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Pyximport

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 add the first line above to your sitecustomize.py. 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"

See help(pyximport.install) to learn its options for controlling the default behavior of import and reload.

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.

Limitations

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

setup.py install does not modify sitecustomize.py 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.