adding a _typing module

this is a place that *must* only be imported inside a if
typing.TYPE_CHECKING block. It is a mixture of smoothing over thinigs
that moved from typing_extensions to typing in later python versions and
useful but typing only code.

This makes typing_extensions required for python versions older than
3.8 *when running mypy*. typing_extensions should *only* be imported
inside an `if typing.TYPE_CHECKING` block (include the new _typing.py
module) to ensure that it doesn't become a runtime dependency

mesonbuild/_typing.py

@@ -0,0 +1,114 @@
+# SPDX-License-Identifer: Apache-2.0
+# Copyright 2020 The Meson development team
+# Copyright © 2020-2021 Intel Corporation
+
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+
+#     http://www.apache.org/licenses/LICENSE-2.0
+
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Meson specific typing helpers.
+
+Holds typing helper classes, such as the ImmutableProtocol classes
+"""
+
+__all__ = [
+    'Protocol',
+    'ImmutableListProtocol'
+]
+
+import typing
+
+# We can change this to typing when we require python 3.8
+from typing_extensions import Protocol
+
+
+T = typing.TypeVar('T')
+
+
+class ImmutableListProtocol(Protocol[T]):
+
+    """A protocol used in cases where a list is returned, but should not be
+    mutated.
+
+    This provides all of the methods of a Sequence, as well as copy(). copy()
+    returns a list, which allows mutation as it's a copy and that's (hopefully)
+    safe.
+
+    One particular case this is important is for cached values, since python is
+    a pass-by-reference language.
+    """
+
+    def __iter__(self) -> typing.Iterator[T]: ...
+
+    @typing.overload
+    def __getitem__(self, index: int) -> T:...
+    @typing.overload
+    def __getitem__(self, index: slice) -> typing.List[T]: ...
+
+    def __contains__(self, item: T) -> bool: ...
+
+    def __reversed__(self) -> typing.Iterator[T]: ...
+
+    def __len__(self) -> int: ...
+
+    def __add__(self, other: typing.List[T]) -> typing.List[T]: ...
+
+    def __eq__(self, other: typing.Any) -> bool: ...
+    def __ne__(self, other: typing.Any) -> bool: ...
+    def __le__(self, other: typing.Any) -> bool: ...
+    def __lt__(self, other: typing.Any) -> bool: ...
+    def __gt__(self, other: typing.Any) -> bool: ...
+    def __ge__(self, other: typing.Any) -> bool: ...
+
+    def count(self, item: T) -> int: ...
+
+    def index(self, item: T) -> int: ...
+
+    def copy(self) -> typing.List[T]: ...
+
+
+class ImmutableSetProtocol(Protocol[T]):
+
+    """A protocol for a set that cannot be mutated.
+
+    This provides for cases where mutation of the set is undesired. Although
+    this will be allowed at runtime, mypy (or another type checker), will see
+    any attempt to use mutative methods as an error.
+    """
+
+    def __iter__(self) -> typing.Iterator[T]: ...
+
+    def __contains__(self, item: T) -> bool: ...
+
+    def __len__(self) -> int: ...
+
+    def __add__(self, other: typing.Set[T]) -> typing.Set[T]: ...
+
+    def __eq__(self, other: typing.Any) -> bool: ...
+    def __ne__(self, other: typing.Any) -> bool: ...
+    def __le__(self, other: typing.Any) -> bool: ...
+    def __lt__(self, other: typing.Any) -> bool: ...
+    def __gt__(self, other: typing.Any) -> bool: ...
+    def __ge__(self, other: typing.Any) -> bool: ...
+
+    def copy(self) -> typing.Set[T]: ...
+
+    def difference(self, other: typing.Set[T]) -> typing.Set[T]: ...
+
+    def intersection(self, other: typing.Set[T]) -> typing.Set[T]: ...
+
+    def issubset(self, other: typing.Set[T]) -> bool: ...
+
+    def issuperset(self, other: typing.Set[T]) -> bool: ...
+
+    def symmetric_difference(self, other: typing.Set[T]) -> typing.Set[T]: ...
+
+    def union(self, other: typing.Set[T]) -> typing.Set[T]: ...

setup.cfg

@@ -43,6 +43,9 @@ console_scripts =
 [options.extras_require]
 progress =
   tqdm
+typing =
+  mypy
+  typing_extensions; python_version <"3.8"
 
 [options.packages.find]
 include = mesonbuild, mesonbuild.*