Agree, for the general case I think it makes sense that this is fixed in pyside.
Hijacking __signature__ python-wide sounds like it can cause other issues, if they need such a hack I would suggest finding a way (in pyside) to allow for mock to work.

I wonder though if this change in __signature__ will affect in any way any C extension that might not allow setting __signature__ if not set as a lack of __dict__ (I have not managed to find any issue with small examples though).