annotate planemo/lib/python3.7/site-packages/packaging/_typing.py @ 1:56ad4e20f292 draft

"planemo upload commit 6eee67778febed82ddd413c3ca40b3183a3898f1"
author guerler
date Fri, 31 Jul 2020 00:32:28 -0400 (2020-07-31)
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1 """For neatly implementing static typing in packaging.
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2
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3 `mypy` - the static type analysis tool we use - uses the `typing` module, which
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4 provides core functionality fundamental to mypy's functioning.
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5
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6 Generally, `typing` would be imported at runtime and used in that fashion -
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7 it acts as a no-op at runtime and does not have any run-time overhead by
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8 design.
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9
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10 As it turns out, `typing` is not vendorable - it uses separate sources for
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11 Python 2/Python 3. Thus, this codebase can not expect it to be present.
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12 To work around this, mypy allows the typing import to be behind a False-y
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13 optional to prevent it from running at runtime and type-comments can be used
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14 to remove the need for the types to be accessible directly during runtime.
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15
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16 This module provides the False-y guard in a nicely named fashion so that a
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17 curious maintainer can reach here to read this.
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18
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19 In packaging, all static-typing related imports should be guarded as follows:
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20
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21 from packaging._typing import TYPE_CHECKING
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23 if TYPE_CHECKING:
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24 from typing import ...
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25
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26 Ref: https://github.com/python/mypy/issues/3216
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27 """
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28
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29 __all__ = ["TYPE_CHECKING", "cast"]
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31 # The TYPE_CHECKING constant defined by the typing module is False at runtime
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32 # but True while type checking.
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33 if False: # pragma: no cover
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34 from typing import TYPE_CHECKING
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35 else:
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36 TYPE_CHECKING = False
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37
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38 # typing's cast syntax requires calling typing.cast at runtime, but we don't
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39 # want to import typing at runtime. Here, we inform the type checkers that
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40 # we're importing `typing.cast` as `cast` and re-implement typing.cast's
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41 # runtime behavior in a block that is ignored by type checkers.
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42 if TYPE_CHECKING: # pragma: no cover
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43 # not executed at runtime
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44 from typing import cast
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45 else:
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46 # executed at runtime
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47 def cast(type_, value): # noqa
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48 return value