Doctest fails to find doctests defined in extension modules.  With tools
like cython (http://cython.org) it's trivially easy to add docstrings to
extension code, a task that is much less pleasant with hand-written
extensions.   The following patch is a minimal fix for the problem:


--- doctest_ori.py      2008-06-20 19:22:56.000000000 -0700
+++ doctest.py  2008-06-20 19:23:53.000000000 -0700
@@ -887,7 +887,8 @@
             for valname, val in obj.__dict__.items():
                 valname = '%s.%s' % (name, valname)
                 # Recurse to functions & classes.
-                if ((inspect.isfunction(val) or inspect.isclass(val)) and
+                if ((inspect.isfunction(val) or inspect.isclass(val) or
+                     inspect.isbuiltin(val) ) and
                     self._from_module(module, val)):
                     self._find(tests, val, valname, module, source_lines,
                                globs, seen)


However, it is likely not sufficient as it doesn't take into account the
__test__ dict, for which probably the same change would work, just a few
lines later.  Furthermore, the real issue is in my view in the
distinction made by inspect between isfunction() and isbuiltin() for the
sake of analyzing docstrings.  isfunction() returns false for a function
that is defined in an extension module (though it *is* a function) while
isbuiltin returns True (though it is *not* a builtin).

For purposes of doctesting, doctest should simply care:

- That it is a function.
- That it has a docstring that can be parsed.

But in too many places in doctest there are currently assumptions about
being able to extract full source, line numbers, etc.  Hopefully this
quick fix can be applied as it will immediately make doctest work with
large swaths of extension code, while a proper rethinking of doctest is
made.  

BTW, in that process doctest will hopefully be made more modular and
flexible: its current structure forces massive copy/paste subclassing
for any kind of alternate use, since it has internally hardwired use of
its own classes.  Doctest is tremendously useful, but it really could
use with some structural reorganization to make it more flexible (cleanly).