cc: Add typing to overloaded function implementations

PEP 484 doesn't require an overloaded function implementation to be
annotated, but mypy will not check it otherwise.

python/mypy#3160

Note that in dispatchlib I replaced overload by type restriction,
since that was a simpler way of expressing the same rules.

Author: mthuurne

src/softfab/dispatchlib.py

@@ -2,8 +2,8 @@
 
 from collections import defaultdict
 from typing import (
-    TYPE_CHECKING, Dict, Iterable, List, Mapping, Optional, Sequence, Set,
-    cast, overload
+    TYPE_CHECKING, DefaultDict, Dict, Iterable, List, Mapping, Optional,
+    Sequence, Set, TypeVar, cast
 )
 
 from softfab.resreq import ResourceClaim, ResourceSpec
@@ -17,18 +17,11 @@
 else:
     Resource = None
 
-@overload
-def _groupByType(
-        items: Iterable[Resource] # pylint: disable=unused-argument
-        ) -> Mapping[str, Sequence[Resource]]:
-    pass
-@overload
-def _groupByType(
-        items: Iterable[ResourceSpec] # pylint: disable=unused-argument
-        ) -> Mapping[str, Sequence[ResourceSpec]]:
-    pass
-def _groupByType(items):
-    grouped = defaultdict(list)
+ResOrSpec = TypeVar('ResOrSpec', Resource, ResourceSpec)
+
+def _groupByType(items: Iterable[ResOrSpec]
+                 ) -> Mapping[str, Sequence[ResOrSpec]]:
+    grouped = defaultdict(list) # type: DefaultDict[str, List[ResOrSpec]]
     for item in items:
         grouped[item.typeName].append(item)
     return grouped

src/softfab/pageargs.py

@@ -468,7 +468,7 @@ def __init__(self, default: _MandatoryValue = mandatory
                  ): # pylint: disable=unused-argument
         ...
 
-    def __init__(self, default=mandatory):
+    def __init__(self, default: Any = mandatory):
         '''Creates a page argument with a given default value.
         '''
         self.__default = default
@@ -503,7 +503,10 @@ def __get__(self,
             ) -> Union[ValueT, DefaultT]:
         pass
 
-    def __get__(self, instance, owner):
+    def __get__(self,
+                instance: Optional[PageArgs],
+                owner: Type[PageArgs]
+                ) -> Any:
         if instance is None:
             # Class attribute access.
             return self
@@ -1234,7 +1237,7 @@ def _externalizeArg(arg: Any, value: Any # pylint: disable=unused-argument
     # type checks we do here just to please mypy.
     pass
 
-def _externalizeArg(arg, value):
+def _externalizeArg(arg: Any, value: Any) -> Sequence[str]:
     if isinstance(arg, SingularArgument):
         return (arg.externalize(value),)
     elif isinstance(arg, CollectionArg):