Add support for attrs.fields (#15021)

This add support for `attrs.fields`, which is a nicer way of accessing
the attrs magic attribute.

Co-authored-by: Ilya Priven <ilya.konstantinov@gmail.com>

mypy/plugins/attrs.py

@@ -69,6 +69,7 @@
     TupleType,
     Type,
     TypeOfAny,
+    TypeType,
     TypeVarType,
     UninhabitedType,
     UnionType,
@@ -935,7 +936,7 @@ def add_method(
 
 def _get_attrs_init_type(typ: Instance) -> CallableType | None:
     """
-    If `typ` refers to an attrs class, gets the type of its initializer method.
+    If `typ` refers to an attrs class, get the type of its initializer method.
     """
     magic_attr = typ.type.get(MAGIC_ATTR_NAME)
     if magic_attr is None or not magic_attr.plugin_generated:
@@ -1009,7 +1010,7 @@ def _get_expanded_attr_types(
 
 def _meet_fields(types: list[Mapping[str, Type]]) -> Mapping[str, Type]:
     """
-    "Meets" the fields of a list of attrs classes, i.e. for each field, its new type will be the lower bound.
+    "Meet" the fields of a list of attrs classes, i.e. for each field, its new type will be the lower bound.
     """
     field_to_types = defaultdict(list)
     for fields in types:
@@ -1026,7 +1027,7 @@ def _meet_fields(types: list[Mapping[str, Type]]) -> Mapping[str, Type]:
 
 def evolve_function_sig_callback(ctx: mypy.plugin.FunctionSigContext) -> CallableType:
     """
-    Generates a signature for the 'attr.evolve' function that's specific to the call site
+    Generate a signature for the 'attr.evolve' function that's specific to the call site
     and dependent on the type of the first argument.
     """
     if len(ctx.args) != 2:
@@ -1060,3 +1061,48 @@ def evolve_function_sig_callback(ctx: mypy.plugin.FunctionSigContext) -> Callabl
         fallback=ctx.default_signature.fallback,
         name=f"{ctx.default_signature.name} of {inst_type_str}",
     )
+
+
+def fields_function_sig_callback(ctx: mypy.plugin.FunctionSigContext) -> CallableType:
+    """Provide the signature for `attrs.fields`."""
+    if not ctx.args or len(ctx.args) != 1 or not ctx.args[0] or not ctx.args[0][0]:
+        return ctx.default_signature
+
+    # <hack>
+    assert isinstance(ctx.api, TypeChecker)
+    inst_type = ctx.api.expr_checker.accept(ctx.args[0][0])
+    # </hack>
+    proper_type = get_proper_type(inst_type)
+
+    # fields(Any) -> Any, fields(type[Any]) -> Any
+    if (
+        isinstance(proper_type, AnyType)
+        or isinstance(proper_type, TypeType)
+        and isinstance(proper_type.item, AnyType)
+    ):
+        return ctx.default_signature
+
+    cls = None
+    arg_types = ctx.default_signature.arg_types
+
+    if isinstance(proper_type, TypeVarType):
+        inner = get_proper_type(proper_type.upper_bound)
+        if isinstance(inner, Instance):
+            # We need to work arg_types to compensate for the attrs stubs.
+            arg_types = [inst_type]
+            cls = inner.type
+    elif isinstance(proper_type, CallableType):
+        cls = proper_type.type_object()
+
+    if cls is not None and MAGIC_ATTR_NAME in cls.names:
+        # This is a proper attrs class.
+        ret_type = cls.names[MAGIC_ATTR_NAME].type
+        assert ret_type is not None
+        return ctx.default_signature.copy_modified(arg_types=arg_types, ret_type=ret_type)
+
+    ctx.api.fail(
+        f'Argument 1 to "fields" has incompatible type "{format_type_bare(proper_type, ctx.api.options)}"; expected an attrs class',
+        ctx.context,
+    )
+
+    return ctx.default_signature

