Antoine> Flattening should only happen for instances of the exact type.

I am attaching a variant of the patch that will only flatten if both nested and wrapping partial is of the exact type. Other possibilities would include flattening partial_subtype(f, ...) if type(f) == partial and if type(f) == partial_subtype, but I'll present only the least and most conservative variants.

Antoine> When people create subclasses [of partial type], there's usually a reason for it.

It is hard not to agree with this thesis, but I don't see how it follows that subclass writers will not benefit from flattening their instances.  Can you suggest a use case where flattening in functools.partial subtype would be undesirable?