pxd files¶
Note
This page uses two different syntax variants:
Cython specific
cdefsyntax, which was designed to make type declarations concise and easily readable from a C/C++ perspective.Pure Python syntax which allows static Cython type declarations in pure Python code, following PEP-484 type hints and PEP 526 variable annotations.
To make use of C data types in Python syntax, you need to import the special
cythonmodule in the Python module that you want to compile, e.g.import cython
If you use the pure Python syntax we strongly recommend you use a recent Cython 3 release, since significant improvements have been made here compared to the 0.29.x releases.
In addition to the .pyx and .py 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:
They can be used for sharing external C declarations.
They can contain functions which are well suited for inlining by the C compiler. Such functions should be marked
inline, example:inline.pxd¶cdef inline int int_min(int a, int b): return b if b < a else a
When accompanying an equally named
.pyx/.pyfile, 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:
Add a
cmath.pxd:cmath.pxd¶cdef extern from "math.h": cpdef double sin(double x)
Then one would simply do
integrate.py¶from cython.cimports.cmath import sin
Warning
The code provided above / on this page uses an external native (non-Python) library through a
cimport(cython.cimports). Cython compilation enables this, but there is no support for this from plain Python. Trying to run this code from Python (without compilation) will fail when accessing the external library. This is described in more detail in Calling C functions.integrate.pyx¶from cmath cimport sin
Add a
integrate.pxdso that other modules written in Cython can define fast custom functions to integrate:integrate.pxd¶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
.pxdfile (if you use one), not the.pyx/.pyfile. 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:
Place the module files in a directory tree as one usually would for Python:
CyIntegration/ ├── __init__.py ├── __init__.pxd ├── integrate.py └── integrate.pxd
CyIntegration/ ├── __init__.pyx ├── __init__.pxd ├── integrate.pyx └── integrate.pxd
In
__init__.pxd, usecimportfor any declarations that one would want to be available from the package’s main namespace:from cython.cimports.CyIntegration import integrate
from CyIntegration cimport integrate
Other modules would then be able to use
cimporton the package in order to recursively gain faster, Cython access to the entire package and the data declared in its modules:from cython.cimports import CyIntegration @cython.ccall def do_integration(f: CyIntegration.integrate.Function): return CyIntegration.integrate.integrate(f, 0., 2., 1)
Warning
The code provided above / on this page uses an external native (non-Python) library through a
cimport(cython.cimports). Cython compilation enables this, but there is no support for this from plain Python. Trying to run this code from Python (without compilation) will fail when accessing the external library. This is described in more detail in Calling C functions.cimport CyIntegration cpdef do_integration(CyIntegration.integrate.Function f): return CyIntegration.integrate.integrate(f, 0., 2., 1)
