Fixed checking of protocols on the class level

Fixes #498.

docs/versionhistory.rst

@@ -13,6 +13,8 @@ This library adheres to
   (`#486 <https://github.com/agronholm/typeguard/pull/486>`_)
 - Fixed ``TypeError`` when checking a class against ``type[Self]``
   (`#481 <https://github.com/agronholm/typeguard/pull/481>`_)
+- Fixed checking of protocols on the class level (against ``type[SomeProtocol]``)
+  (`#498 <https://github.com/agronholm/typeguard/pull/498>`_)
 
 **4.4.0** (2024-10-27)
 

src/typeguard/_checkers.py

@@ -635,10 +635,8 @@ def check_io(
         raise TypeCheckError("is not an I/O object")
 
 
-def check_signature_compatible(
-    subject_callable: Callable[..., Any], protocol: type, attrname: str
-) -> None:
-    subject_sig = inspect.signature(subject_callable)
+def check_signature_compatible(subject: type, protocol: type, attrname: str) -> None:
+    subject_sig = inspect.signature(getattr(subject, attrname))
     protocol_sig = inspect.signature(getattr(protocol, attrname))
     protocol_type: typing.Literal["instance", "class", "static"] = "instance"
     subject_type: typing.Literal["instance", "class", "static"] = "instance"
@@ -652,12 +650,12 @@ def check_signature_compatible(
             protocol_type = "class"
 
     # Check if the subject-side method is a class method or static method
-    if inspect.ismethod(subject_callable) and inspect.isclass(
-        subject_callable.__self__
-    ):
-        subject_type = "class"
-    elif not hasattr(subject_callable, "__self__"):
-        subject_type = "static"
+    if attrname in subject.__dict__:
+        descriptor = subject.__dict__[attrname]
+        if isinstance(descriptor, staticmethod):
+            subject_type = "static"
+        elif isinstance(descriptor, classmethod):
+            subject_type = "class"
 
     if protocol_type == "instance" and subject_type != "instance":
         raise TypeCheckError(
@@ -714,6 +712,10 @@ def check_signature_compatible(
         if protocol_type == "instance":
             protocol_args.pop(0)
 
+        # Remove the "self" parameter from the subject arguments to match
+        if subject_type == "instance":
+            subject_args.pop(0)
+
         for protocol_arg, subject_arg in zip_longest(protocol_args, subject_args):
             if protocol_arg is None:
                 if subject_arg.default is Parameter.empty:
@@ -818,8 +820,9 @@ def check_protocol(
 
             # TODO: implement assignability checks for parameter and return value
             #  annotations
+            subject = value if isclass(value) else value.__class__
             try:
-                check_signature_compatible(subject_member, origin_type, attrname)
+                check_signature_compatible(subject, origin_type, attrname)
             except TypeCheckError as exc:
                 raise TypeCheckError(
                     f"is not compatible with the {origin_type.__qualname__} "

tests/test_checkers.py

@@ -1069,7 +1069,11 @@ def test_raises_for_non_member(self, subject: object, predicate_type: type) -> N
 
 
 class TestProtocol:
-    def test_success(self, typing_provider: Any) -> None:
+    @pytest.mark.parametrize(
+        "instantiate",
+        [pytest.param(True, id="instance"), pytest.param(False, id="class")],
+    )
+    def test_success(self, typing_provider: Any, instantiate: bool) -> None:
         class MyProtocol(Protocol):
             member: int
 
@@ -1129,7 +1133,10 @@ def my_static_method(cls, x: int, y: str) -> None:
             def my_class_method(x: int, y: str) -> None:
                 pass
 
-        check_type(Foo(), MyProtocol)
+        if instantiate:
+            check_type(Foo(), MyProtocol)
+        else:
+            check_type(Foo, type[MyProtocol])
 
     @pytest.mark.parametrize("has_member", [True, False])
     def test_member_checks(self, has_member: bool) -> None: