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bpo-42195: Override _CallableGenericAlias's __getitem__ by Fidget-Spinner · Pull Request #23915 · python/cpython · GitHub

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12 changes: 11 additions & 1 deletion Lib/_collections_abc.py
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters. Learn more about bidirectional Unicode characters
Original file line number Diff line number Diff line change
Expand Up @@ -434,7 +434,7 @@ def __create_ga(cls, origin, args):
raise TypeError(
"Callable must be used as Callable[[arg, ...], result].")
t_args, t_result = args
if isinstance(t_args, list):
if isinstance(t_args, (list, tuple)):
ga_args = tuple(t_args) + (t_result,)
# This relaxes what t_args can be on purpose to allow things like
# PEP 612 ParamSpec. Responsibility for whether a user is using
Expand All @@ -456,6 +456,16 @@ def __reduce__(self):
args = list(args[:-1]), args[-1]
return _CallableGenericAlias, (Callable, args)

def __getitem__(self, item):
# Called during TypeVar substitution, returns the custom subclass
# rather than the default types.GenericAlias object.
ga = super().__getitem__(item)
args = ga.__args__
t_result = args[-1]
t_args = args[:-1]
args = (t_args, t_result)
return _CallableGenericAlias(Callable, args)


def _type_repr(obj):
"""Return the repr() of an object, special-casing types (internal helper).
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6 changes: 6 additions & 0 deletions Lib/test/test_genericalias.py
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Original file line number Diff line number Diff line change
Expand Up @@ -347,6 +347,12 @@ def test_abc_callable(self):
self.assertEqual(C2[int, float, str], Callable[[int, float], str])
self.assertEqual(C3[int], Callable[..., int])

# multi chaining
C4 = C2[int, V, str]
self.assertEqual(repr(C4).split(".")[-1], "Callable[[int, ~V], str]")
self.assertEqual(repr(C4[dict]).split(".")[-1], "Callable[[int, dict], str]")
self.assertEqual(C4[dict], Callable[[int, dict], str])

with self.subTest("Testing type erasure"):
class C1(Callable):
def __call__(self):
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