8000 gh-102433: Use `inspect.getattr_static` in `typing._ProtocolMeta.__in… · python/cpython@6d59c9e · GitHub
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gh-102433: Use inspect.getattr_static in typing._ProtocolMeta.__instancecheck__ (#103034)
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‎Doc/library/typing.rst

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@@ -1598,6 +1598,15 @@ These are not used in annotations. They are building blocks for creating generic
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import threading
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assert isinstance(threading.Thread(name='Bob'), Named)
16001600

1601+
.. versionchanged:: 3.12
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The internal implementation of :func:`isinstance` checks against
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runtime-checkable protocols now uses :func:`inspect.getattr_static`
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to look up attributes (previously, :func:`hasattr` was used).
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As a result, some objects which used to be considered instances
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of a runtime-checkable protocol may no longer be considered instances
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of that protocol on Python 3.12+, and vice versa.
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Most users are unlikely to be affected by this change.
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.. note::
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:func:`!runtime_checkable` will check only the presence of the required

‎Doc/whatsnew/3.12.rst

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@@ -391,6 +391,17 @@ typing
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same name on a base class, as per :pep:`698`. (Contributed by Steven Troxler in
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:gh:`101564`.)
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* :func:`isinstance` checks against
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:func:`runtime-checkable protocols <typing.runtime_checkable>` now use
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:func:`inspect.getattr_static` rather than :func:`hasattr` to lookup whether
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attributes exist. This means that descriptors and :meth:`~object.__getattr__`
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methods are no longer unexpectedly evaluated during ``isinstance()`` checks
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against runtime-checkable protocols. However, it may also mean that some
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objects which used to be considered instances of a runtime-checkable protocol
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may no longer be considered instances of that protocol on Python 3.12+, and
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vice versa. Most users are unlikely to be affected by this change.
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(Contributed by Alex Waygood in :gh:`102433`.)
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sys
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---
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‎Lib/test/test_typing.py

Lines changed: 91 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -2637,7 +2637,15 @@ def attr(self): ...
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class PG1(Protocol[T]):
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attr: T
26392639

2640-
for protocol_class in P, P1, PG, PG1:
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@runtime_checkable
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class MethodP(Protocol):
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def attr(self): ...
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@runtime_checkable
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class MethodPG(Protocol[T]):
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def attr(self) -> T: ...
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for protocol_class in P, P1, PG, PG1, MethodP, MethodPG:
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for klass in C, D, E, F:
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with self.subTest(
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klass=klass.__name__,
@@ -2662,7 +2670,12 @@ def attr(self): ...
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class BadPG1(Protocol[T]):
26632671
attr: T
26642672

2665-
for obj in PG[T], PG[C], PG1[T], PG1[C], BadP, BadP1, BadPG, BadPG1:
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cases = (
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PG[T], PG[C], PG1[T], PG1[C], MethodPG[T],
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MethodPG[C], BadP, BadP1, BadPG, BadPG1
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)
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for obj in cases:
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for klass in C, D, E, F, Empty:
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with self.subTest(klass=klass.__name__, obj=obj):
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with self.assertRaises(TypeError):
@@ -2685,6 +2698,82 @@ def __dir__(self):
26852698
self.assertIsInstance(CustomDirWithX(), HasX)
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self.assertNotIsInstance(CustomDirWithoutX(), HasX)
26872700

