Fixed check_callable() not counting positional args with defaults pro…

…perly

Fixes #400.

docs/versionhistory.rst

@@ -4,6 +4,12 @@ Version history
 This library adheres to
 `Semantic Versioning 2.0 <https://semver.org/#semantic-versioning-200>`_.
 
+**UNRELEASED**
+
+- Fixed ``Callable`` erroneously rejecting a callable that has the requested amount of
+  positional arguments but they have defaults
+  (`#400 <https://github.com/agronholm/typeguard/issues/400>`_)
+
 **4.1.4** (2023-09-10)
 
 - Fixed ``AttributeError`` where the transformer removed elements from a PEP 604 union

src/typeguard/_checkers.py

@@ -172,31 +172,29 @@ def check_callable(
                     f'{", ".join(unfulfilled_kwonlyargs)}'
                 )
 
-            num_mandatory_args = len(
-                [
-                    param.name
-                    for param in signature.parameters.values()
-                    if param.kind
-                    in (Parameter.POSITIONAL_ONLY, Parameter.POSITIONAL_OR_KEYWORD)
-                    and param.default is Parameter.empty
-                ]
-            )
-            has_varargs = any(
-                param
-                for param in signature.parameters.values()
-                if param.kind == Parameter.VAR_POSITIONAL
-            )
-
-            if num_mandatory_args > len(argument_types):
+            num_positional_args = num_mandatory_pos_args = 0
+            has_varargs = False
+            for param in signature.parameters.values():
+                if param.kind in (
+                    Parameter.POSITIONAL_ONLY,
+                    Parameter.POSITIONAL_OR_KEYWORD,
+                ):
+                    num_positional_args += 1
+                    if param.default is Parameter.empty:
+                        num_mandatory_pos_args += 1
+                elif param.kind == Parameter.VAR_POSITIONAL:
+                    has_varargs = True
+
+            if num_mandatory_pos_args > len(argument_types):
                 raise TypeCheckError(
-                    f"has too many arguments in its declaration; expected "
-                    f"{len(argument_types)} but {num_mandatory_args} argument(s) "
-                    f"declared"
+                    f"has too many mandatory positional arguments in its declaration; "
+                    f"expected {len(argument_types)} but {num_mandatory_pos_args} "
+                    f"mandatory positional argument(s) declared"
                 )
-            elif not has_varargs and num_mandatory_args < len(argument_types):
+            elif not has_varargs and num_positional_args < len(argument_types):
                 raise TypeCheckError(
                     f"has too few arguments in its declaration; expected "
-                    f"{len(argument_types)} but {num_mandatory_args} argument(s) "
+                    f"{len(argument_types)} but {num_positional_args} argument(s) "
                     f"declared"
                 )
 

tests/test_checkers.py

@@ -180,8 +180,8 @@ def some_callable(x: int, y: str, z: float) -> int:
         pytest.raises(
             TypeCheckError, check_type, some_callable, Callable[[int, str], int]
         ).match(
-            r"has too many arguments in its declaration; expected 2 but 3 "
-            r"argument\(s\) declared"
+            r"has too many mandatory positional arguments in its declaration; expected "
+            r"2 but 3 mandatory positional argument\(s\) declared"
         )
 
     def test_mandatory_kwonlyargs(self):
@@ -264,6 +264,12 @@ def test_concatenate(self):
         """Test that ``Concatenate`` in the arglist is ignored."""
         check_type([].append, Callable[Concatenate[object, P], Any])
 
+    def test_positional_only_arg_with_default(self):
+        def some_callable(x: int = 1, /) -> None:
+            pass
+
+        check_type(some_callable, Callable[[int], Any])
+
 
 class TestLiteral:
     def test_literal_union(self):