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Merged
JelleZijlstra merged 15 commits into from
Apr 5, 2022
Merged

bpo-46998: Allow subclassing Any at runtime #31841

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bc71456
bpo-46998: Allow subclassing Any at runtime
Mar 13, 2022
abea39a
make error consistent
Mar 13, 2022
771f492
remove functools checks
Mar 13, 2022
900f2c7
📜🤖 Added by blurb_it.
blurb-it[bot] Mar 13, 2022
6e76441
more tests
Mar 13, 2022
b00ab17
Update Misc/NEWS.d/next/Library/2022-03-13-08-52-58.bpo-46998.cHh-9O.rst
hauntsaninja Mar 13, 2022
4d913ad
oops, fix test
Mar 13, 2022
1c1ab38
cooperative inheritance
Mar 13, 2022
4927e78
fix tests
Mar 13, 2022
68ea3a8
okay, make instancecheck work with multiple inheritance
Mar 13, 2022
f94b7e0
mention in docs
Mar 30, 2022
05db029
Merge remote-tracking branch 'upstream/main' into subclass-any
Mar 30, 2022
e11f405
fix pydoc test
Mar 30, 2022
63b00e4
add a sentence on reasoning
Mar 30, 2022
ac7b4a8
preposition
Mar 31, 2022
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bpo-46998: Allow subclassing Any at runtime
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hauntsaninja committed Mar 13, 2022
commit bc7145697c188e6876257c1de0d545a27b6d7b89
16 changes: 3 additions & 13 deletions Lib/test/test_typing.py
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Original file line number Diff line number Diff line change
Expand Up @@ -89,12 +89,6 @@ def test_any_instance_type_error(self):
with self.assertRaises(TypeError):
isinstance(42, Any)

def test_any_subclass_type_error(self):
with self.assertRaises(TypeError):
issubclass(Employee, Any)
with self.assertRaises(TypeError):
issubclass(Any, Employee)

def test_repr(self):
self.assertEqual(repr(Any), 'typing.Any')

Expand All @@ -104,13 +98,9 @@ def test_errors(self):
with self.assertRaises(TypeError):
Any[int] # Any is not a generic type.

def test_cannot_subclass(self):
with self.assertRaises(TypeError):
class A(Any):
pass
with self.assertRaises(TypeError):
class A(type(Any)):
pass
def test_can_subclass(self):
class X(Any): pass
X()
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def test_cannot_instantiate(self):
with self.assertRaises(TypeError):
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19 changes: 15 additions & 4 deletions Lib/typing.py
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Expand Up @@ -428,8 +428,15 @@ def __getitem__(self, parameters):
return self._getitem(self, *parameters)


@_SpecialForm
def Any(self, parameters):
class _AnyMeta(type):
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@JelleZijlstra JelleZijlstra Mar 13, 2022

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The metaclass is unfortunate because it restricts what classes can double-inherit from Any (due to metaclass conflicts). Seems unavoidable though.

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@hauntsaninja hauntsaninja Mar 13, 2022 •
edited
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Not entirely unavoidable, if we were willing to give up on instancecheck (and repr), I'd say we could just remove the metaclass entirely

def __instancecheck__(self, obj):
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@JelleZijlstra JelleZijlstra Mar 13, 2022

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Should we even have this? isinstance(X, Any) is now a meaningful operation.

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@hauntsaninja hauntsaninja Mar 13, 2022 •
edited
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I'm fine with removing it (as this PR currently does for issubclass). Doing so would also allow us to get rid of the metaclass, which will help remove restrictions on what classes can inherit from Any.

My reasoning for keeping it is that isinstance is very commonly used, potentially by typing / Python novices, and isinstance(..., Any) doesn't correspond well to the notion of Any at type check time. Sophisticated users have workarounds available to them for the equivalent isinstance check.

raise TypeError("Any cannot be used with isinstance()")

def __repr__(self):
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return "typing.Any"


class Any(metaclass=_AnyMeta):
"""Special type indicating an unconstrained type.

- Any is compatible with every type.
Expand All @@ -438,9 +445,13 @@ def Any(self, parameters):

Note that all the above statements are true from the point of view of
static type checkers. At runtime, Any should not be used with instance
or class checks.
checks.
"""
raise TypeError(f"{self} is not subscriptable")
def __new__(cls, *args, **kwargs):
if cls is Any:
raise TypeError("Any cannot be instantiated")
return object.__new__(cls, *args, **kwargs)
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@_SpecialForm
def NoReturn(self, parameters):
Expand Down