Attempt to reduce cyclical dependencies between types and importlib. (#335)

- Move Loader and ModuleType into _importlib_modulespec.pyi.
- Add "import X as X" for these to types.pyi and importlib/abc.pyi.

The goal is to ensure mypy -i still works, to fix https://github.com/python/mypy/issues/1797.

stdlib/3/_importlib_modulespec.pyi

@@ -0,0 +1,44 @@
+# ModuleSpec, ModuleType, Loader are part of a dependency cycle.
+# They are officially defined/exported in other places:
+#
+# - ModuleType in types
+# - Loader in importlib.abc
+# - ModuleSpec in importlib.machinery (3.4 and later only)
+
+import abc
+import sys
+from typing import Any, Optional
+
+if sys.version_info >= (3, 4):
+    class ModuleSpec:
+        def __init__(self, name: str, loader: Optional['Loader'], *,
+                     origin: str = None, loader_state: Any = None,
+                     is_package: bool = None) -> None: ...
+        name = ... # type: str
+        loader = ... # type: Optional[Loader]
+        origin = ... # type: Optional[str]
+        submodule_search_locations = ... # type: Optional[List[str]]
+        loader_state = ... # type: Any
+        cached = ... # type: Optional[str]
+        parent = ... # type: Optional[str]
+        has_location = ... # type: bool
+
+class ModuleType:
+    __name__ = ... # type: str
+    __file__ = ...  # type: str
+    __doc__ = ... # type: Optional[str]
+    if sys.version_info >= (3, 4):
+        __loader__ = ... # type: Optional[Loader]
+        __package__ = ... # type: Optional[str]
+        __spec__ = ... # type: Optional[ModuleSpec]
+    def __init__(self, name: str, doc: str) -> None: ...
+
+class Loader(metaclass=abc.ABCMeta):
+    def load_module(self, fullname: str) -> ModuleType: ...
+    if sys.version_info >= (3, 3):
+        def module_repr(self, module: ModuleType) -> str: ...
+    if sys.version_info >= (3, 4):
+        def create_module(self, spec: ModuleSpec) -> Optional[ModuleType]: ...
+        # Not defined on the actual class for backwards-compatibility reasons,
+        # but expected in new code.
+        def exec_module(self, module: ModuleType) -> None: ...

stdlib/3/importlib/_modulespec.pyi

@@ -1,19 +0,0 @@
-# ModuleSpec is in its own file to deal with import loops; defined in
-# importlib.machinery.
-import importlib.abc
-import sys
-from typing import Any, Optional
-
-if sys.version_info >= (3, 4):
-    class ModuleSpec:
-        def __init__(self, name: str, loader: Optional[importlib.abc.Loader], *,
-                     origin: str = None, loader_state: Any = None,
-                     is_package: bool = None) -> None: ...
-        name = ... # type: str
-        loader = ... # type: Optional[importlib.abc.Loader]
-        origin = ... # type: Optional[str]
-        submodule_search_locations = ... # type: Optional[List[str]]
-        loader_state = ... # type: Any
-        cached = ... # type: Optional[str]
-        parent = ... # type: Optional[str]
-        has_location = ... # type: bool

stdlib/3/importlib/abc.pyi

@@ -1,21 +1,14 @@
 import abc
-from importlib._modulespec import ModuleSpec
+from _importlib_modulespec import ModuleSpec
 import sys
 import types
-from typing import Mapping, Optional, Sequence, Union
+from typing import Any, Mapping, Optional, Sequence, Tuple, Union
 
 _Path = Union[bytes, str]
 
-class Loader(metaclass=abc.ABCMeta):
-    def load_module(self, fullname: str) -> types.ModuleType: ...
-    if sys.version_info >= (3, 3):
-        def module_repr(self, module: types.ModuleType) -> str: ...
-    if sys.version_info >= (3, 4):
-        def create_module(self, spec: ModuleSpec) -> Optional[types.ModuleType]:
-            ...
-        # Not defined on the actual class for backwards-compatibility reasons,
-        # but expected in new code.
-        def exec_module(self, module: types.ModuleType) -> None: ...
+# Loader is exported from this module, but for circular import reasons
+# exists in its own stub file (with ModuleSpec and ModuleType).
+from _importlib_modulespec import Loader as Loader  # Exported
 
 class Finder(metaclass=abc.ABCMeta): ...
     # Technically this class defines the following method, but its subclasses

stdlib/3/importlib/machinery.pyi

@@ -3,9 +3,9 @@ import sys
 import types
 from typing import Any, Callable, List, Optional, Sequence, Tuple, Union
 
-# ModuleSpec is defined in this module, but for circular import reasons exists
-# in its own stub file.
-from importlib._modulespec import ModuleSpec
+# ModuleSpec is exported from this module, but for circular import
+# reasons exists in its own stub file (with Loader and ModuleType).
+from _importlib_modulespec import ModuleSpec  # Exported
 
 class BuiltinImporter(importlib.abc.MetaPathFinder,
                       importlib.abc.InspectLoader):

stdlib/3/types.pyi

@@ -3,8 +3,6 @@
 
 # TODO parts of this should be conditional on version
 
-import importlib.abc
-from importlib._modulespec import ModuleSpec
 import sys
 from typing import (
     Any, Callable, Dict, Generic, Iterator, Mapping, Optional, Tuple, TypeVar,
@@ -108,17 +106,9 @@ class BuiltinFunctionType:
     def __call__(self, *args: Any, **kwargs: Any) -> Any: ...
 BuiltinMethodType = BuiltinFunctionType
 
-class ModuleType:
-    __name__ = ... # type: str
-    __file__ = ...  # type: str
-    __doc__ = ... # type: Optional[str]
-    if sys.version_info >= (3, 4):
-        __loader__ = ... # type: Optional[importlib.abc.Loader]
-        __package__ = ... # type: Optional[str]
-        # Should be Optional[ModuleSpec], but importlib.machinery has no stub
-        # yet.
-        __spec__ = ... # type: Optional[ModuleSpec]
-    def __init__(self, name: str, doc: str) -> None: ...
+# ModuleType is exported from this module, but for circular import
+# reasons exists in its own stub file (with ModuleSpec and Loader).
+from _importlib_modulespec import ModuleType as ModuleType  # Exported
 
 class TracebackType:
     tb_frame = ... # type: FrameType