> To avoid the wasted memory and import time, it's better to use:
>
> try:
>    from _cmodule import *
> except ImportError:
>    from _pymodule import *
>

.. also this makes it harder to prototype things in Python or have mixed Python/C modules.  The goal is to use Python implementation unless native implementation exists on per function/class basis.  The syntax above makes it all or nothing.