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"""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
if MYPY_CHECK_RUNNING:
from typing import ...
Ref: https://github.com/python/mypy/issues/3216
"""
MYPY_CHECK_RUNNING = False
if MYPY_CHECK_RUNNING: # pragma: no cover
import typing
cast = typing.cast
else:
# typing's cast() is needed at runtime, but we don't want to import typing.
# Thus, we use a dummy no-op version, which we tell mypy to ignore.
def cast(type_, value): # type: ignore
return value
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