Mypy improvements (#890)

* Mypy improvements

* Change protocol, add a test

* Relax fields return type

* Test fixes

Co-authored-by: Hynek Schlawack <hs@ox.cx>

src/attr/__init__.pyi

@@ -3,11 +3,13 @@ import sys
 from typing import (
     Any,
     Callable,
+    ClassVar,
     Dict,
     Generic,
     List,
     Mapping,
     Optional,
+    Protocol,
     Sequence,
     Tuple,
     Type,
@@ -22,8 +24,8 @@ from . import exceptions as exceptions
 from . import filters as filters
 from . import setters as setters
 from . import validators as validators
-from ._version_info import VersionInfo
 from ._cmp import cmp_using as cmp_using
+from ._version_info import VersionInfo
 
 __version__: str
 __version_info__: VersionInfo
@@ -57,6 +59,10 @@ _FieldTransformer = Callable[
 # _ValidatorType from working when passed in a list or tuple.
 _ValidatorArgType = Union[_ValidatorType[_T], Sequence[_ValidatorType[_T]]]
 
+# A protocol to be able to statically accept an attrs class.
+class AttrsInstance(Protocol):
+    __attrs_attrs__: ClassVar[Any]
+
 # _make --
 
 NOTHING: object
@@ -399,13 +405,9 @@ def define(
 mutable = define
 frozen = define  # they differ only in their defaults
 
-# TODO: add support for returning NamedTuple from the mypy plugin
-class _Fields(Tuple[Attribute[Any], ...]):
-    def __getattr__(self, name: str) -> Attribute[Any]: ...
-
-def fields(cls: type) -> _Fields: ...
-def fields_dict(cls: type) -> Dict[str, Attribute[Any]]: ...
-def validate(inst: Any) -> None: ...
+def fields(cls: Type[AttrsInstance]) -> Any: ...
+def fields_dict(cls: Type[AttrsInstance]) -> Dict[str, Attribute[Any]]: ...
+def validate(inst: AttrsInstance) -> None: ...
 def resolve_types(
     cls: _C,
     globalns: Optional[Dict[str, Any]] = ...,
@@ -449,7 +451,7 @@ def make_class(
 # https://github.com/python/typing/issues/253
 # XXX: remember to fix attrs.asdict/astuple too!
 def asdict(
-    inst: Any,
+    inst: AttrsInstance,
     recurse: bool = ...,
     filter: Optional[_FilterType[Any]] = ...,
     dict_factory: Type[Mapping[Any, Any]] = ...,
@@ -462,7 +464,7 @@ def asdict(
 
 # TODO: add support for returning NamedTuple from the mypy plugin
 def astuple(
-    inst: Any,
+    inst: AttrsInstance,
     recurse: bool = ...,
     filter: Optional[_FilterType[Any]] = ...,
     tuple_factory: Type[Sequence[Any]] = ...,

tests/test_mypy.yml

@@ -1,7 +1,7 @@
 - case: attr_s_with_type_argument
   parametrized:
-    - val: 'a = attr.ib(type=int)'
-    - val: 'a: int = attr.ib()'
+    - val: "a = attr.ib(type=int)"
+    - val: "a: int = attr.ib()"
   main: |
     import attr
     @attr.s
@@ -12,7 +12,7 @@
     C(a=1)
     C(a="hi")  # E: Argument "a" to "C" has incompatible type "str"; expected "int"
 - case: attr_s_with_type_annotations
-  main : |
+  main: |
     import attr
     @attr.s
     class C:
@@ -97,7 +97,7 @@
 
 - case: testAttrsUntypedNoUntypedDefs
   mypy_config: |
-     disallow_untyped_defs = True
+    disallow_untyped_defs = True
   main: |
     import attr
     @attr.s
@@ -316,7 +316,6 @@
     class Confused:
        ...
 
-
 - case: testAttrsInheritance
   main: |
     import attr
@@ -469,14 +468,13 @@
             return self.x  # E: Incompatible return value type (got "List[T]", expected "T")
     reveal_type(A) # N: Revealed type is "def [T] (x: builtins.list[T`1], y: T`1) -> main.A[T`1]"
     a = A([1], 2)
-    reveal_type(a)  # N: Revealed type is "main.A[builtins.int*]"
-    reveal_type(a.x)  # N: Revealed type is "builtins.list[builtins.int*]"
-    reveal_type(a.y)  # N: Revealed type is "builtins.int*"
+    reveal_type(a)  # N: Revealed type is "main.A[builtins.int]"
+    reveal_type(a.x)  # N: Revealed type is "builtins.list[builtins.int]"
+    reveal_type(a.y)  # N: Revealed type is "builtins.int"
 
     A(['str'], 7)  # E: Cannot infer type argument 1 of "A"
     A([1], '2')  # E: Cannot infer type argument 1 of "A"
 
-
 - case: testAttrsUntypedGenericInheritance
   main: |
     from typing import Generic, TypeVar
@@ -514,12 +512,12 @@
         pass
 
     sub_int = Sub[int](attr=1)
-    reveal_type(sub_int)  # N: Revealed type is "main.Sub[builtins.int*]"
-    reveal_type(sub_int.attr)  # N: Revealed type is "builtins.int*"
+    reveal_type(sub_int)  # N: Revealed type is "main.Sub[builtins.int]"
+    reveal_type(sub_int.attr)  # N: Revealed type is "builtins.int"
 
     sub_str = Sub[str](attr='ok')
-    reveal_type(sub_str)  # N: Revealed type is "main.Sub[builtins.str*]"
-    reveal_type(sub_str.attr)  # N: Revealed type is "builtins.str*"
+    reveal_type(sub_str)  # N: Revealed type is "main.Sub[builtins.str]"
+    reveal_type(sub_str.attr)  # N: Revealed type is "builtins.str"
 
 - case: testAttrsGenericInheritance2
   main: |
@@ -764,8 +762,7 @@
             return 'hello'
 
 - case: testAttrsUsingBadConverter
-  mypy_config:
-    strict_optional = False
+  mypy_config: strict_optional = False
   main: |
     import attr
     from typing import overload
@@ -792,8 +789,7 @@
     main:17: note: Revealed type is "def (bad: Any, bad_overloaded: Any) -> main.A"
 
 - case: testAttrsUsingBadConverterReprocess
-  mypy_config:
-    strict_optional = False
+  mypy_config: strict_optional = False
   main: |
     import attr
     from typing import overload
@@ -1169,7 +1165,6 @@
         c = attr.ib(15)
     D(b=17)
 
-
 - case: testAttrsKwOnlySubclass
   main: |
     import attr
@@ -1211,8 +1206,7 @@
     reveal_type(B)  # N: Revealed type is "def (x: main.C) -> main.B"
 
 - case: testDisallowUntypedWorksForwardBad
-  mypy_config:
-    disallow_untyped_defs = True
+  mypy_config: disallow_untyped_defs = True
   main: |
     import attr
 
@@ -1357,3 +1351,24 @@
         foo = x
 
     reveal_type(B)  # N: Revealed type is "def (foo: builtins.int) -> main.B"
+
+- case: testFields
+  main: |
+    from attrs import define, fields
+
+    @define
+    class A:
+      a: int
+      b: str
+
+    reveal_type(fields(A))  # N: Revealed type is "Any"
+
+- case: testFieldsError
+  main: |
+    from attrs import fields
+
+    class A:
+      a: int
+      b: str
+
+    fields(A)  # E: Argument 1 to "fields" has incompatible type "Type[A]"; expected "Type[AttrsInstance]"