For neatly implementing static typing in packaging.

mypy - the static type analysis tool we use - uses the typing module, which provides core functionality fundamental to mypy’s functioning.

Generally, typing would be imported at runtime and used in that fashion - it acts as a no-op at runtime and does not have any run-time overhead by design.

As it turns out, typing is not vendorable - it uses separate sources for Python 2/Python 3. Thus, this codebase can not expect it to be present. To work around this, mypy allows the typing import to be behind a False-y optional to prevent it from running at runtime and type-comments can be used to remove the need for the types to be accessible directly during runtime.

This module provides the False-y guard in a nicely named fashion so that a curious maintainer can reach here to read this.

In packaging, all static-typing related imports should be guarded as follows:

from packaging._typing import MYPY_CHECK_RUNNING


from typing import …


lib.packaging._typing.cast(type_, value)[source]