fix contextmanager decorator treating all context managers as if they can suppress exceptions when that's no longer the case since python 3.7

Author: DetachHead

packages/pyright-internal/src/tests/samples/withBased.py

@@ -1,10 +1,10 @@
-import contextlib
+from contextlib import AbstractContextManager, contextmanager
 from types import TracebackType
-from typing import Iterator, Literal
+from typing import Iterator, Literal, Iterator
 
 from typing_extensions import assert_never
 
-class BoolOrNone(contextlib.AbstractContextManager[None]):
+class BoolOrNone(AbstractContextManager[None]):
     def __exit__(
         self,
         __exc_type: type[BaseException] | None,
@@ -18,7 +18,7 @@ def _():
         raise Exception
     print(1)  # reachable
 
-class TrueOrNone(contextlib.AbstractContextManager[None]):
+class TrueOrNone(AbstractContextManager[None]):
     def __exit__(
         self,
         __exc_type: type[BaseException] | None,
@@ -33,7 +33,7 @@ def _():
     print(1)  # reachable
 
 
-class FalseOrNone(contextlib.AbstractContextManager[None]):
+class FalseOrNone(AbstractContextManager[None]):
     def __exit__(
         self,
         __exc_type: type[BaseException] | None,
@@ -48,7 +48,7 @@ def _():
     print(1)  # unreachable
 
 
-class OnlyNone(contextlib.AbstractContextManager[None]):
+class OnlyNone(AbstractContextManager[None]):
     def __exit__(
         self,
         __exc_type: type[BaseException] | None,
@@ -60,4 +60,15 @@ def __exit__(
 def _():
     with OnlyNone():
         raise Exception
-    print(1)  # unreachable
+    print(1)  # unreachable
+
+
+@contextmanager
+def foo() -> Iterator[None]: ...
+
+
+with foo():
+    a = 1
+
+# no reportPossiblyUnboundVariable since _GeneratorContextManager cannot suppress exceptions anymore as of python 3.7
+print(a)

packages/pyright-internal/typeshed-fallback/stdlib/contextlib.pyi

@@ -4,7 +4,7 @@ from _typeshed import FileDescriptorOrPath, Unused
 from abc import abstractmethod
 from collections.abc import AsyncGenerator, AsyncIterator, Awaitable, Callable, Generator, Iterator
 from types import TracebackType
-from typing import IO, Any, Generic, Protocol, TypeVar, overload, runtime_checkable
+from typing import IO, Any, Generic, Literal, Protocol, TypeVar, overload, runtime_checkable
 from typing_extensions import ParamSpec, Self, TypeAlias
 
 __all__ = [
@@ -67,8 +67,23 @@ class _GeneratorContextManager(AbstractContextManager[_T_co], ContextDecorator):
     if sys.version_info >= (3, 9):
         def __exit__(
             self, typ: type[BaseException] | None, value: BaseException | None, traceback: TracebackType | None
-        ) -> bool | None: ...
+        ) -> Literal[False]: ...
+    elif sys.version_info >= (3, 7):
+        def __exit__(
+            self, type: type[BaseException] | None, value: BaseException | None, traceback: TracebackType | None
+        ) -> Literal[False]: ...
     else:
+        # python 3.7 fixes an issue where generators could incorrectly suppress StopIteration exceptions
+        # (see https://peps.python.org/pep-0479/). while it's still technically possible to craft a generator that
+        # does, it's an extreme edge case that you need to go out of your way to cause. so we change the return type
+        # to `Literal[False]` to prevent it from assuming every context manager can suppress exceptions. the only
+        # reason this isn't an issue in upstream pyright/mypy is because they both "fix" this problem in the
+        # stupidest way imaginable, by special-casing `bool | None` to mean `Literal[False]` instead.
+
+        # the tradeoff with my fix is that python <=3.7 users will experience plenty of annoying errors caused by
+        # basedpyright being overly strict and assuming variables set inside context managers can be unbound in case
+        # the exception gets suppressed. i could do some special casing to address this, but python 3.6 is deprecated
+        # anyway and you should just update to a supported version of python instead.
         def __exit__(
             self, type: type[BaseException] | None, value: BaseException | None, traceback: TracebackType | None
         ) -> bool | None: ...