protobuf: Bring back Message.Extensions (#7154)

Author: nipunn1313

stubs/protobuf/@tests/stubtest_allowlist.txt

@@ -2,14 +2,24 @@
 # carefully in mypy-protobuf which internally runs stubtest. Skip those here.
 google.protobuf\..*_pb2\..*
 
-google.protobuf.internal.containers.BaseContainer.sort
 # While Message and Descriptor are both defined with a null DESCRIPTOR,
 # subclasses of Message and instances of EnumTypeWrapper require this value to
 # be set, and since these type stubs are intended for use with protoc-generated
 # python it's more accurate to make them non-nullable.
 google.protobuf.internal.enum_type_wrapper.EnumTypeWrapper.DESCRIPTOR
 google.protobuf.message.Message.DESCRIPTOR
+
+# Exists at runtime, but via a __getitem__/__setitem__ hack
+# See https://github.com/protocolbuffers/protobuf/blob/3ea30d80847cd9561db570ae7f673afc15523545/python/google/protobuf/message.py#L67
+google.protobuf.message.Message.Extensions
+
+# These are typed as (self, **kwargs) at runtime as thin wrapper functions
+# around the underlying true typing. We prefer the true typing
+google.protobuf.internal.containers.BaseContainer.sort
 google.protobuf.message.Message.SerializePartialToString
 google.protobuf.message.Message.SerializeToString
-google.protobuf.service.Service.GetDescriptor
 google.protobuf.text_format.MessageToBytes
+
+# Stubbed as static method, but actually exists as a property that's
+# a function. Typeshed's typing is more useful
+google.protobuf.service.Service.GetDescriptor

stubs/protobuf/google/protobuf/message.pyi

@@ -2,7 +2,7 @@ from _typeshed import Self
 from typing import Any, Sequence, TypeVar
 
 from .descriptor import Descriptor, FieldDescriptor
-from .internal.extension_dict import _ExtensionFieldDescriptor
+from .internal.extension_dict import _ExtensionDict, _ExtensionFieldDescriptor
 
 class Error(Exception): ...
 class DecodeError(Error): ...
@@ -28,6 +28,9 @@ class Message:
     # The TypeVar must be bound to `Message` or we get mypy errors, so we cannot use `Self` for `HasExtension` & `ClearExtension`
     def HasExtension(self: _M, extension_handle: _ExtensionFieldDescriptor[_M, Any]) -> bool: ...
     def ClearExtension(self: _M, extension_handle: _ExtensionFieldDescriptor[_M, Any]) -> None: ...
+    # The TypeVar must be bound to `Message` or we get mypy errors, so we cannot use `Self` for `Extensions`
+    @property
+    def Extensions(self: _M) -> _ExtensionDict[_M]: ...
     def ByteSize(self) -> int: ...
     @classmethod
     def FromString(cls: type[Self], s: bytes) -> Self: ...