Remove _Val alias for MultiValueDict so that generic evaluate (#36)

* remove _Val alias for MultiValueDict so that generic evaluate

* fix multivaluedict init argument

django-stubs/utils/datastructures.pyi

@@ -12,8 +12,11 @@ from typing import (
     Union,
     overload,
     Iterator,
+    Optional,
 )
 
+from typing_extensions import Literal
+
 _K = TypeVar("_K")
 _V = TypeVar("_V")
 
@@ -27,24 +30,22 @@ class OrderedSet(MutableSet[_K]):
 
 class MultiValueDictKeyError(KeyError): ...
 
-_Val = Union[_V, List[_V]]
-
 class MultiValueDict(MutableMapping[_K, _V]):
     @overload
-    def __init__(self, key_to_list_mapping: Iterable[Tuple[_K, _Val]] = ...) -> None: ...
+    def __init__(self, key_to_list_mapping: Mapping[_K, Optional[List[_V]]] = ...) -> None: ...
     @overload
-    def __init__(self, key_to_list_mapping: Mapping[_K, _Val] = ...) -> None: ...
+    def __init__(self, key_to_list_mapping: Iterable[Tuple[_K, List[_V]]] = ...) -> None: ...
     def getlist(self, key: _K, default: List[_V] = None) -> List[_V]: ...
     def setlist(self, key: _K, list_: List[_V]) -> None: ...
     def setlistdefault(self, key: _K, default_list: List[_V] = None) -> List[_V]: ...
     def appendlist(self, key: _K, value: _V) -> None: ...
     def lists(self) -> Iterable[Tuple[_K, List[_V]]]: ...
-    def dict(self) -> Dict[_K, _Val]: ...
+    def dict(self) -> Dict[_K, Union[_V, List[_V]]]: ...
     def copy(self) -> MultiValueDict[_K, _V]: ...
     # These overrides are needed to convince mypy that this isn't an abstract class
     def __delitem__(self, item: _K) -> None: ...
-    def __getitem__(self, item: _K) -> _Val: ...  # type: ignore
-    def __setitem__(self, k: _K, v: _Val) -> None: ...
+    def __getitem__(self, item: _K) -> Union[_V, Literal[[]]]: ...  # type: ignore
+    def __setitem__(self, k: _K, v: Union[_V, List[_V]]) -> None: ...
     def __len__(self) -> int: ...
     def __iter__(self) -> Iterator[_K]: ...