Remove error when passing a "non-dataclass" to asdict to reduce false…

…-positives when type-checking code that calls asdict on "non-specific" or "generic" dataclasses. Small cleanup of control flow and comment for clarity.
python · syastrov · Mar 26, 2020 · Mar 26, 2020 · Mar 26, 2020 · Mar 26, 2020
commit d7e03101a5b9bbf49cfcb5c0edd9b35cf5ab1fdb
diff --git a/mypy/plugins/dataclasses.py b/mypy/plugins/dataclasses.py
@@ -403,15 +403,9 @@ def asdict_callback(ctx: FunctionContext) -> Type:
         if isinstance(dataclass_instance, Instance):
             if is_type_dataclass(dataclass_instance.type):
                 if len(ctx.arg_types) == 1:
+                    # Can only infer a more precise type for calls where dict_factory is not set.
                     return _asdictify(ctx.api, dataclass_instance)
-                else:
-                    # We can't infer a more precise type for calls where dict_factory is set.
-                    # At least for now, typeshed stubs for asdict don't allow you to pass in
-                    # `dict` as dict_factory, so we can't special-case that.
-                    return ctx.default_return_type
 
-    ctx.api.fail("'dataclasses.asdict' should be called on dataclass instances",
-                 ctx.context)
     return ctx.default_return_type
 
 

diff --git a/test-data/unit/check-dataclasses.test b/test-data/unit/check-dataclasses.test
@@ -1122,13 +1122,14 @@ class NonDataclass:
     pass
 
 reveal_type(asdict(Person('John', 32)))  # N: Revealed type is 'TypedDict({'name': builtins.str, 'age': builtins.int})'
-asdict(Person) # E: 'dataclasses.asdict' should be called on dataclass instances
-asdict(NonDataclass()) # E: 'dataclasses.asdict' should be called on dataclass instances
+# It's OK to call on a non-dataclass, to reduce false-positives.
+reveal_type(asdict(Person)) # N: Revealed type is 'builtins.dict*[builtins.str, Any]'
+reveal_type(asdict(NonDataclass())) # N: Revealed type is 'builtins.dict*[builtins.str, Any]'
 
 def my_dict_factory(seq: List[Tuple[str, Any]]) -> Dict[str, Any]:
     pass
 
-asdict(NonDataclass(), dict_factory=my_dict_factory) # E: 'dataclasses.asdict' should be called on dataclass instances
+reveal_type(asdict(NonDataclass(), dict_factory=my_dict_factory)) # N: Revealed type is 'builtins.dict*[builtins.str, Any]'
 
 # Passing in a dict_factory should not return a TypedDict
 reveal_type(asdict(Person('John', 32), dict_factory=my_dict_factory))  # N: Revealed type is 'builtins.dict*[builtins.str, Any]'