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.
