Deprecate `mypy_extensions.TypedDict`?

[AlexWaygood]

mypy_extensions.TypedDict is problematic for several reasons:

• mypy_extensions.TypedDict is missing quite a few features that have been added to typing.TypedDict at runtime (and have been backported to typing_extensions.TypedDict).
• I think the existence of both mypy_extensions.TypedDict and typing_extensions.TypedDict has the potential to be pretty confusing for users.
• It's quite annoying and error-prone that at typeshed, we have to remember to keep typing._TypedDict, typing_extensions._TypedDict and mypy_extensions._TypedDict all in sync. Unfortunately, we can't put them all in _typeshed and have all three modules import _TypedDict from _typeshed, as the three classes all have slightly subtle differences. (E.g. mypy_extensions._TypedDict doesn't have the __required_keys__ and __optional_keys__ ClassVars that both typing._TypedDict and typing_extensions._TypedDict have.)
• Mypy now maintains its own copy of the mypy-extensions stubs anyway, so its stubs for mypy_extensions are no longer automatically updated with each typeshed sync. That means that even if we update the stubs for mypy_extensions.TypedDict in typeshed (as we did in Improve __(r)or__ signatures for TypedDict classes typeshed#10565), those updates are no longer of any benefit to mypy users unless mypy maintainers remember to copy across the changes to their forked version of the mypy_extensions stubs.

I propose that we deprecate mypy_extensions.TypedDict, and steer people towards typing_extensions.TypedDict instead: it's up-to-date with the features on typing.TypedDict, it's much more comprehensively tested, and it has up-to-date stubs.

Thoughts?

[JelleZijlstra]

Agree, there's no need for mypy_extensions.TypedDict any more. As an implementation strategy, we can:

• Raise a runtime DeprecationWarning when a new mypy_extensions.TypedDict class is created
• Have mypy show a warning whenever mypy_extensions.TypedDict is used

[AlexWaygood]

I'm taking a look at the runtime side of things. One issue: it seems we'd really need stacklevel=3 if you're constructing a TypedDict class like this:

Foo = TypedDict('Foo', {'x': int})

...But you'd need stacklevel=2 if you're constructing a TypedDict like this:

class Bar(TypedDict):
    x: int

I'm not sure if there's a way to distinguish between the two, if we emit the DeprecationWarning in _TypedDictMeta.__new__

[AlexWaygood]

I'm not sure if there's a way to distinguish between the two, if we emit the DeprecationWarning in _TypedDictMeta.__new__

Hmm, I think I have a solution