I'd like to propose a new function for inclusion to the inspect module
-- getcallargs(func, *args, **kwds) -- that returns a dict which maps
the formal arguments of a function (or other callable) to the values
passed as args and kwds, just as Python has to do when calling
func(*args, **kwds). For example:

>>> def func(a, b='foo', c=None, *x, **y):
...         pass
>>> sorted(getcallargs(func, 5, z=3, b=2).items())
 [('a', 5), ('b', 2), ('c', None), ('x', ()), ('y', {'z': 3})]

This is handy when writing decorators, or more generally when one would
want to do some minimal type checking without actually calling the function.

I have posted a recipe at
http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/551779; I can
clean it up and submit a proper patch if it's deemed useful enough for
the stdlib.

I updated the recipe to also return a `missing_args` tuple - the tuple
of the formal parameters whose value was not provided. This is useful in
cases where one want to distinguish f() from f(None) given "def f(x=None)".

Also updated url: http://code.activestate.com/recipes/551779/

I reverted the function to the original API (return just the dict with the bindings), cleaned it up, wrote thorough unit tests and made a patch against Python 2.7a4.

Renamed the Testcase classes to conform with the rest in test_inspect.py, added a few more tests for tuple args and patched against the latest trunk (r79086).

The patch will also need docs in inspect.rst.

- Added docs in inspect.rst
- Fixed TypeError message for zero-arg functions ("takes no arguments" instead of "takes exactly 0 arguments") + added test.

Would you upload this patch to Rietveld for review?

Uploaded at http://codereview.appspot.com/659041/show

Applied in r79500. Note I removed the error checking that a bound method received an instance of the class as the first argument because that error checking is a function of the calling of the function, not the binding of the arguments.