python
@@ -1598,6 +1598,15 @@ These are not used in annotations. They are building blocks for creating generic
       import threading
       assert isinstance(threading.Thread(name='Bob'), Named)
 
+   .. versionchanged:: 3.12
+      The internal implementation of :func:`isinstance` checks against
+      runtime-checkable protocols now uses :func:`inspect.getattr_static`
+      to look up attributes (previously, :func:`hasattr` was used).
+      As a result, some objects which used to be considered instances
+      of a runtime-checkable protocol may no longer be considered instances
+      of that protocol on Python 3.12+, and vice versa.
+      Most users are unlikely to be affected by this change.
+
    .. note::
 
         :func:`!runtime_checkable` will check only the presence of the required

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

@@ -2637,7 +2637,15 @@ def attr(self): ...
         class PG1(Protocol[T]):
             attr: T
 
-        for protocol_class in P, P1, PG, PG1:
+        @runtime_checkable
+        class MethodP(Protocol):
+            def attr(self): ...
+
+        @runtime_checkable
+        class MethodPG(Protocol[T]):
+            def attr(self) -> T: ...
+
+        for protocol_class in P, P1, PG, PG1, MethodP, MethodPG:
             for klass in C, D, E, F:
                 with self.subTest(
                     klass=klass.__name__,
@@ -2662,7 +2670,12 @@ def attr(self): ...
         class BadPG1(Protocol[T]):
             attr: T
 
-        for obj in PG[T], PG[C], PG1[T], PG1[C], BadP, BadP1, BadPG, BadPG1:
+        cases = (
+            PG[T], PG[C], PG1[T], PG1[C], MethodPG[T],
+            MethodPG[C], BadP, BadP1, BadPG, BadPG1
+        )
+
+        for obj in cases:
             for klass in C, D, E, F, Empty:
                 with self.subTest(klass=klass.__name__, obj=obj):
                     with self.assertRaises(TypeError):
@@ -2685,6 +2698,82 @@ def __dir__(self):
         self.assertIsInstance(CustomDirWithX(), HasX)
         self.assertNotIsInstance(CustomDirWithoutX(), HasX)
 
+    def test_protocols_isinstance_attribute_access_with_side_effects(self):
+        class C:
+            @property
+            def attr(self):
+                raise AttributeError('no')
+
+        class CustomDescriptor:
+            def __get__(self, obj, objtype=None):
+                raise RuntimeError("NO")
+
+        class D:
+            attr = CustomDescriptor()
+
+        # Check that properties set on superclasses
+        # are still found by the isinstance() logic
+        class E(C): ...
+        class F(D): ...
+
+        class WhyWouldYouDoThis:
+            def __getattr__(self, name):
+                raise RuntimeError("wut")
+
+        T = TypeVar('T')
+
+        @runtime_checkable
+        class P(Protocol):
+            @property
+            def attr(self): ...
+
+        @runtime_checkable
+        class P1(Protocol):
+            attr: int
+
+        @runtime_checkable
+        class PG(Protocol[T]):
+            @property
+            def attr(self): ...
+
+        @runtime_checkable
+        class PG1(Protocol[T]):
+            attr: T
+
+        @runtime_checkable
+        class MethodP(Protocol):
+            def attr(self): ...
+
+        @runtime_checkable
+        class MethodPG(Protocol[T]):
+            def attr(self) -> T: ...
+
+        for protocol_class in P, P1, PG, PG1, MethodP, MethodPG:
+            for klass in C, D, E, F:
+                with self.subTest(
+                    klass=klass.__name__,
+                    protocol_class=protocol_class.__name__
+                ):
+                    self.assertIsInstance(klass(), protocol_class)
+
+            with self.subTest(
+                klass="WhyWouldYouDoThis",
+                protocol_class=protocol_class.__name__
+            ):
+                self.assertNotIsInstance(WhyWouldYouDoThis(), protocol_class)
+
+    def test_protocols_isinstance___slots__(self):
+        # As per the consensus in https://github.com/python/typing/issues/1367,
+        # this is desirable behaviour
+        @runtime_checkable
+        class HasX(Protocol):
+            x: int
+
+        class HasNothingButSlots:
+            __slots__ = ("x",)
+
+        self.assertIsInstance(HasNothingButSlots(), HasX)
+
     def test_protocols_isinstance_py36(self):
         class APoint:
             def __init__(self, x, y, label):

@@ -1998,6 +1998,17 @@ def _allow_reckless_class_checks(depth=3):
 }
 
 
+@functools.cache
+def _lazy_load_getattr_static():
+    # Import getattr_static lazily so as not to slow down the import of typing.py
+    # Cache the result so we don't slow down _ProtocolMeta.__instancecheck__ unnecessarily
+    from inspect import getattr_static
+    return getattr_static
+
+
+_cleanups.append(_lazy_load_getattr_static.cache_clear)
+
+
 class _ProtocolMeta(ABCMeta):
     # This metaclass is really unfortunate and exists only because of
     # the lack of __instancehook__.
@@ -2025,12 +2036,17 @@ def __instancecheck__(cls, instance):
             return True
 
         if is_protocol_cls:
-            if all(hasattr(instance, attr) and
-                    # All *methods* can be blocked by setting them to None.
-                    (not callable(getattr(cls, attr, None)) or
-                     getattr(instance, attr) is not None)
-                    for attr in protocol_attrs):
+            getattr_static = _lazy_load_getattr_static()
+            for attr in protocol_attrs:
+                try:
+                    val = getattr_static(instance, attr)
+                except AttributeError:
+                    break
+                if callable(getattr(cls, attr, None)) and val is None:
+                    break
+            else:
                 return True
+
         return super().__instancecheck__(instance)
 
 

diff --git a/Misc/NEWS.d/next/Library/2023-03-25-16-57-18.gh-issue-102433.L-7x2Q.rst b/Misc/NEWS.d/next/Library/2023-03-25-16-57-18.gh-issue-102433.L-7x2Q.rst
@@ -0,0 +1,10 @@
+:func:`isinstance` checks against :func:`runtime-checkable protocols
+<typing.runtime_checkable>` now use :func:`inspect.getattr_static` rather
+than :func:`hasattr` to lookup whether attributes exist. This means that
+descriptors and :meth:`~object.__getattr__` methods are no longer
+unexpectedly evaluated during ``isinstance()`` checks against
+runtime-checkable protocols. However, it may also mean that some objects
+which used to be considered instances of a runtime-checkable protocol may no
+longer be considered instances of that protocol on Python 3.12+, and vice
+versa. Most users are unlikely to be affected by this change. Patch by Alex
+Waygood.