {
margin: 5em }
div.system-messages h1 {
color: red }
div.system-message {
border: medium outset ;
padding: 1em }
div.system-message p.system-message-title {
color: red ;
font-weight: bold }
div.topic {
margin: 2em }
h1.section-subtitle, h2.section-subtitle, h3.section-subtitle,
h4.section-subtitle, h5.section-subtitle, h6.section-subtitle {
margin-top: 0.4em }
h1.title {
text-align: center }
h2.subtitle {
text-align: center }
hr.docutils {
width: 75% }
img.align-left, .figure.align-left, object.align-left {
clear: left ;
float: left ;
margin-right: 1em }
img.align-right, .figure.align-right, object.align-right {
clear: right ;
float: right ;
margin-left: 1em }
img.align-center, .figure.align-center, object.align-center {
display: block;
margin-left: auto;
margin-right: auto;
}
.align-left {
text-align: left }
.align-center {
clear: both ;
text-align: center }
.align-right {
text-align: right }
/* reset inner alignment in figures */
div.align-right {
text-align: inherit }
/* div.align-center * { */
/* text-align: left } */
ol.simple, ul.simple {
margin-bottom: 1em }
ol.arabic {
list-style: decimal }
ol.loweralpha {
list-style: lower-alpha }
ol.upperalpha {
list-style: upper-alpha }
ol.lowerroman {
list-style: lower-roman }
ol.upperroman {
list-style: upper-roman }
p.attribution {
text-align: right ;
margin-left: 50% }
p.caption {
font-style: italic }
p.credits {
font-style: italic ;
font-size: smaller }
p.label {
white-space: nowrap }
p.rubric {
font-weight: bold ;
font-size: larger ;
color: maroon ;
text-align: center }
p.sidebar-title {
font-family: sans-serif ;
font-weight: bold ;
font-size: larger }
p.sidebar-subtitle {
font-family: sans-serif ;
font-weight: bold }
p.topic-title {
font-weight: bold }
pre.address {
margin-bottom: 0 ;
margin-top: 0 ;
font: inherit }
pre.literal-block, pre.doctest-block, pre.math, pre.code {
margin-left: 2em ;
margin-right: 2em }
pre.code .ln { color: grey; } /* line numbers */
pre.code, code { background-color: #eeeeee }
pre.code .comment, code .comment { color: #5C6576 }
pre.code .keyword, code .keyword { color: #3B0D06; font-weight: bold }
pre.code .literal.string, code .literal.string { color: #0C5404 }
pre.code .name.builtin, code .name.builtin { color: #352B84 }
pre.code .deleted, code .deleted { background-color: #DEB0A1}
pre.code .inserted, code .inserted { background-color: #A3D289}
span.classifier {
font-family: sans-serif ;
font-style: oblique }
span.classifier-delimiter {
font-family: sans-serif ;
font-weight: bold }
span.interpreted {
font-family: sans-serif }
span.option {
white-space: nowrap }
span.pre {
white-space: pre }
span.problematic {
color: red }
span.section-subtitle {
/* font-size relative to parent (h1..h6 element) */
font-size: 80% }
table.citation {
border-left: solid 1px gray;
margin-left: 1px }
table.docinfo {
margin: 2em 4em }
table.docutils {
margin-top: 0.5em ;
margin-bottom: 0.5em }
table.footnote {
border-left: solid 1px black;
margin-left: 1px }
table.docutils td, table.docutils th,
table.docinfo td, table.docinfo th {
padding-left: 0.5em ;
padding-right: 0.5em ;
vertical-align: top }
table.docutils th.field-name, table.docinfo th.docinfo-name {
font-weight: bold ;
text-align: left ;
white-space: nowrap ;
padding-left: 0 }
/* "booktabs" style (no vertical lines) */
table.docutils.booktabs {
border: 0px;
border-top: 2px solid;
border-bottom: 2px solid;
border-collapse: collapse;
}
table.docutils.booktabs * {
border: 0px;
}
table.docutils.booktabs th {
border-bottom: thin solid;
text-align: left;
}
h1 tt.docutils, h2 tt.docutils, h3 tt.docutils,
h4 tt.docutils, h5 tt.docutils, h6 tt.docutils {
font-size: 100% }
ul.auto-toc {
list-style-type: none }
