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.

__init__.pxd

Cython also supports __init__.pxd files for declarations in package’s namespaces, similar to __init__.py files in Python.

Continuing the integration example, we could package the module as follows:

1. Place the module files in a directory tree as one usually would for Python:

   CyIntegration/
   ├── __init__.pyx
   ├── __init__.pxd
   ├── integrate.pyx
   └── integrate.pxd
   

2. In __init__.pxd, use cimport for any declarations that one would want to be available from the package’s main namespace:

   from CyIntegration cimport integrate
   

   Other modules would then be able to use cimport on the package in order to recursively gain faster, Cython access to the entire package and the data declared in its modules:

   cimport CyIntegration
   
   cpdef do_integration(CyIntegration.integrate.Function f):
       return CyIntegration.integrate.integrate(f, 0., 2., 1)