Return Dict from MappingProxyType.copy()

[Dreamsorcerer]

Is it OK to change this to a Dict? Not sure why it would return Mapping.
Or is there a way to use a TypeVar here, such that the return type is the same as the type passed in at creation time?

I keep encountering typing errors with code like:

TEMPLATE = MappingProxyType({...})

...

data = TEMPLATE.copy()
data["foo"] = ...  # Error, can't assign to Mapping.

[gvanrossum]

I think this is reasonable. MappingProxy is an implementation detail of class.__dict__ that prevents changes to the class attributes via a backdoor (you must use the front door, i.e. setattr). And MappingProxy.copy() indeed returns a dict. The key here is that the returned object is owned by the caller and can be freely mutated -- returning a Mapping prevents that.

[JelleZijlstra]

Looking at the C implementation (https://github.com/python/cpython/blob/384621c42f9102e31ba2c47feba144af09c989e5/Objects/descrobject.c#L1114), it just calls .copy() on the underlying mapping. Strictly speaking, then, we should make MappingProxyType generic over its underlying mapping type and return that type. But I'm not sure we can preserve the key and value types correctly in that case, and in practice MappingProxyType usually just maps over a dict, so I'm OK with annotating it as returning a dict.

[Dreamsorcerer]

Do I need to nudge someone to merge this? Doesn't seems like there's anyone against it...

[srittau]

Thanks for the nudge, I merged it.