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<center> <h1> <hr width="100%">Cython FAQ
<hr width="100%"></h1>
</center>
<h2> Contents</h2>
<ul>
<li> <b><a href="#CallCAPI">How do I call Python/C API routines?</a></b></li>
<li> <b><a href="#NullBytes">How do I convert a C string containing null
bytes to a Python string?</a></b></li>
<li> <b><a href="#NumericAccess">How do I access the data inside a Numeric
array object?</a></b></li>
<li><b><a href="#Rhubarb">Cython says my extension type object has no attribute
'rhubarb', but I know it does. What gives?</a></b></li><li><a style="font-weight: bold;" href="#Quack">Python says my extension type has no method called 'quack', but I know it does. What gives?</a><br>
</li>
</ul>
<hr width="100%"> <h2> <a name="CallCAPI"></a>How do I call Python/C API routines?</h2>
Declare them as C functions inside a <tt>cdef extern from</tt> block.
Use the type name <tt>object</tt> for any parameters and return types which
are Python object references. Don't use the word <tt>const</tt> anywhere.
Here is an example which defines and uses the <tt>PyString_FromStringAndSize</tt> routine:
<blockquote><tt>cdef extern from "Python.h":</tt> <br>
<tt>&nbsp;&nbsp;&nbsp; object PyString_FromStringAndSize(char *, int)</tt> <p><tt>cdef char buf[42]</tt> <br>
<tt>my_string = PyString_FromStringAndSize(buf, 42)</tt></p>
</blockquote>
<h2> <a name="NullBytes"></a>How do I convert a C string containing null
bytes to a Python string?</h2>
Put in a declaration for the <tt>PyString_FromStringAndSize</tt> API routine
and use that<tt>.</tt> See <a href="#CallCAPI">How do I call Python/C API
routines?</a> <h2> <a name="NumericAccess"></a>How do I access the data inside a Numeric
array object?</h2>
Use a <tt>cdef extern from</tt> block to include the Numeric header file
and declare the array object as an external extension type. The following
code illustrates how to do this:
<blockquote><tt>cdef extern from "Numeric/arrayobject.h":</tt> <p><tt>&nbsp;&nbsp;&nbsp; struct PyArray_Descr:</tt> <br>
<tt>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; int type_num, elsize</tt> <br>
<tt>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; char type</tt> </p>
<p><tt>&nbsp;&nbsp;&nbsp; ctypedef class Numeric.ArrayType [object PyArrayObject]</tt><tt>:</tt> <br>
<tt>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; cdef char *data</tt> <br>
<tt>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; cdef int nd</tt> <br>
<tt>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; cdef int *dimensions,
*strides</tt> <br>
<tt>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; cdef object base</tt>
<br>
<tt>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; cdef PyArray_Descr *descr</tt> <br>
<tt>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; cdef int flags<br>
</tt></p>
</blockquote>
<p>For more information about external extension types, see the <a href="extension_types.html#ExternalExtTypes">"External Extension Types"</a>
section of the <a href="extension_types.html">"Extension Types"</a> documentation
page.<br>
<tt> </tt> </p>
<h2><a name="Rhubarb"></a>Cython says my extension type object has no attribute
'rhubarb', but I know it does. What gives?</h2>
You're probably trying to access it through a reference which Cython thinks
is a generic Python object. You need to tell Cython that it's a reference
to your extension type by means of a declaration,<br>
for example,<br>
<blockquote><tt>cdef class Vegetables:</tt><br>
<tt>&nbsp; &nbsp; cdef int rhubarb</tt><br>
<br>
<tt>...</tt><br>
<tt>cdef Vegetables veg</tt><br>
<tt>veg.rhubarb = 42</tt><br>
</blockquote>
Also see the <a href="extension_types.html#ExtTypeAttrs">"Attributes"</a>
section of the <a href="extension_types.html">"Extension
Types"</a> documentation page.<br>
<h2><a name="Quack"></a>Python says my extension type has no method called 'quack', but I know it does. What gives?</h2>
You may have declared the method using <span style="font-family: monospace;">cdef</span> instead of <span style="font-family: monospace;">def</span>. Only functions and methods declared with <span style="font-family: monospace;">def</span> are callable from Python code.<br>
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