Introduce Mapping-like protocols

typing.Mapping is not a protocol, which has caused problems in the past.
(E.g. python#3569, see also python#3576.) This
introduces a few narrow protocols to _typeshed.pyi that can be used in
place of Mapping.

Not all uses of Mapping can be replaced. For example, cgi.FieldStorage
explictly checks whether the supplied headers argument is a Mapping
instance.

stdlib/2and3/_typeshed/__init__.pyi

@@ -15,12 +15,27 @@
 import array
 import mmap
 import sys
-from typing import Protocol, Text, TypeVar, Union
+from typing import AbstractSet, Container, Protocol, Text, Tuple, TypeVar, Union
 from typing_extensions import Literal
 
+_KT = TypeVar("_KT")
+_VT = TypeVar("_VT")
+_VT_co = TypeVar("_VT_co", covariant=True)
 _T_co = TypeVar("_T_co", covariant=True)
 _T_contra = TypeVar("_T_contra", contravariant=True)
 
+# Mapping-like protocols
+
+class SupportsItems(Protocol[_KT, _VT_co]):
+    def items(self) -> AbstractSet[Tuple[_KT, _VT_co]]: ...
+
+class SupportsGetItem(Container[_KT], Protocol[_KT, _VT_co]):
+    def __getitem__(self, __k: _KT) -> _VT_co: ...
+
+class SupportsItemAccess(SupportsGetItem[_KT, _VT], Protocol[_KT, _VT]):
+    def __setitem__(self, __k: _KT, __v: _VT) -> None: ...
+    def __delitem__(self, __v: _KT) -> None: ...
+
 # StrPath and AnyPath can be used in places where a
 # path can be used instead of a string, starting with Python 3.6.
 if sys.version_info >= (3, 6):

stdlib/2and3/cgi.pyi

@@ -1,10 +1,14 @@
 import sys
-from typing import IO, Any, AnyStr, Dict, Iterator, List, Mapping, Optional, Tuple, TypeVar, Union
+from _typeshed import SupportsGetItem, SupportsItemAccess
+from typing import IO, Any, AnyStr, Dict, Iterable, Iterator, List, Mapping, Optional, Protocol, Tuple, TypeVar, Union
 
 _T = TypeVar("_T", bound=FieldStorage)
 
 def parse(
-    fp: Optional[IO[Any]] = ..., environ: Mapping[str, str] = ..., keep_blank_values: bool = ..., strict_parsing: bool = ...
+    fp: Optional[IO[Any]] = ...,
+    environ: SupportsItemAccess[str, str] = ...,
+    keep_blank_values: bool = ...,
+    strict_parsing: bool = ...,
 ) -> Dict[str, List[str]]: ...
 
 if sys.version_info < (3, 8):
@@ -13,15 +17,19 @@ if sys.version_info < (3, 8):
 
 if sys.version_info >= (3, 7):
     def parse_multipart(
-        fp: IO[Any], pdict: Mapping[str, bytes], encoding: str = ..., errors: str = ...
+        fp: IO[Any], pdict: SupportsGetItem[str, bytes], encoding: str = ..., errors: str = ...
     ) -> Dict[str, List[Any]]: ...
 
 else:
-    def parse_multipart(fp: IO[Any], pdict: Mapping[str, bytes]) -> Dict[str, List[bytes]]: ...
+    def parse_multipart(fp: IO[Any], pdict: SupportsGetItem[str, bytes]) -> Dict[str, List[bytes]]: ...
+
+class _Environ(Protocol):
+    def __getitem__(self, __k: str) -> str: ...
+    def keys(self) -> Iterable[str]: ...
 
 def parse_header(line: str) -> Tuple[str, Dict[str, str]]: ...
-def test(environ: Mapping[str, str] = ...) -> None: ...
-def print_environ(environ: Mapping[str, str] = ...) -> None: ...
+def test(environ: _Environ = ...) -> None: ...
+def print_environ(environ: _Environ = ...) -> None: ...
 def print_form(form: Dict[str, Any]) -> None: ...
 def print_directory() -> None: ...
 def print_environ_usage() -> None: ...
@@ -77,7 +85,7 @@ class FieldStorage(object):
             fp: Optional[IO[Any]] = ...,
             headers: Optional[Mapping[str, str]] = ...,
             outerboundary: bytes = ...,
-            environ: Mapping[str, str] = ...,
+            environ: SupportsGetItem[str, str] = ...,
             keep_blank_values: int = ...,
             strict_parsing: int = ...,
             limit: Optional[int] = ...,
@@ -91,7 +99,7 @@ class FieldStorage(object):
             fp: Optional[IO[Any]] = ...,
             headers: Optional[Mapping[str, str]] = ...,
             outerboundary: bytes = ...,
-            environ: Mapping[str, str] = ...,
+            environ: SupportsGetItem[str, str] = ...,
             keep_blank_values: int = ...,
             strict_parsing: int = ...,
             limit: Optional[int] = ...,
@@ -104,7 +112,7 @@ class FieldStorage(object):
             fp: IO[Any] = ...,
             headers: Mapping[str, str] = ...,
             outerboundary: bytes = ...,
-            environ: Mapping[str, str] = ...,
+            environ: SupportsGetItem[str, str] = ...,
             keep_blank_values: int = ...,
             strict_parsing: int = ...,
         ) -> None: ...

stdlib/3.9/graphlib.pyi

@@ -1,9 +1,10 @@
-from typing import Generic, Iterable, Mapping, Optional, Tuple, TypeVar
+from _typeshed import SupportsItems
+from typing import Generic, Iterable, Optional, Tuple, TypeVar
 
 _T = TypeVar("_T")
 
 class TopologicalSorter(Generic[_T]):
-    def __init__(self, graph: Optional[Mapping[_T, Iterable[_T]]] = ...) -> None: ...
+    def __init__(self, graph: Optional[SupportsItems[_T, Iterable[_T]]] = ...) -> None: ...
     def add(self, node: _T, *predecessors: _T) -> None: ...
     def prepare(self) -> None: ...
     def is_active(self) -> bool: ...