gh-130164: Fix inspect.Signature.bind() handling of positional-only args without defaults

Lib/inspect.py

@@ -3077,6 +3077,9 @@ def _bind(self, args, kwargs, *, partial=False):
                         break
                     elif param.name in kwargs:
                         if param.kind == _POSITIONAL_ONLY:
+                            if param.default is _empty:
+                                msg = f'missing a required positional-only argument: {param.name!r}'
+                                raise TypeError(msg)
                             # Raise a TypeError once we are sure there is no
                             # **kwargs param later.
                             pos_only_param_in_kwargs.append(param)

Lib/test/test_inspect/test_inspect.py

@@ -5149,7 +5149,11 @@ class TestSignatureBind(unittest.TestCase):
     def call(func, *args, **kwargs):
         sig = inspect.signature(func)
         ba = sig.bind(*args, **kwargs)
-        return func(*ba.args, **ba.kwargs)
+        # Prevent unexpected success of assertRaises(TypeError, ...)
+        try:
+            return func(*ba.args, **ba.kwargs)
+        except TypeError as e:
+            raise AssertionError from e
 
     def test_signature_bind_empty(self):
         def test():
@@ -5349,7 +5353,7 @@ def test(a_po, b_po, c_po=3, /, foo=42, *, bar=50, **kwargs):
         self.assertEqual(self.call(test, 1, 2, c_po=4),
                          (1, 2, 3, 42, 50, {'c_po': 4}))
 
-        with self.assertRaisesRegex(TypeError, "missing 2 required positional arguments"):
+        with self.assertRaisesRegex(TypeError, "missing a required positional-only argument: 'a_po'"):
             self.call(test, a_po=1, b_po=2)
 
         def without_var_kwargs(c_po=3, d_po=4, /):

Misc/NEWS.d/next/Library/2025-02-16-08-56-48.gh-issue-130164.vvoaU2.rst

@@ -0,0 +1,3 @@
+Fixed failure to raise :exc:`TypeError` in :meth:`inspect.Signature.bind`
+for positional-only arguments provided by keyword when a variadic keyword
+argument (e.g. ``**kwargs``) is present.