2701+
def test_protocols_isinstance_attribute_access_with_side_effects(self):
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class C:
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@property
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def attr(self):
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raise AttributeError('no')
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class CustomDescriptor:
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def __get__(self, obj, objtype=None):
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raise RuntimeError("NO")
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class D:
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attr = CustomDescriptor()
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# Check that properties set on superclasses
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# are still found by the isinstance() logic
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class E(C): ...
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class F(D): ...
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class WhyWouldYouDoThis:
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def __getattr__(self, name):
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raise RuntimeError("wut")
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2723+
T = TypeVar('T')
2724+
2725+
@runtime_checkable
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class P(Protocol):
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@property
2728+
def attr(self): ...
2729+
2730+
@runtime_checkable
2731+
class P1(Protocol):
2732+
attr: int
2733+
2734+
@runtime_checkable
2735+
class PG(Protocol[T]):
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@property
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def attr(self): ...
2738+
2739+
@runtime_checkable
2740+
class PG1(Protocol[T]):
2741+
attr: T
2742+
2743+
@runtime_checkable
2744+
class MethodP(Protocol):
2745+
def attr(self): ...
2746+
2747+
@runtime_checkable
2748+
class MethodPG(Protocol[T]):
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def attr(self) -> T: ...
2750+
2751+
for protocol_class in P, P1, PG, PG1, MethodP, MethodPG:
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for klass in C, D, E, F:
2753+
with self.subTest(
2754+
klass=klass.__name__,
2755+
protocol_class=protocol_class.__name__
2756+
):
2757+
self.assertIsInstance(klass(), protocol_class)
2758+
2759+
with self.subTest(
2760+
klass="WhyWouldYouDoThis",
2761+
protocol_class=protocol_class.__name__
2762+
):
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self.assertNotIsInstance(WhyWouldYouDoThis(), protocol_class)
2764+
2765+
def test_protocols_isinstance___slots__(self):
2766+
# As per the consensus in https://github.com/python/typing/issues/1367,
2767+
# this is desirable behaviour
2768+
@runtime_checkable
2769+
class HasX(Protocol):
2770+
x: int
2771+
2772+
class HasNothingButSlots:
2773+
__slots__ = ("x",)
2774+
2775+
self.assertIsInstance(HasNothingButSlots(), HasX)
2776+
26882777
def test_protocols_isinstance_py36(self):
26892778
class APoint:
26902779
def __init__(self, x, y, label):

‎Lib/typing.py

Lines changed: 21 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -1998,6 +1998,17 @@ def _allow_reckless_class_checks(depth=3):
19981998
}
19991999

20002000

2001+
@functools.cache
2002+
def _lazy_load_getattr_static():
2003+
# Import getattr_static lazily so as not to slow down the import of typing.py
2004+
# Cache the result so we don't slow down _ProtocolMeta.__instancecheck__ unnecessarily
2005+
from inspect import getattr_static
2006+
return getattr_static
2007+
2008+
2009+
_cleanups.append(_lazy_load_getattr_static.cache_clear)
2010+
2011+
20012012
class _ProtocolMeta(ABCMeta):
20022013
# This metaclass is really unfortunate and exists only because of
20032014
# the lack of __instancehook__.
@@ -2025,12 +2036,17 @@ def __instancecheck__(cls, instance):
20252036
return True
20262037

20272038
if is_protocol_cls:
2028-
if all(hasattr(instance, attr) and
2029-
# All *methods* can be blocked by setting them to None.
2030-
(not callable(getattr(cls, attr, None)) or
2031-
getattr(instance, attr) is not None)
2032-
for attr in protocol_attrs):
2039+
getattr_static = _lazy_load_getattr_static()
2040+
for attr in protocol_attrs:
2041+
try:
2042+
val = getattr_static(instance, attr)
2043+
except AttributeError:
2044+
break
2045+
if callable(getattr(cls, attr, None)) and val is None:
2046+
break
2047+
else:
20332048
return True
2049+
20342050
return super().__instancecheck__(instance)
20352051

20362052

‎Misc/NEWS.d/next/Library/2023-03-25-16-57-18.gh-issue-102433.L-7x2Q.rst

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Original file line numberDiff line numberDiff line change
@@ -0,0 +1,10 @@
1+
:func:`isinstance` checks against :func:`runtime-checkable protocols
2+
<typing.runtime_checkable>` now use :func:`inspect.getattr_static` rather
3+
than :func:`hasattr` to lookup whether attributes exist. This means that
4+
descriptors and :meth:`~object.__getattr__` methods are no longer
5+
unexpectedly evaluated during ``isinstance()`` checks against
6+
runtime-checkable protocols. However, it may also mean that some objects
7+
which used to be considered instances of a runtime-checkable protocol may no
8+
longer be considered instances of that protocol on Python 3.12+, and vice
9+
versa. Most users are unlikely to be affected by this change. Patch by Alex
10+
Waygood.

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