We'll have to wait for Nick to chime in, but I'll make a couple of comments.  

First, I think this is a bug in partial, so I think we need to decide what, if anything, to do about that first, before we decide if signature needs to compensate for it or not.  

The other comment is about ==/__hash__, in case that is involved in the solution: two objects may very well have the same __hash__ and *not* be equal, but they cannot have different __hash__es and *be* equal.  The equality comparison algorithm uses __hash__ as a shortcut.  If two objects have different hashes, they are not equal and we return False.  If they have the same hash, then a full equality comparison is done before the result of __eq__ is returned.  You will understand that this must be the case if you think about the nature of hashing: it throws away information.  Cryptographic hashes used for identification are constructed such that the *probability* of a collision is very low, but it is still not zero, so how they are used is just as important as how they work.  Our __hash__es are generally not that strong, so collision is likely.