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Editing: _typing.py

"""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 TYPE_CHECKING

if TYPE_CHECKING:
        from typing import ...

Ref: https://github.com/python/mypy/issues/3216
"""

__all__ = ["TYPE_CHECKING", "cast"]

# The TYPE_CHECKING constant defined by the typing module is False at runtime
# but True while type checking.
if False:  # pragma: no cover
    from typing import TYPE_CHECKING
else:
    TYPE_CHECKING = False

# typing's cast syntax requires calling typing.cast at runtime, but we don't
# want to import typing at runtime. Here, we inform the type checkers that
# we're importing `typing.cast` as `cast` and re-implement typing.cast's
# runtime behavior in a block that is ignored by type checkers.
if TYPE_CHECKING:  # pragma: no cover
    # not executed at runtime
    from typing import cast
else:
    # executed at runtime
    def cast(type_, value):  # noqa
        return value

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