Fixed signature check raising ValueError for a built-in function

Fixes #504.

docs/versionhistory.rst

@@ -13,6 +13,9 @@ This library adheres to
   (`#492 <https://github.com/agronholm/typeguard/pull/492>`_; PR by @JelleZijlstra)
 - Fixed ``TypeError`` when using an assignment expression
   (`#510 <https://github.com/agronholm/typeguard/issues/510>`_; PR by @JohannesK71083)
+- Fixed ``ValueError: no signature found for builtin`` when checking against a protocol
+  and a matching attribute in the subject is a built-in function
+  (`#504 <https://github.com/agronholm/typeguard/issues/504>`_)
 
 **4.4.1** (2024-11-03)
 

src/typeguard/_checkers.py

@@ -648,7 +648,12 @@ def check_io(
 
 
 def check_signature_compatible(subject: type, protocol: type, attrname: str) -> None:
-    subject_sig = inspect.signature(getattr(subject, attrname))
+    subject_attr = getattr(subject, attrname)
+    try:
+        subject_sig = inspect.signature(subject_attr)
+    except ValueError:
+        return  # this can happen with builtins where the signature cannot be retrieved
+
     protocol_sig = inspect.signature(getattr(protocol, attrname))
     protocol_type: typing.Literal["instance", "class", "static"] = "instance"
     subject_type: typing.Literal["instance", "class", "static"] = "instance"

tests/test_checkers.py

@@ -2,6 +2,7 @@
 import sys
 import types
 from contextlib import nullcontext
+from datetime import timedelta
 from functools import partial
 from io import BytesIO, StringIO
 from pathlib import Path
@@ -1383,6 +1384,18 @@ def meth(self) -> None:
             f"be a class method but it's an instance method"
         )
 
+    def test_builtin_signature_check(self) -> None:
+        class MyProtocol(Protocol):
+            def attr(self) -> None:
+                pass
+
+        class Foo:
+            attr = timedelta
+
+        # Foo.attr is incompatible but timedelta has not inspectable signature so the
+        # check is skipped
+        check_type(Foo(), MyProtocol)
+
 
 class TestRecursiveType:
     def test_valid(self):