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typing.py
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"""For neatly implementing static typing in pip. `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 pip, all static-typing related imports should be guarded as follows: from pip._internal.utils.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: from typing import 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