</style>
</head>
<body>
<div class="document" id="extending-embedding-faq">
<h1 class="title">Extending/Embedding FAQ</h1>
<table class="docinfo" frame="void" rules="none">
<col class="docinfo-name" />
<col class="docinfo-content" />
<tbody valign="top">
<tr><th class="docinfo-name">Date:</th>
<td>2004-04-08</td></tr>
<tr><th class="docinfo-name">Version:</th>
<td>7294</td></tr>
<tr class="field"><th class="docinfo-name">Web site:</th><td class="field-body"><a class="reference external" href="http://www.python.org/">http://www.python.org/</a></td>
</tr>
</tbody>
</table>
<div class="contents topic" id="contents">
<p class="topic-title first">Contents</p>
<ul class="auto-toc simple">
<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>
</ul>
</div>
<div class="section" id="can-i-create-my-own-functions-in-c">
<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">extern "C" { ... }</tt> around the Python include files and put
<tt class="docutils literal">extern "C"</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">
<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>
</div>
<div class="section" id="how-can-i-execute-arbitrary-python-statements-from-c">
<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">PyRun_SimpleString()</tt> which takes
a single string argument to be executed in the context of the module
<tt class="docutils literal">__main__</tt> and returns 0 for success and -1 when an exception occurred
(including <tt class="docutils literal">SyntaxError</tt>). If you want more control, use <tt class="docutils literal">PyRun_String()</tt>;
see the source for <tt class="docutils literal">PyRun_SimpleString()</tt> in Python/pythonrun.c.</p>
</div>
<div class="section" id="how-can-i-evaluate-an-arbitrary-python-expression-from-c">
<h1><a class="toc-backref" href="#id11">5 How can I evaluate an arbitrary Python expression from C?</a></h1>
<p>Call the function <tt class="docutils literal">PyRun_String()</tt> from the previous question with the
start symbol <tt class="docutils literal">Py_eval_input</tt>; 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">
<h1><a class="toc-backref" href="#id12">6 How do I extract C values from a Python object?</a></h1>
<p>That depends on the object's type. If it's a tuple,
<tt class="docutils literal">PyTupleSize(o)</tt> returns its length and <tt class="docutils literal">PyTuple_GetItem(o, i)</tt>
returns its i'th item. Lists have similar functions, <tt class="docutils literal">PyListSize(o)</tt>
and <tt class="docutils literal">PyList_GetItem(o, i)</tt>.</p>
<p>For strings, <tt class="docutils literal">PyString_Size(o)</tt> returns
its length and <tt class="docutils literal">PyString_AsString(o)</tt> a pointer to its value.
Note that Python strings may contain null bytes so C's <tt class="docutils literal">strlen()</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">PyString_Check(o)</tt>, <tt class="docutils literal">PyTuple_Check(o)</tt>,
<tt class="docutils literal">PyList_Check(o)</tt>, 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">Include/abstract.h</tt> for further details. It allows
interfacing with any kind of Python sequence
using calls like <tt class="docutils literal">PySequence_Length()</tt>, <tt class="docutils literal">PySequence_GetItem()</tt>, 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">
<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">t = PyTuple_New(n)</tt> instead, and fill it with
objects using <tt class="docutils literal">PyTuple_SetItem(t, i, o)</tt> -- note that this "eats" a
reference count of <tt class="docutils literal">o</tt>, so you have to <tt class="docutils literal">Py_INCREF</tt> it.
