`mypy` option to error if there's a missing `py.typed`

[jamesbraza]

Feature

I often see people making packages, type checking them with mypy, and forgetting to include a py.typed file.

Pitch

Can we add a new option to mypy to error if someone has a package without a py.typed file? It would be nice if mypy could catch this common type of error (forgetting a py.typed file).

[emmatyping]

Unfortunately I don't think we can do this. If type checkers could tell if a package is missing a py.typed file, the file would not need to exist in the first place. The file exists to signal that a package should be used as a source of type information.

It is entirely reasonable to start using types internally in a package but not want them to be used externally.

As a stop-gap solution, you can use --follow-untyped-imports (recently merged, unreleased) for when a package is published without py.typed.