Implement type-aware get for TypedDict

Previously, `get` would simply fallback to the type of the underlying dictionary which made
TypedDicts hard to use with code that's parsing objects where fields may or may not be
present (for example, parsing a response).

This implementation _explicitly_ ignores the default parameter's type as it's quite useful to
chain together get calls (Until something like PEP 505 hits 😄)

```python
foo.get('a', {}).get('b', {}).get('c')
```

This fixes #2612

Author: Roy Williams

mypy/checkexpr.py

@@ -9,7 +9,7 @@
     PartialType, DeletedType, UnboundType, UninhabitedType, TypeType,
     true_only, false_only, is_named_instance, function_type, FunctionLike,
     get_typ_args, set_typ_args,
-)
+    TypedDictGetFunction)
 from mypy.nodes import (
     NameExpr, RefExpr, Var, FuncDef, OverloadedFuncDef, TypeInfo, CallExpr,
     MemberExpr, IntExpr, StrExpr, BytesExpr, UnicodeExpr, FloatExpr,
@@ -341,6 +341,10 @@ def check_call(self, callee: Type, args: List[Expression],
         """
         arg_messages = arg_messages or self.msg
         if isinstance(callee, CallableType):
+            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])
+                    return return_type, callee
             if callee.is_concrete_type_obj() and callee.type_object().is_abstract:
                 type = callee.type_object()
                 self.msg.cannot_instantiate_abstract_class(
@@ -1484,11 +1488,13 @@ def _get_value(self, index: Expression) -> Optional[int]:
         return None
 
     def visit_typeddict_index_expr(self, td_type: TypedDictType, index: Expression):
+        return self.get_typeddict_index_type(td_type, index)
+
+    def get_typeddict_index_type(self, td_type: TypedDictType, index: Expression):
         if not isinstance(index, (StrExpr, UnicodeExpr)):
             self.msg.typeddict_item_name_must_be_string_literal(td_type, index)
             return AnyType()
         item_name = index.value
-
         item_type = td_type.items.get(item_name)
         if item_type is None:
             self.msg.typeddict_item_name_not_found(td_type, item_name, index)

mypy/checkmember.py

@@ -5,8 +5,8 @@
 from mypy.types import (
     Type, Instance, AnyType, TupleType, TypedDictType, CallableType, FunctionLike, TypeVarDef,
     Overloaded, TypeVarType, UnionType, PartialType,
-    DeletedType, NoneTyp, TypeType, function_type
-)
+    DeletedType, NoneTyp, TypeType, function_type,
+    TypedDictGetFunction)
 from mypy.nodes import (
     TypeInfo, FuncBase, Var, FuncDef, SymbolNode, Context, MypyFile, TypeVarExpr,
     ARG_POS, ARG_STAR, ARG_STAR2,
@@ -120,9 +120,12 @@ def analyze_member_access(name: str,
                                      original_type=original_type, chk=chk)
     elif isinstance(typ, TypedDictType):
         # Actually look up from the fallback instance type.
-        return analyze_member_access(name, typ.fallback, node, is_lvalue, is_super,
-                                     is_operator, builtin_type, not_ready_callback, msg,
-                                     original_type=original_type, chk=chk)
+        result = analyze_member_access(name, typ.fallback, node, is_lvalue, is_super,
+                                       is_operator, builtin_type, not_ready_callback, msg,
+                                       original_type=original_type, chk=chk)
+        if name == 'get' and isinstance(result, CallableType):
+            result = TypedDictGetFunction(typ, result)
+        return result
     elif isinstance(typ, FunctionLike) and typ.is_type_obj():
         # Class attribute.
         # TODO super?

mypy/types.py

@@ -980,6 +980,26 @@ def zipall(self, right: 'TypedDictType') \
             yield (item_name, None, right_item_type)
 
