Assume unannotated return values to be None instead of Any (behind flag)

[berzi]

Feature

Behind a config flag like untyped_return_type_is_none = true, Mypy assumes that a function with no return type, e.g.:

def my_func(i_am_annotated: str):  # Note: no "-> int" annotation
    return 5

Returns None, instead of accepting any return type at face value.

Additionally, while untyped_return_type_is_none is true, disallow_untyped_defs allows functions to have no return annotation if their return type is None.

Pitch

Adoption for the disallow_untyped_defs is difficult, because it requires None to be explicitly annotated. This makes code more wordy in many places where the annotation adds zero value (think of all test functions/methods for extremely popular frameworks like pytest or unittest). This is especially overwhelming for maintainers of large codebases with many tests and existing functions.

Having the flag would give users an option to leave -> None functions unannotated.

Current behaviour would be unchanged if the new flag were not activated, but activating the flag would force users to annotate those functions that do return a value, providing a large but progressive step towards disallow_untyped_defs, reducing the essentially meaningless menial work of adding -> None everywhere.

[tmke8]

Duplicate of #9413
(Though your proposal is fleshed out more)

[AlexWaygood]

Duplicate of #9413 (Though your proposal is fleshed out more)

Closing as a duplicate of the older issue so that we can keep the discussion in one place rather than split across two issue threads