Revert "Add support for attrs.fields (python#15021)"

This reverts commit 391ed85.

The use of a Union item type for `attr.Attribute` would likely be
more useful to callsites of `attr.fields` - otherwise this currently
results in inferring `builtins.object` in many cases where we treat
the fields as a variable-length tuple, rather than a fixed-length
one. Applications of the fixed-length tuple seem limited since you
would need to access it by index.

@Tinche as the original author of the commit.

Author: jhance

mypy/plugins/attrs.py

@@ -69,7 +69,6 @@
     TupleType,
     Type,
     TypeOfAny,
-    TypeType,
     TypeVarType,
     UninhabitedType,
     UnionType,
@@ -945,7 +944,7 @@ def add_method(
 
 def _get_attrs_init_type(typ: Instance) -> CallableType | None:
     """
-    If `typ` refers to an attrs class, get the type of its initializer method.
+    If `typ` refers to an attrs class, gets 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:
@@ -1019,7 +1018,7 @@ def _get_expanded_attr_types(
 
 def _meet_fields(types: list[Mapping[str, Type]]) -> Mapping[str, Type]:
     """
-    "Meet" the fields of a list of attrs classes, i.e. for each field, its new type will be the lower bound.
+    "Meets" 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:
@@ -1036,7 +1035,7 @@ def _meet_fields(types: list[Mapping[str, Type]]) -> Mapping[str, Type]:
 
 def evolve_function_sig_callback(ctx: mypy.plugin.FunctionSigContext) -> CallableType:
     """
-    Generate a signature for the 'attr.evolve' function that's specific to the call site
+    Generates 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:
@@ -1070,48 +1069,3 @@ 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

@@ -45,7 +45,6 @@ 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(
@@ -55,8 +54,6 @@ 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

@@ -1553,59 +1553,6 @@ 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

@@ -247,5 +247,3 @@ 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

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