 
+class TypedDictGetFunction(CallableType):
+    """A special callable type containing a reference to the TypedDict `get` callable instance.
+    This is needed to delay determining the signature of a TypedDict's `get` method until the
+    method is actually called.  This allows `get` to behave just as indexing into the TypedDict
+    would.
+
+    This is not a real type, but is needed to allow TypedDict.get to behave as expected.
+    """
+    def __init__(self, typed_dict: TypedDictType, fallback_callable: CallableType) -> None:
+        super().__init__(fallback_callable.arg_types, fallback_callable.arg_kinds,
+                         fallback_callable.arg_names, fallback_callable.ret_type,
+                         fallback_callable.fallback, fallback_callable.name,
+                         fallback_callable.definition, fallback_callable.variables,
+                         fallback_callable.line, fallback_callable.column,
+                         fallback_callable.is_ellipsis_args, fallback_callable.implicit,
+                         fallback_callable.is_classmethod_class, fallback_callable.special_sig)
+        self.typed_dict = typed_dict
+        self.fallback_callable = fallback_callable
+
+
 class StarType(Type):
     """The star type *type_parameter.
 

test-data/unit/check-typeddict.test

@@ -431,6 +431,38 @@ def set_coordinate(p: TaggedPoint, key: str, value: int) -> None:
 
 -- Special Method: get
 
+[case testCanUseGetMethodWithStringLiteralKey]
+from mypy_extensions import TypedDict
+TaggedPoint = TypedDict('TaggedPoint', {'type': str, 'x': int, 'y': int})
+p = TaggedPoint(type='2d', x=42, y=1337)
+reveal_type(p.get('type'))  # E: Revealed type is 'builtins.str'
+reveal_type(p.get('x'))     # E: Revealed type is 'builtins.int'
+reveal_type(p.get('y'))     # E: Revealed type is 'builtins.int'
+[builtins fixtures/dict.pyi]
+
+[case testCannotGetMethodWithInvalidStringLiteralKey]
+from mypy_extensions import TypedDict
+TaggedPoint = TypedDict('TaggedPoint', {'type': str, 'x': int, 'y': int})
+p = TaggedPoint(type='2d', x=42, y=1337)
+p.get('z')  # E: 'z' is not a valid item name; expected one of ['type', 'x', 'y']
+[builtins fixtures/dict.pyi]
+
+[case testGetMethodWithVariableKeyFallsBack]
+from mypy_extensions import TypedDict
+TaggedPoint = TypedDict('TaggedPoint', {'type': str, 'x': int, 'y': int})
+p = TaggedPoint(type='2d', x=42, y=1337)
+key = 'type'
+reveal_type(p.get(key))  # E: Revealed type is 'builtins.object*'
+[builtins fixtures/dict.pyi]
+
+[case testChainedGetMethodWithFallback]
+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))
+reveal_type(p.get('first_point', {}).get('x'))  # E: Revealed type is 'builtins.int'
+[builtins fixtures/dict.pyi]
+
 -- TODO: Implement support for these cases:
 --[case testGetOfTypedDictWithValidStringLiteralKeyReturnsPreciseType]
 --[case testGetOfTypedDictWithInvalidStringLiteralKeyIsError]

test-data/unit/fixtures/dict.pyi

@@ -18,6 +18,7 @@ class dict(Iterable[KT], Mapping[KT, VT], Generic[KT, VT]):
     def __init__(self, arg: Iterable[Tuple[KT, VT]], **kwargs: VT) -> None: pass
     def __setitem__(self, k: KT, v: VT) -> None: pass
     def __iter__(self) -> Iterator[KT]: pass
+    def get(self, k: KT, default: VT=None) -> VT: pass
     def update(self, a: Mapping[KT, VT]) -> None: pass
 
 class int: # for convenience

test-data/unit/lib-stub/typing.pyi

@@ -78,7 +78,9 @@ class Sequence(Iterable[T], Generic[T]):
     @abstractmethod
     def __getitem__(self, n: Any) -> T: pass
 
-class Mapping(Generic[T, U]): pass
+class Mapping(Generic[T, U]):
+    @abstractmethod
+    def get(self, k: T, default: U=None) -> U: pass
 
 class MutableMapping(Generic[T, U]): pass