mypy/plugins/default.py

@@ -45,6 +45,7 @@ def get_function_hook(self, fullname: str) -> Callable[[FunctionContext], Type]
             return ctypes.array_constructor_callback
         elif fullname == "functools.singledispatch":
             return singledispatch.create_singledispatch_function_callback
+
         return None
 
     def get_function_signature_hook(
@@ -54,6 +55,8 @@ def get_function_signature_hook(
 
         if fullname in ("attr.evolve", "attrs.evolve", "attr.assoc", "attrs.assoc"):
             return attrs.evolve_function_sig_callback
+        elif fullname in ("attr.fields", "attrs.fields"):
+            return attrs.fields_function_sig_callback
         return None
 
     def get_method_signature_hook(

test-data/unit/check-plugin-attrs.test

@@ -1553,6 +1553,59 @@ takes_attrs_cls(A(1, ""))  # E: Argument 1 to "takes_attrs_cls" has incompatible
 takes_attrs_instance(A)  # E: Argument 1 to "takes_attrs_instance" has incompatible type "Type[A]"; expected "AttrsInstance" # N: ClassVar protocol member AttrsInstance.__attrs_attrs__ can never be matched by a class object
 [builtins fixtures/plugin_attrs.pyi]
 
+[case testAttrsFields]
+import attr
+from attrs import fields as f  # Common usage.
+
+@attr.define
+class A:
+    b: int
+    c: str
+
+reveal_type(f(A))  # N: Revealed type is "Tuple[attr.Attribute[builtins.int], attr.Attribute[builtins.str], fallback=__main__.A.____main___A_AttrsAttributes__]"
+reveal_type(f(A)[0])  # N: Revealed type is "attr.Attribute[builtins.int]"
+reveal_type(f(A).b)  # N: Revealed type is "attr.Attribute[builtins.int]"
+f(A).x  # E: "____main___A_AttrsAttributes__" has no attribute "x"
+
+[builtins fixtures/plugin_attrs.pyi]
+
+[case testAttrsGenericFields]
+from typing import TypeVar
+
+import attr
+from attrs import fields
+
+@attr.define
+class A:
+    b: int
+    c: str
+
+TA = TypeVar('TA', bound=A)
+
+def f(t: TA) -> None:
+    reveal_type(fields(t))  # N: Revealed type is "Tuple[attr.Attribute[builtins.int], attr.Attribute[builtins.str], fallback=__main__.A.____main___A_AttrsAttributes__]"
+    reveal_type(fields(t)[0])  # N: Revealed type is "attr.Attribute[builtins.int]"
+    reveal_type(fields(t).b)  # N: Revealed type is "attr.Attribute[builtins.int]"
+    fields(t).x  # E: "____main___A_AttrsAttributes__" has no attribute "x"
+
+
+[builtins fixtures/plugin_attrs.pyi]
+
+[case testNonattrsFields]
+from typing import Any, cast, Type
+from attrs import fields
+
+class A:
+    b: int
+    c: str
+
+fields(A)  # E: Argument 1 to "fields" has incompatible type "Type[A]"; expected an attrs class
+fields(None)  # E: Argument 1 to "fields" has incompatible type "None"; expected an attrs class
+fields(cast(Any, 42))
+fields(cast(Type[Any], 43))
+
+[builtins fixtures/plugin_attrs.pyi]
+
 [case testAttrsInitMethodAlwaysGenerates]
 from typing import Tuple
 import attr

test-data/unit/lib-stub/attr/__init__.pyi

@@ -247,3 +247,5 @@ def field(
 
 def evolve(inst: _T, **changes: Any) -> _T: ...
 def assoc(inst: _T, **changes: Any) -> _T: ...
+
+def fields(cls: type) -> Any: ...

test-data/unit/lib-stub/attrs/__init__.pyi

@@ -131,3 +131,5 @@ def field(
 
 def evolve(inst: _T, **changes: Any) -> _T: ...
 def assoc(inst: _T, **changes: Any) -> _T: ...
+
+def fields(cls: type) -> Any: ...