Loosen some types to avoid pain in real-life situations (#4870)

In this diff:
* Loosen `set.__[i]sub__()` to allow typical use cases (that work at runtime). Namely, allow removing `unicode` from a set of `str`, and allow removing optional values from non-optional sets.
* Avoid using union return types in `cryptography` deserialization functions.
* Tune `SupportsItems` so that `dict` implements it on Python 2.

Co-authored-by: Ivan Levkivskyi <ilevkivskyi@dropbox.com>

stdlib/2/__builtin__.pyi

@@ -737,8 +737,14 @@ class set(MutableSet[_T], Generic[_T]):
     def __iand__(self, s: AbstractSet[object]) -> Set[_T]: ...
     def __or__(self, s: AbstractSet[_S]) -> Set[Union[_T, _S]]: ...
     def __ior__(self, s: AbstractSet[_S]) -> Set[Union[_T, _S]]: ...
-    def __sub__(self, s: AbstractSet[_T]) -> Set[_T]: ...
-    def __isub__(self, s: AbstractSet[_T]) -> Set[_T]: ...
+    @overload
+    def __sub__(self: Set[str], s: AbstractSet[Optional[Text]]) -> Set[_T]: ...
+    @overload
+    def __sub__(self, s: AbstractSet[Optional[_T]]) -> Set[_T]: ...
+    @overload  # type: ignore
+    def __isub__(self: Set[str], s: AbstractSet[Optional[Text]]) -> Set[_T]: ...
+    @overload
+    def __isub__(self, s: AbstractSet[Optional[_T]]) -> Set[_T]: ...
     def __xor__(self, s: AbstractSet[_S]) -> Set[Union[_T, _S]]: ...
     def __ixor__(self, s: AbstractSet[_S]) -> Set[Union[_T, _S]]: ...
     def __le__(self, s: AbstractSet[object]) -> bool: ...

stdlib/2/builtins.pyi

@@ -737,8 +737,14 @@ class set(MutableSet[_T], Generic[_T]):
     def __iand__(self, s: AbstractSet[object]) -> Set[_T]: ...
     def __or__(self, s: AbstractSet[_S]) -> Set[Union[_T, _S]]: ...
     def __ior__(self, s: AbstractSet[_S]) -> Set[Union[_T, _S]]: ...
-    def __sub__(self, s: AbstractSet[_T]) -> Set[_T]: ...
-    def __isub__(self, s: AbstractSet[_T]) -> Set[_T]: ...
+    @overload
+    def __sub__(self: Set[str], s: AbstractSet[Optional[Text]]) -> Set[_T]: ...
+    @overload
+    def __sub__(self, s: AbstractSet[Optional[_T]]) -> Set[_T]: ...
+    @overload  # type: ignore
+    def __isub__(self: Set[str], s: AbstractSet[Optional[Text]]) -> Set[_T]: ...
+    @overload
+    def __isub__(self, s: AbstractSet[Optional[_T]]) -> Set[_T]: ...
     def __xor__(self, s: AbstractSet[_S]) -> Set[Union[_T, _S]]: ...
     def __ixor__(self, s: AbstractSet[_S]) -> Set[Union[_T, _S]]: ...
     def __le__(self, s: AbstractSet[object]) -> bool: ...

stdlib/2and3/_typeshed/__init__.pyi

@@ -34,7 +34,11 @@ SupportsLessThanT = TypeVar("SupportsLessThanT", bound=SupportsLessThan)  # noqa
 # Mapping-like protocols
 
 class SupportsItems(Protocol[_KT_co, _VT_co]):
-    def items(self) -> AbstractSet[Tuple[_KT_co, _VT_co]]: ...
+    if sys.version_info >= (3,):
+        def items(self) -> AbstractSet[Tuple[_KT_co, _VT_co]]: ...
+    else:
+        # We want dictionaries to support this on Python 2.
+        def items(self) -> Iterable[Tuple[_KT_co, _VT_co]]: ...
 
 class SupportsKeysAndGetItem(Protocol[_KT, _VT_co]):
     def keys(self) -> Iterable[_KT]: ...

stdlib/3/builtins.pyi

@@ -807,8 +807,8 @@ class set(MutableSet[_T], Generic[_T]):
     def __iand__(self, s: AbstractSet[object]) -> Set[_T]: ...
     def __or__(self, s: AbstractSet[_S]) -> Set[Union[_T, _S]]: ...
     def __ior__(self, s: AbstractSet[_S]) -> Set[Union[_T, _S]]: ...
-    def __sub__(self, s: AbstractSet[_T]) -> Set[_T]: ...
-    def __isub__(self, s: AbstractSet[_T]) -> Set[_T]: ...
+    def __sub__(self, s: AbstractSet[Optional[_T]]) -> Set[_T]: ...
+    def __isub__(self, s: AbstractSet[Optional[_T]]) -> Set[_T]: ...
     def __xor__(self, s: AbstractSet[_S]) -> Set[Union[_T, _S]]: ...
     def __ixor__(self, s: AbstractSet[_S]) -> Set[Union[_T, _S]]: ...
     def __le__(self, s: AbstractSet[object]) -> bool: ...

third_party/2and3/cryptography/hazmat/primitives/serialization/__init__.pyi

@@ -1,6 +1,6 @@
 from abc import ABCMeta
 from enum import Enum
-from typing import Optional, Union
+from typing import Any, Optional, Union
 
 from cryptography.hazmat.backends.interfaces import (
     DERSerializationBackend,
@@ -17,19 +17,19 @@ from cryptography.hazmat.primitives.asymmetric.rsa import RSAPrivateKey, RSAPubl
 
 def load_pem_private_key(
     data: bytes, password: Optional[bytes], backend: Optional[PEMSerializationBackend] = ...
-) -> Union[RSAPrivateKey, DSAPrivateKey, DHPrivateKey, EllipticCurvePrivateKey]: ...
+) -> Any: ...  # actually Union[RSAPrivateKey, DSAPrivateKey, DHPrivateKey, EllipticCurvePrivateKey]
 def load_pem_public_key(
     data: bytes, backend: Optional[PEMSerializationBackend] = ...
-) -> Union[RSAPublicKey, DSAPublicKey, DHPublicKey, EllipticCurvePublicKey]: ...
+) -> Any: ...  # actually Union[RSAPublicKey, DSAPublicKey, DHPublicKey, EllipticCurvePublicKey]
 def load_der_private_key(
     data: bytes, password: Optional[bytes], backend: Optional[DERSerializationBackend] = ...
-) -> Union[RSAPrivateKey, DSAPrivateKey, DHPrivateKey, EllipticCurvePrivateKey]: ...
+) -> Any: ...  # actually Union[RSAPrivateKey, DSAPrivateKey, DHPrivateKey, EllipticCurvePrivateKey]
 def load_der_public_key(
     data: bytes, backend: Optional[DERSerializationBackend] = ...
-) -> Union[RSAPublicKey, DSAPublicKey, DHPublicKey, EllipticCurvePublicKey]: ...
+) -> Any: ...  # actually Union[RSAPublicKey, DSAPublicKey, DHPublicKey, EllipticCurvePublicKey]
 def load_ssh_public_key(
     data: bytes, backend: Union[RSABackend, DSABackend, EllipticCurveBackend, None]
-) -> Union[RSAPublicKey, DSAPublicKey, DHPublicKey, EllipticCurvePublicKey, Ed25519PublicKey]: ...
+) -> Any: ...  # actually Union[RSAPublicKey, DSAPublicKey, DHPublicKey, EllipticCurvePublicKey, Ed25519PublicKey]
 
 class Encoding(Enum):
     PEM: str