Python type hints: typing.Mapping vs. typing.Dict

Question

I'm working on a python3 project where we use the typing module type hints throughout.

It seems that we use typing.Dict and typing.Mapping pretty much interchangeably.

Is there a reason to prefer one over the other?

Solution

Managed to answer this myself.

typing.Dict should be used to indicate a literal dict type with support for element type hinting i.e. Dict[bytes, str]

typing.Mapping is an object which defines the __getitem__,__len__,__iter__ magic methods

typing.MutableMapping is an object which defines same as Mapping but with __setitem__,__delitem__ magic methods as well.

typing.Mapping et al. are based on the abc types in this table

Comments

[Hubert Grzeskowiak]

Rule of a thumb: expect generic types, return specific types. This gives users of your functions and classes the biggest flexibility and safety.

[Dmitry Vyal]

One more difference. Dict is invariant, Mapping is covariant. It's important when e.g. you have a function receiving Dict[A,B] and try to pass Dict[A,C] where C is a subtype of B. You will get typing error which can be avoided by using Mapping[A,B], See realpython.com/python-type-checking/….

[Saddle Point]

@HubertGrzeskowiak Besides the rule, if a function preprocess on the inputs, should I use Dict as both input and output types? Receiving a MutableMapping and then returns a Dict seems a bit inconvenient in this case ...