gh-103556: [inspect.Signature] disallow pos-or-kw params without default after pos-only with default

Lib/inspect.py

@@ -3021,7 +3021,12 @@ def __init__(self, parameters=None, *, return_annotation=_empty,
                                          kind.description)
                         raise ValueError(msg)
                     elif kind > top_kind:
-                        kind_defaults = False
+                        if (top_kind != _POSITIONAL_ONLY
+                                and kind != _POSITIONAL_OR_KEYWORD):
+                            # We still have to maintain defaults in cases like
+                            #    def some(pod=42, /, pk=1): ...
+                            # Here `pk` must have a default value.
+                            kind_defaults = False
                         top_kind = kind
 
                     if kind in (_POSITIONAL_ONLY, _POSITIONAL_OR_KEYWORD):

Lib/test/test_inspect.py

@@ -2462,18 +2462,43 @@ def test_signature_object(self):
         self.assertEqual(str(S()), '()')
         self.assertEqual(repr(S().parameters), 'mappingproxy(OrderedDict())')
 
-        def test(po, pk, pod=42, pkd=100, *args, ko, **kwargs):
+        def test(po, /, pk, pkd=100, *args, ko, kod=10, **kwargs):
             pass
+
         sig = inspect.signature(test)
-        po = sig.parameters['po'].replace(kind=P.POSITIONAL_ONLY)
-        pod = sig.parameters['pod'].replace(kind=P.POSITIONAL_ONLY)
+        self.assertTrue(repr(sig).startswith('<Signature'))
+        self.assertTrue('(po, /, pk' in repr(sig))
+
+        # We need two functions, because it is impossible to represent
+        # all param kinds in a single one.
+        def test2(pod=42, /):
+            pass
+
+        sig2 = inspect.signature(test2)
+        self.assertTrue(repr(sig2).startswith('<Signature'))
+        self.assertTrue('(pod=42, /)' in repr(sig2))
+
+        po = sig.parameters['po']
+        pod = sig2.parameters['pod']
         pk = sig.parameters['pk']
         pkd = sig.parameters['pkd']
         args = sig.parameters['args']
         ko = sig.parameters['ko']
+        kod = sig.parameters['kod']
         kwargs = sig.parameters['kwargs']
 
         S((po, pk, args, ko, kwargs))
+        S((po, pk, ko, kod))
+        S((po, pod, ko))
+        S((po, pod, kod))
+        S((pod, ko, kod))
+        S((pod, kod))
+        S((pod, args, kod, kwargs))
+        # keyword-only parameters without default values
+        # can follow keyword-only parameters with default values:
+        S((kod, ko))
+        S((kod, ko, kwargs))
+        S((args, kod, ko))
 
         with self.assertRaisesRegex(ValueError, 'wrong parameter order'):
             S((pk, po, args, ko, kwargs))
@@ -2494,15 +2519,18 @@ def test(po, pk, pod=42, pkd=100, *args, ko, **kwargs):
         with self.assertRaisesRegex(ValueError, 'follows default argument'):
             S((pod, po))
 
+        with self.assertRaisesRegex(ValueError, 'follows default argument'):
+            S((pod, pk))
+
+        with self.assertRaisesRegex(ValueError, 'follows default argument'):
+            S((po, pod, pk))
+
         with self.assertRaisesRegex(ValueError, 'follows default argument'):
             S((po, pkd, pk))
 
         with self.assertRaisesRegex(ValueError, 'follows default argument'):
             S((pkd, pk))
 
-        self.assertTrue(repr(sig).startswith('<Signature'))
-        self.assertTrue('(po, pk' in repr(sig))
-
     def test_signature_object_pickle(self):
         def foo(a, b, *, c:1={}, **kw) -> {42:'ham'}: pass
         foo_partial = functools.partial(foo, a=1)

Misc/NEWS.d/next/Library/2023-04-15-12-19-14.gh-issue-103556.TEf-2m.rst

@@ -0,0 +1,3 @@
+Now creating :class:`inspect.Signature` objects with positional-only
+parameter with a default followed by a positional-or-keyword parameter
+without one is impossible.