pxd files

In addition to the .pyx source files, Cython uses .pxd files which work like C header files – they contain Cython declarations (and sometimes code sections) which are only meant for inclusion by Cython modules. A pxd file is imported into a pyx module by using the cimport keyword.

pxd files have many use-cases:

1. They can be used for sharing external C declarations.

2. They can contain functions which are well suited for inlining by the C compiler. Such functions should be marked inline, example:

   cdef inline int int_min(int a, int b):
       return b if b < a else a
   

3. When accompanying an equally named pyx file, they provide a Cython interface to the Cython module so that other Cython modules can communicate with it using a more efficient protocol than the Python one.

In our integration example, we might break it up into pxd files like this:

1. Add a cmath.pxd function which defines the C functions available from the C math.h header file, like sin. Then one would simply do from cmath cimport sin in integrate.pyx.

2. Add a integrate.pxd so that other modules written in Cython can define fast custom functions to integrate.

   cdef class Function:
       cpdef evaluate(self, double x)
   cpdef integrate(Function f, double a,
                   double b, int N)
   

   Note that if you have a cdef class with attributes, the attributes must be declared in the class declaration pxd file (if you use one), not the pyx file. The compiler will tell you about this.