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gh-105237: Allow calling `issubclass(X, typing.Protocol)` again by AlexWaygood · Pull Request #105239 · python/cpython · GitHub
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gh-105237: Allow calling issubclass(X, typing.Protocol) again #105239

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Merged
JelleZijlstra merged 6 commits into from
Jun 5, 2023
Merged

gh-105237: Allow calling issubclass(X, typing.Protocol) again #105239

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a6b11b0
Allow calling `issubclass(X, Protocol)`
AlexWaygood Jun 2, 2023
36b2ac9
Add back some implementation details so we don't unnecessarily break …
AlexWaygood Jun 2, 2023
db51c8d
Revert unnecessary changes to tests
AlexWaygood Jun 2, 2023
776bf2b
Fix longstanding bug and add many tests
AlexWaygood Jun 2, 2023
ffb7f1c
Small micro-optimisation
AlexWaygood Jun 2, 2023
b85a320
Cleaner solution
AlexWaygood Jun 2, 2023
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59 changes: 59 additions & 0 deletions Lib/test/test_typing.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2758,6 +2758,65 @@ def x(self): ...
with self.assertRaisesRegex(TypeError, only_classes_allowed):
issubclass(1, BadPG)

def test_issubclass_and_isinstance_on_Protocol_itself(self):
class C:
def x(self): pass

self.assertNotIsSubclass(object, Protocol)
self.assertNotIsInstance(object(), Protocol)

self.assertNotIsSubclass(str, Protocol)
self.assertNotIsInstance('foo', Protocol)

self.assertNotIsSubclass(C, Protocol)
self.assertNotIsInstance(C(), Protocol)

only_classes_allowed = r"issubclass\(\) arg 1 must be a class"

with self.assertRaisesRegex(TypeError, only_classes_allowed):
issubclass(1, Protocol)
with self.assertRaisesRegex(TypeError, only_classes_allowed):
issubclass('foo', Protocol)
with self.assertRaisesRegex(TypeError, only_classes_allowed):
issubclass(C(), Protocol)

T = TypeVar('T')

@runtime_checkable
class EmptyProtocol(Protocol): pass

@runtime_checkable
class SupportsStartsWith(Protocol):
def startswith(self, x: str) -> bool: ...

@runtime_checkable
class SupportsX(Protocol[T]):
def x(self): ...

for proto in EmptyProtocol, SupportsStartsWith, SupportsX:
with self.subTest(proto=proto.__name__):
self.assertIsSubclass(proto, Protocol)

# gh-105237 / PR #105239:
# check that the presence of Protocol subclasses
# where `issubclass(X, <subclass>)` evaluates to True
# doesn't influence the result of `issubclass(X, Protocol)`

self.assertIsSubclass(object, EmptyProtocol)
self.assertIsInstance(object(), EmptyProtocol)
self.assertNotIsSubclass(object, Protocol)
self.assertNotIsInstance(object(), Protocol)

self.assertIsSubclass(str, SupportsStartsWith)
self.assertIsInstance('foo', SupportsStartsWith)
self.assertNotIsSubclass(str, Protocol)
self.assertNotIsInstance('foo', Protocol)

self.assertIsSubclass(C, SupportsX)
self.assertIsInstance(C(), SupportsX)
self.assertNotIsSubclass(C, Protocol)
self.assertNotIsInstance(C(), Protocol)

def test_protocols_issubclass_non_callable(self):
class C:
x = 1
Expand Down
6 changes: 4 additions & 2 deletions Lib/typing.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1782,6 +1782,8 @@ def __init__(cls, *args, **kwargs):
)

def __subclasscheck__(cls, other):
if cls is Protocol:
return type.__subclasscheck__(cls, other)
if not isinstance(other, type):
# Same error message as for issubclass(1, int).
raise TypeError('issubclass() arg 1 must be a class')
Expand Down Expand Up @@ -1862,7 +1864,7 @@ def meth(self) -> T:
...
"""
__slots__ = ()
_is_protocol = True
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_is_protocol = False
_is_runtime_protocol = False

def __init_subclass__(cls, *args, **kwargs):
Expand Down Expand Up @@ -1904,7 +1906,7 @@ def _proto_hook(other):

# ... otherwise check consistency of bases, and prohibit instantiation.
for base in cls.__bases__:
if not (base in (object, Generic) or
if not (base in {object, Generic, Protocol} or
base.__module__ in _PROTO_ALLOWLIST and
base.__name__ in _PROTO_ALLOWLIST[base.__module__] or
issubclass(base, Generic) and getattr(base, '_is_protocol', False)):
Expand Down
2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2023-06-02-14-57-11.gh-issue-105239.SAmuuj.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Fix longstanding bug where ``issubclass(object, typing.Protocol)`` would
evaluate to ``True`` in some edge cases. Patch by Alex Waygood.