I am still working through this and thinking about implications, but my first impression is that the functools.partial object should provide an attribute (property) __signature__ which yields the correct result.

When you think about it, any user who wants to implement a function wrapper using a class to do so rather than using functools.update_wrapper(), has to implement __signature__ if the wrapper is a signature changing decorator. So why shouldn't Python itself follow the same mechanism that is forced on users in their own wrappers.

If functools.partial were to implement __signature__, then the part of PEP 362 where it says:

> If the object is an instance of functools.partial, construct a new Signature from its partial.func attribute, and account for already bound partial.args and partial.kwargs

becomes redundant as the code to deal with it is localised within the functools.partial implementation by virtue of __signature__ on that type rather than having a special case in inspect.signature().

If this was seen as making more sense, one might even argue that FunctionType and the bound variant could implement __signature__ and so localise things to those implementations as well, which would further simplify inspect.signature().

This would set a good precedent going forward that if any special callable wrapper objects are added to the Python core in the future, that they implement __signature__, rather than someone thinking that further special cases could be added to inspect.signature() to deal with them.

I have yet to do some actual code experiments so might have more thoughts on the matter later.