The following code:

  def foo(a, *, b=None):
    pass

  unittest.mock.create_autospec(foo)

fails with this traceback:

    Traceback (most recent call last):
      File "temp.py", line 6, in <module>
        unittest.mock.create_autospec(foo)
      File "/home/rdmurray/python/p33/Lib/unittest/mock.py", line 2026, in create_autospec
        mock = _set_signature(mock, spec)
      File "/home/rdmurray/python/p33/Lib/unittest/mock.py", line 162, in _set_signature
        result = _getsignature(original, skipfirst, instance)
      File "/home/rdmurray/python/p33/Lib/unittest/mock.py", line 81, in _getsignature
        regargs, varargs, varkwargs, defaults = inspect.getargspec(func)
      File "/home/rdmurray/python/p33/Lib/inspect.py", line 808, in getargspec
        raise ValueError("Function has keyword-only arguments or annotations"
    ValueError: Function has keyword-only arguments or annotations, use getfullargspec() API which can support them

This is non-trivial to fix. Although inspect.getfullargspec can be used, which does support keyword only arguments, inspect.formatargspec *doesn't* support them. (mock.create_autospec uses these to rebuild a compatible signature for generated mocks.)

The easiest route to fixing would be to extend formatargspec to optionally take extra arguments for kwonlyargs and kwonlydefaults.

Hmmm... looks like formatargspec does support these features but they aren't documented. If it works out I'll update the docs for inspect.formatargspec too.

New changeset 6f478a4aa137 by Michael Foord in branch 'default':
Closes issue 14634. unittest.mock.create_autospec now supports keyword only arguments.
http://hg.python.org/cpython/rev/6f478a4aa137