Refactor to support typevars, and more tests

mypy/plugins/attrs.py

@@ -2,7 +2,7 @@
 
 from __future__ import annotations
 
-from typing import Iterable, List, Optional, cast
+from typing import Iterable, List, cast
 from typing_extensions import Final, Literal
 
 import mypy.plugin  # To avoid circular imports.
@@ -43,7 +43,7 @@
     Var,
     is_class_var,
 )
-from mypy.plugin import FunctionContext, SemanticAnalyzerPluginInterface
+from mypy.plugin import SemanticAnalyzerPluginInterface
 from mypy.plugins.common import (
     _get_argument,
     _get_bool_argument,
@@ -990,27 +990,42 @@ def evolve_function_sig_callback(ctx: mypy.plugin.FunctionSigContext) -> Callabl
     )
 
 
-def _get_cls_from_init(t: Type) -> Optional[TypeInfo]:
-    proper_type = get_proper_type(t)
-    if isinstance(proper_type, CallableType):
-        return proper_type.type_object()
-    return None
+def fields_function_sig_callback(ctx: mypy.plugin.FunctionSigContext) -> CallableType:
+    """Provide the proper signature for `attrs.fields`."""
+    if ctx.args and len(ctx.args) == 1 and ctx.args[0] and ctx.args[0][0]:
 
+        # <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)
+
+        if isinstance(proper_type, AnyType):  # fields(Any) -> Any
+            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()
 
-def fields_function_callback(ctx: FunctionContext) -> Type:
-    """Provide the proper return value for `attrs.fields`."""
-    if ctx.arg_types and ctx.arg_types[0] and ctx.arg_types[0][0]:
-        first_arg_type = ctx.arg_types[0][0]
-        cls = _get_cls_from_init(first_arg_type)
         if cls is not None:
             if MAGIC_ATTR_NAME in cls.names:
                 # This is a proper attrs class.
                 ret_type = cls.names[MAGIC_ATTR_NAME].type
                 if ret_type is not None:
-                    return ret_type
-            else:
-                ctx.api.fail(
-                    f'Argument 1 to "fields" has incompatible type "{format_type_bare(first_arg_type)}"; expected an attrs class',
-                    ctx.context,
-                )
-    return ctx.default_return_type
+                    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)}"; expected an attrs class',
+            ctx.context,
+        )
+    return ctx.default_signature

mypy/plugins/default.py

@@ -39,14 +39,13 @@ class DefaultPlugin(Plugin):
     """Type checker plugin that is enabled by default."""
 
     def get_function_hook(self, fullname: str) -> Callable[[FunctionContext], Type] | None:
-        from mypy.plugins import attrs, ctypes, singledispatch
+        from mypy.plugins import ctypes, singledispatch
 
         if fullname == "ctypes.Array":
             return ctypes.array_constructor_callback
         elif fullname == "functools.singledispatch":
             return singledispatch.create_singledispatch_function_callback
-        elif fullname in ("attr.fields", "attrs.fields"):
-            return attrs.fields_function_callback
+
         return None
 
     def get_function_signature_hook(
@@ -56,6 +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-attr.test

@@ -1549,6 +1549,24 @@ takes_attrs_instance(A)  # E: Argument 1 to "takes_attrs_instance" has incompati
 [builtins fixtures/attr.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/attr.pyi]
+
+[case testAttrsGenericFields]
+from typing import TypeVar
+
 import attr
 from attrs import fields
 
@@ -1557,21 +1575,28 @@ class A:
     b: int
     c: str
 
-reveal_type(fields(A))  # N: Revealed type is "Tuple[attr.Attribute[builtins.int], attr.Attribute[builtins.str], fallback=__main__.A.____main___A_AttrsAttributes__]"
-reveal_type(fields(A)[0])  # N: Revealed type is "attr.Attribute[builtins.int]"
-reveal_type(fields(A).b)  # N: Revealed type is "attr.Attribute[builtins.int]"
-fields(A).x  # E: "____main___A_AttrsAttributes__" has no attribute "x"
+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/attr.pyi]
 
 [case testNonattrsFields]
+from typing import Any, cast
 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))
 
 [builtins fixtures/attr.pyi]
 

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

@@ -248,4 +248,4 @@ def field(
 def evolve(inst: _T, **changes: Any) -> _T: ...
 def assoc(inst: _T, **changes: Any) -> _T: ...
 
-def fields(cls: _C) -> Any: ...
+def fields(cls: type) -> Any: ...

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

@@ -130,4 +130,4 @@ def field(
 def evolve(inst: _T, **changes: Any) -> _T: ...
 def assoc(inst: _T, **changes: Any) -> _T: ...
 
-def fields(cls: _C) -> Any: ...
+def fields(cls: type) -> Any: ...