Lists have similar functions <tt class="docutils literal">PyList_New(n)</tt> and
<tt class="docutils literal">PyList_SetItem(l, i, o)</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">PyTuple_New(n)</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">
<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">PyObject_CallMethod()</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">Py_BuildValue()</tt>, and the argument values:</p>
<pre class="literal-block">
PyObject *
PyObject_CallMethod(PyObject *object, char *method_name,
char *arg_format, ...);
</pre>
<p>This works for any object that has methods -- whether built-in or
user-defined. You are responsible for eventually <tt class="docutils literal">Py_DECREF</tt>'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>
<pre class="literal-block">
res = PyObject_CallMethod(f, "seek", "(ii)", 10, 0);
if (res == NULL) {
... an exception occurred ...
}
else {
Py_DECREF(res);
}
</pre>
<p>Note that since <tt class="docutils literal">PyObject_CallObject()</tt> <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">
<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">write()</tt> method.
Assign this object to <tt class="docutils literal">sys.stdout</tt> and <tt class="docutils literal">sys.stderr</tt>.
Call print_error, or just allow the standard traceback mechanism to
work. Then, the output will go wherever your <tt class="docutils literal">write()</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>
<pre class="literal-block">
>>> 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 class="section" id="how-do-i-access-a-module-written-in-python-from-c">
<h1><a class="toc-backref" href="#id16">10 How do I access a module written in Python from C?</a></h1>
<p>You can get a pointer to the module object as follows:</p>
<pre class="literal-block">
module = PyImport_ImportModule("<modulename>");
</pre>
<p>If the module hasn't been imported yet (i.e. it is not yet present in
<tt class="docutils literal">sys.modules</tt>), 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 <tt class="docutils literal">sys.modules</tt>.</p>
<p>You can then access the module's attributes (i.e. any name defined in
the module) as follows:</p>
<pre class="literal-block">
attr = PyObject_GetAttrString(module, "<attrname>");
</pre>
<p>Calling <tt class="docutils literal">PyObject_SetAttrString()</tt> to assign to variables in the module also works.</p>
</div>
<div class="section" id="how-do-i-interface-to-c-objects-from-python">
<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>
</div>
<div class="section" id="i-added-a-module-using-the-setup-file-and-the-make-fails-why">
<h1><a class="toc-backref" href="#id18">12 I added a module using the Setup file and the make fails; why?</a></h1>
<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">
<h1><a class="toc-backref" href="#id19">13 How do I debug an extension?</a></h1>
<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">.gdbinit</tt> file (or interactively), add the command:</p>
<pre class="literal-block">
br _PyImport_LoadDynamicModule
</pre>
<p>Then, when you run GDB:</p>
<pre class="literal-block">
$ 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 class="section" id="i-want-to-compile-a-python-module-on-my-linux-system-but-some-files-are-missing-why">
<h1><a class="toc-backref" href="#id20">14 I want to compile a Python module on my Linux system, but some files are missing. Why?</a></h1>
<p>Most packaged versions of Python don't include the
/usr/lib/python2.x/config/ 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> install <span class="pre">python-dev</span></tt>.</p>
</div>
<div class="section" id="what-does-systemerror-pyimport-fixupextension-module-yourmodule-not-loaded-mean">
<h1><a class="toc-backref" href="#id21">15 What does "SystemError: _PyImport_FixupExtension: module yourmodule not loaded" mean?</a></h1>
<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>
<pre class="literal-block">
module = Py_InitModule("yourmodule", yourmodule_functions);
</pre>
<p>If the string passed to this function is not the same name as your
extenion module, the <tt class="docutils literal">SystemError</tt> exception will be raised.</p>
