How to create Python module distribution to gracefully fall-back to pure Python code

Question

I have written a Python module, and I have two versions: a pure Python implementation and a C extension. I've written the __init__.py file so that it tries to import the C extension, and if that fails, it imports the pure Python code (is that reasonable?).

Now, I'd like to know what is the best way to distribute this module (e.g. write setup.py) so it can be easily used by people with or without the facility to build, or use, the C extension, just by running:

python setup.py install

My experience is limited, but I see two possible cases:

• User does not have MS Visual Studio, or the GCC compiler suite, installed on their machine, to build the C extension
• User is running IronPython, Jython, or anything other than CPython. I only have used CPython. So I'm not sure how I could distribute this module so that it would work smoothly and be easy to install on those platforms, if they're unable to use the C extension.

Answer 1

(is that reasonable?).

Yep, perfectly sensible.

To catch the "no suitable C compiler case": the call to setup(...) will do a sys.exit in case of problems. So, first try it with the ext_modules argument set as desired, within a try:

try:
  setup(..., ext_modules=...)
except SystemExit: ...

and in the except clause, call setup(...) again without the ext_modules (so it gives up on building and installing the extensions). The user who's installing will still see messages like "unable to execute gcc-4.0: No such file or directory", but you can appropriately add your own messages to inform the user that it's no big deal and that you're trying again without the extension modules.

To support non-CPython implementations, in your setup.py you can test sys.version (I'm not sure what the value will be for each non-CPython implementation, but IronPython has an 'IronPython' substring there, for example), to avoid even trying the ext_modules part. If you miss some such implementation in your checks, the try/except should probably catch most others anyway, just with a modest amount of wasted work;-).