Pyrex FAQ


How do I call Python/C API routines?

Declare them as C functions inside a cdef extern from block. Use the type name object for any parameters and return types which are Python object references. Don't use the word const anywhere. Here is an example which defines and uses the PyString_FromStringAndSize routine:
cdef extern from "Python.h":
    object PyString_FromStringAndSize(char *, int)

cdef char buf[42]
my_string = PyString_FromStringAndSize(buf, 42)

How do I convert a C string containing null bytes to a Python string?

Put in a declaration for the PyString_FromStringAndSize API routine and use that. See How do I call Python/C API routines?

How do I access the data inside a Numeric array object?

Use a cdef extern from 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:
cdef extern from "Numeric/arrayobject.h":

    struct PyArray_Descr:
        int type_num, elsize
        char type

    ctypedef class Numeric.ArrayType [object PyArrayObject]:
        cdef char *data
        cdef int nd
        cdef int *dimensions, *strides
        cdef object base
        cdef PyArray_Descr *descr
        cdef int flags

For more information about external extension types, see the "External Extension Types" section of the "Extension Types" documentation page.

Pyrex says my extension type object has no attribute 'rhubarb', but I know it does. What gives?

You're probably trying to access it through a reference which Pyrex thinks is a generic Python object. You need to tell Pyrex that it's a reference to your extension type by means of a declaration. For example,
cdef class Vegetables:
    cdef int rhubarb

cdef Vegetables veg
veg.rhubarb = 42
Also see the "Attributes" section of the "Extension Types" documentation page.

Python says my extension type has no method called 'quack', but I know it does. What gives?

You may have declared the method using cdef instead of def. Only functions and methods declared with def are callable from Python code.