</div>
<div class="section" id="how-do-i-tell-incomplete-input-from-invalid-input">
<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">codeop</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">PyRun_InteractiveLoop()</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">PyOS_ReadlineFunctionPointer</tt>
to point at your custom input function. See <tt class="docutils literal">Modules/readline.c</tt> and
<tt class="docutils literal">Parser/myreadline.c</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">PyRun_InteractiveLoop()</tt> to stop while waiting for user input. The
one solution then is to call <tt class="docutils literal">PyParser_ParseString()</tt> and test for
<tt class="docutils literal">e.error</tt> equal to <tt class="docutils literal">E_EOF</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">Py_CompileString()</tt>. If it compiles without errors, try to execute the returned
code object by calling <tt class="docutils literal">PyEval_EvalCode()</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>
<pre class="literal-block">
#include <stdio.h>
#include <readline.h>
#include <Python.h>
#include <object.h>
#include <compile.h>
#include <eval.h>
int main (int argc, char* argv[])
{
int i, j, done = 0; /* lengths of line, code */
char ps1[] = ">>> ";
char ps2[] = "... ";
char *prompt = ps1;
char *msg, *line, *code = NULL;
PyObject *src, *glb, *loc;
PyObject *exc, *val, *trb, *obj, *dum;
Py_Initialize ();
loc = PyDict_New ();
glb = PyDict_New ();
PyDict_SetItemString (glb, "__builtins__", PyEval_GetBuiltins ());
while (!done)
{
line = readline (prompt);
if (NULL == line) /* CTRL-D pressed */
{
done = 1;
}
else
{
i = strlen (line);
if (i > 0)
add_history (line); /* save non-empty lines */
if (NULL == code) /* nothing in code yet */
j = 0;
else
j = strlen (code);
code = realloc (code, i + j + 2);
if (NULL == code) /* out of memory */
exit (1);
if (0 == j) /* code was empty, so */
code[0] = '\0'; /* keep strncat happy */
strncat (code, line, i); /* append line to code */
code[i + j] = '\n'; /* append '\n' to code */
code[i + j + 1] = '\0';
src = Py_CompileString (code, "<stdin>", Py_single_input);
if (NULL != src) /* compiled just fine - */
{
if (ps1 == prompt || /* ">>> " or */
'\n' == code[i + j - 1]) /* "... " and double '\n' */
{ /* so execute it */
dum = PyEval_EvalCode ((PyCodeObject *)src, glb, loc);
Py_XDECREF (dum);
Py_XDECREF (src);
free (code);
code = NULL;
if (PyErr_Occurred ())
PyErr_Print ();
prompt = ps1;
}
} /* syntax error or E_EOF? */
else if (PyErr_ExceptionMatches (PyExc_SyntaxError))
{
PyErr_Fetch (&exc, &val, &trb); /* clears exception! */
if (PyArg_ParseTuple (val, "sO", &msg, &obj) &&
!strcmp (msg, "unexpected EOF while parsing")) /* E_EOF */
{
Py_XDECREF (exc);
Py_XDECREF (val);
Py_XDECREF (trb);
prompt = ps2;
}
else /* some other syntax error */
{
PyErr_Restore (exc, val, trb);
PyErr_Print ();
free (code);
code = NULL;
prompt = ps1;
}
}
else /* some non-syntax error */
{
PyErr_Print ();
free (code);
code = NULL;
prompt = ps1;
}
free (line);
}
}
Py_XDECREF(glb);
Py_XDECREF(loc);
Py_Finalize();
exit(0);
}
</pre>
</div>
<div class="section" id="how-do-i-find-undefined-g-symbols-builtin-new-or-pure-virtual">
<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>
</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">
<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>
<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">
<h1><a class="toc-backref" href="#id25">19 When importing module X, why do I get "undefined symbol: PyUnicodeUCS2*"?</a></h1>
<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">PyUnicodeUCS4</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
<tt class="docutils literal">Py_BuildValue</tt> (or similar) or parameter specifications for
<tt class="docutils literal">PyArg_ParseTuple()</tt>.</p>
<p>You can check the size of the Unicode character a Python interpreter is
using by checking the value of sys.maxunicode:</p>
<pre class="literal-block">
>>> import sys
>>> if sys.maxunicode > 65535:
... print 'UCS4 build'
... else:
... print 'UCS2 build'
</pre>
<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>
</div>
</body>
</html>