Source Files and Compilation

Cython 0.19.1

Source Files and Compilation


See Compilation reference section for more details

Cython source file names consist of the name of the module followed by a .pyx extension, for example a module called primes would have a source file named primes.pyx.

Once you have written your .pyx file, there are a couple of ways of turning it into an extension module. One way is to compile it manually with the Cython compiler, e.g.:

$ cython primes.pyx

This will produce a file called primes.c, which then needs to be compiled with the C compiler using whatever options are appropriate on your platform for generating an extension module. For these options look at the official Python documentation.

The other, and probably better, way is to use the distutils extension provided with Cython. The benifit of this method is that it will give the platform specific compilation options, acting like a stripped down autotools.


The distutils extension provided with Cython allows you to pass .pyx files directly to the Extension constructor in your setup file.

If you have a single Cython file that you want to turn into a compiled extension, say with filename example.pyx the associated would be:

from distutils.core import setup
from distutils.extension import Extension
from Cython.Distutils import build_ext

    cmdclass = {'build_ext': build_ext},
    ext_modules = [Extension("example", ["example.pyx"])]

To understand the more fully look at the official distutils documentation. To compile the extension for use in the current directory use:

$ python build_ext --inplace

Cython Files Depending on C Files

When you have come C files that have been wrapped with cython and you want to compile them into your extension the basic file to do this would be:

from distutils.core import setup
from distutils.extension import Extension
from Cython.Distutils import build_ext

sourcefiles = ['example.pyx', 'helper.c', 'another_helper.c']

    cmdclass = {'build_ext': build_ext},
    ext_modules = [Extension("example", sourcefiles)]

Notice that the files have been given a name, this is not necessary, but it makes the file easier to format if the list gets long.

The Extension class takes many options, and a fuller explanation can be found in the distutils documentation. Some useful options to know about are include_dirs, libraries, and library_dirs which specify where to find the .h and library files when linking to external libraries.

Multiple Cython Files in a Package


Distributing Cython modules

It is strongly recommended that you distribute the generated .c files as well as your Cython sources, so that users can install your module without needing to have Cython available.

It is also recommended that Cython compilation not be enabled by default in the version you distribute. Even if the user has Cython installed, he probably doesn’t want to use it just to install your module. Also, the version he has may not be the same one you used, and may not compile your sources correctly.

This simply means that the file that you ship with will just be a normal distutils file on the generated .c files, for the basic example we would have instead:

from distutils.core import setup
from distutils.extension import Extension

    ext_modules = [Extension("example", ["example.c"])]


Cython is a compiler. Therefore it is natural that people tend to go through an edit/compile/test cycle with Cython modules. But my personal opinion is that one of the deep insights in Python’s implementation is that a language can be compiled (Python modules are compiled to .pyc) files and hide that compilation process from the end-user so that they do not have to worry about it. Pyximport does this for Cython modules. For instance if you write a Cython module called foo.pyx, with Pyximport you can import it in a regular Python module like this:

import pyximport; pyximport.install()
import foo

Doing so will result in the compilation of foo.pyx (with appropriate exceptions if it has an error in it).

If you would always like to import Cython files without building them specially, you can also the first line above to your That will install the hook every time you run Python. Then you can use Cython modules just with simple import statements. I like to test my Cython modules like this:

$ python -c "import foo"

Dependency Handling

In Pyximport 1.1 it is possible to declare that your module depends on multiple files, (likely .h and .pxd files). If your Cython module is named foo and thus has the filename foo.pyx then you should make another file in the same directory called foo.pyxdep. The modname.pyxdep file can be a list of filenames or “globs” (like *.pxd or include/*.h). Each filename or glob must be on a separate line. Pyximport will check the file date for each of those files before deciding whether to rebuild the module. In order to keep track of the fact that the dependency has been handled, Pyximport updates the modification time of your ”.pyx” source file. Future versions may do something more sophisticated like informing distutils of the dependencies directly.


Pyximport does not give you any control over how your Cython file is compiled. Usually the defaults are fine. You might run into problems if you wanted to write your program in half-C, half-Cython and build them into a single library. Pyximport 1.2 will probably do this.

Pyximport does not hide the Distutils/GCC warnings and errors generated by the import process. Arguably this will give you better feedback if something went wrong and why. And if nothing went wrong it will give you the warm fuzzy that pyximport really did rebuild your module as it was supposed to.

For further thought and discussion

I don’t think that Python’s reload() will do anything for changed .so‘s on some (all?) platforms. It would require some (easy) experimentation that I haven’t gotten around to. But reload is rarely used in applications outside of the Python interactive interpreter and certainly not used much for C extension modules. Info about Windows install does not modify for you. Should it? Modifying Python’s “standard interpreter” behaviour may be more than most people expect of a package they install..

Pyximport puts your .c file beside your .pyx file (analogous to .pyc beside .py). But it puts the platform-specific binary in a build directory as per normal for Distutils. If I could wave a magic wand and get Cython or distutils or whoever to put the build directory I might do it but not necessarily: having it at the top level is VERY HELPFUL for debugging Cython problems.