Simplify logic with overriding callee in cases of TypedDict.get functions.

After poking around with this a bunch today I realized it would be much simplier to simply create
a context-specific Callable as opposed to attemping to hijack the rest of the typechecking.

The original implementation had problems in places, for example where a TypedDict had a List field.
A default empty list was not being coerced correctly.

Author: Roy Williams

mypy/checkexpr.py

@@ -347,6 +347,24 @@ def check_call(self, callee: Type, args: List[Expression],
                     callee.type_object().name(), type.abstract_attributes,
                     context)
 
+            if isinstance(callee, TypedDictGetFunction):
+                if 1 <= len(args) <= 2 and isinstance(args[0], (StrExpr, UnicodeExpr)):
+                    return_type = self.get_typeddict_index_type(callee.typed_dict, args[0])
+                    arg_types = callee.arg_types
+                    if len(args) == 1:
+                        return_type = UnionType.make_union([
+                            return_type, NoneTyp()])
+                    elif isinstance(return_type, TypedDictType) and len(callee.arg_types) == 2:
+                        # Explicitly set the type of the default parameter to
+                        # Union[typing.Mapping, <return type>] in cases where the return value
+                        # is a typed dict.  This special case allows for chaining of `get` methods
+                        # when accessing elements deep within nested dictionaries in a safe and
+                        # concise way without having to set up exception handlers.
+                        arg_types = [callee.arg_types[0],
+                                     UnionType.make_union([return_type,
+                                                           self.named_type('typing.Mapping')])]
+                    callee = callee.copy_modified(ret_type=return_type, arg_types=arg_types)
+
             formal_to_actual = map_actuals_to_formals(
                 arg_kinds, arg_names,
                 callee.arg_kinds, callee.arg_names,
@@ -362,24 +380,6 @@ def check_call(self, callee: Type, args: List[Expression],
             arg_types = self.infer_arg_types_in_context2(
                 callee, args, arg_kinds, formal_to_actual)
 
-            if isinstance(callee, TypedDictGetFunction):
-                if 1 <= len(args) <= 2 and isinstance(args[0], (StrExpr, UnicodeExpr)):
-                    return_type = self.get_typeddict_index_type(callee.typed_dict, args[0])
-                    if len(args) == 1:
-                        return_type = UnionType.make_union([
-                            return_type, NoneTyp()])
-                    else:
-                        # Explicitly set the return type to be a the TypedDict in cases where the
-                        # call site is of the form `x.get('key', {})` and x['key'] is another
-                        # TypedDict.  This special case allows for chaining of `get` methods when
-                        # accessing elements deep within nested dictionaries in a safe and
-                        # concise way without having to set up exception handlers.
-                        if not (isinstance(return_type, TypedDictType) and
-                                is_subtype(arg_types[1], self.named_type('typing.Mapping'))):
-                            return_type = UnionType.make_simplified_union(
-                                [return_type, arg_types[1]])
-                    return return_type, callee
-
             self.check_argument_count(callee, arg_types, arg_kinds,
                                       arg_names, formal_to_actual, context, self.msg)
 

test-data/unit/check-typeddict.test

@@ -438,8 +438,6 @@ p = TaggedPoint(type='2d', x=42, y=1337)
 reveal_type(p.get('type'))  # E: Revealed type is 'Union[builtins.str, builtins.None]'
 reveal_type(p.get('x'))     # E: Revealed type is 'Union[builtins.int, builtins.None]'
 reveal_type(p.get('y', 0))     # E: Revealed type is 'builtins.int'
-reveal_type(p.get('y', 'hello'))     # E: Revealed type is 'Union[builtins.int, builtins.str]'
-reveal_type(p.get('y', {}))     # E: Revealed type is 'Union[builtins.int, builtins.dict[builtins.None, builtins.None]]'
 [builtins fixtures/dict.pyi]
 
 [case testDefaultParameterStillTypeChecked]
@@ -472,12 +470,12 @@ p = PointSet(first_point=TaggedPoint(type='2d', x=42, y=1337))
 reveal_type(p.get('first_point', {}).get('x', 0))  # E: Revealed type is 'builtins.int'
 [builtins fixtures/dict.pyi]
 
-[case testChainedGetMethodWithNonDictFallback]
+[case testGetMethodInvalidDefaultType]
 from mypy_extensions import TypedDict
 TaggedPoint = TypedDict('TaggedPoint', {'type': str, 'x': int, 'y': int})
 PointSet = TypedDict('PointSet', {'first_point': TaggedPoint})
 p = PointSet(first_point=TaggedPoint(type='2d', x=42, y=1337))
-p.get('first_point', 32).get('x', 0)  # E: Some element of union has no attribute "get"
+p.get('first_point', 32)  # E: Argument 2 to "get" of "Mapping" has incompatible type "int"; expected "Union[TypedDict(type=str, x=int, y=int), Mapping]"
 [builtins fixtures/dict.pyi]
 
 [case testGetMethodOnList]
@@ -489,6 +487,14 @@ p = PointSet(points=[TaggedPoint(type='2d', x=42, y=1337)])
 reveal_type(p.get('points', []))  # E: Revealed type is 'builtins.list[TypedDict(type=builtins.str, x=builtins.int, y=builtins.int, _fallback=__main__.TaggedPoint)]'
 [builtins fixtures/dict.pyi]
 
+[case testGetMethodWithListOfStrUnifies]
+from typing import List
+from mypy_extensions import TypedDict
+Items = TypedDict('Items', {'name': str, 'values': List[str]})
+def foo(i: Items) -> None:
+  reveal_type(i.get('values', []))  # E: Revealed type is 'builtins.list[builtins.str]'
+[builtins fixtures/dict.pyi]
+
 [case testDictGetMethodStillCallable]
 from typing import Callable
 from mypy_extensions import TypedDict