Emulate generic types in runtime (#289)

* Emulate generic types in runtime

* Fix flake8

Author: asvetlov

multidict/_abc.py

@@ -1,53 +1,42 @@
 import abc
 
-from typing import (Mapping, MutableMapping, List, Union, Iterable,
-                    TypeVar, Tuple, Dict, Optional)
+from collections.abc import Mapping, MutableMapping
 
 
-_T = TypeVar('_T')
+class _TypingMeta(abc.ABCMeta):
+    # A fake metaclass to satisfy typing deps in runtime
+    # basically MultiMapping[str] and other generic-like type instantiations
+    # are emulated.
+    # Note: real type hints are provided by __init__.pyi stub file
+    def __getitem__(self, key):
+        return self
 
-_D = TypeVar('_D')
 
-
-# Note: type defs are slightly different from __init__.pyi version The
-# correct one (and checked by mypy) is the later.  Type checks here
-# exists for sake of consistency and allowing to instantiate
-# MultiMapiing[_T] in inline python code
-
-
-class MultiMapping(Mapping[str, _T]):
+class MultiMapping(Mapping, metaclass=_TypingMeta):
 
     @abc.abstractmethod
-    def getall(self, key: str,
-               default: Optional[_D]=None) -> Union[List[_T], _D, None]:
+    def getall(self, key, default=None):
         raise KeyError
 
     @abc.abstractmethod
-    def getone(self, key: str,
-               default: Optional[_D]=None) -> Union[_T, _D, None]:
+    def getone(self, key, default=None):
         raise KeyError
 
 
-_Arg = Union[Mapping[str, _T],
-             Dict[str, _T],
-             MultiMapping[_T],
-             Iterable[Tuple[str, _T]]]
-
-
-class MutableMultiMapping(MultiMapping[_T], MutableMapping[str, _T]):
+class MutableMultiMapping(MultiMapping, MutableMapping):
 
     @abc.abstractmethod
-    def add(self, key: str, value: _T) -> None:
+    def add(self, key, value):
         raise NotImplementedError
 
     @abc.abstractmethod
-    def extend(self, *args: _Arg[_T], **kwargs: _T) -> None:
+    def extend(self, *args, **kwargs):
         raise NotImplementedError
 
     @abc.abstractmethod
-    def popone(self, key: str, default: Optional[_T]=None) -> _T:
+    def popone(self, key, default=None):
         raise KeyError
 
     @abc.abstractmethod
-    def popall(self, key: str, default: Optional[_T]=None) -> List[_T]:
+    def popall(self, key, default=None):
         raise KeyError

tests/test_abc.py

@@ -157,3 +157,8 @@ def test_proxy_inheritance(proxy_classes):
     proxy, _ = proxy_classes
     assert issubclass(proxy, MultiMapping)
     assert not issubclass(proxy, MutableMultiMapping)
+
+
+def test_generic_type_in_runtime():
+    MultiMapping[str]
+    MutableMultiMapping[str]