If a nested function has doctests, they won't be run:
{{{
def f():
  '''
  >>> 'a'
  'a'
  '''

  def g():
    '''
    >>> 'a'
    'b'
    '''
    pass

  pass
}}}

DocTestFinder will only find f's doctest and won't recurse to find g's, surprising the programmer when they (hopefully) discover that their inner doctest is incorrect!

DocTestFinder uses object inspection to find docstrings, which doesn't really allow this kind of thing:  g isn't created until f is actually run.

Making this possible would probably require implementing an alternate doctest finder based on source code inspection (via the new _ast module?).

Here is a patch that implements this new functionality.
I don't know how desireable this feature is, I just
wanted to see if it could be done.
File Added: doctest_nested_functions.diff

I believe that there could be interest in this, has anyone taken over doctest from tim_one?