gh-107831: Fix inspect.getcallargs accepting positional-only args by keyword

getcallargs() built the set of acceptable keyword names from getfullargspec's
'args', which includes positional-only parameters, so a positional-only
parameter could be filled by keyword without error -- unlike an actual call
or Signature.bind(). Exclude positional-only names (derived from the code
object's co_posonlyargcount) from keyword matching and raise the same
TypeError the interpreter does.

Author: iamsharduld

Lib/inspect.py

@@ -1466,16 +1466,26 @@ def getcallargs(func, /, *positional, **named):
     num_args = len(args)
     num_defaults = len(defaults) if defaults else 0
 
+    # Positional-only parameters cannot be filled by keyword, matching the
+    # behaviour of an actual call and Signature.bind() (gh-107831).
+    func_obj = func.__func__ if ismethod(func) else func
+    code = getattr(func_obj, '__code__', None)
+    posonlyargs = set(args[:code.co_posonlyargcount]) if code is not None else set()
+
     n = min(num_pos, num_args)
     for i in range(n):
         arg2value[args[i]] = positional[i]
     if varargs:
         arg2value[varargs] = tuple(positional[n:])
-    possible_kwargs = set(args + kwonlyargs)
+    possible_kwargs = set(args + kwonlyargs) - posonlyargs
     if varkw:
         arg2value[varkw] = {}
+    posonly_passed_as_kwarg = []
     for kw, value in named.items():
         if kw not in possible_kwargs:
+            if kw in posonlyargs and not varkw:
+                posonly_passed_as_kwarg.append(kw)
+                continue
             if not varkw:
                 raise TypeError("%s() got an unexpected keyword argument %r" %
                                 (f_name, kw))
@@ -1485,6 +1495,10 @@ def getcallargs(func, /, *positional, **named):
             raise TypeError("%s() got multiple values for argument %r" %
                             (f_name, kw))
         arg2value[kw] = value
+    if posonly_passed_as_kwarg:
+        raise TypeError(
+            "%s() got some positional-only arguments passed as keyword "
+            "arguments: %r" % (f_name, ', '.join(posonly_passed_as_kwarg)))
     if num_pos > num_args and not varargs:
         _too_many(f_name, args, kwonlyargs, varargs, num_defaults,
                    num_pos, arg2value)

Lib/test/test_inspect/test_inspect.py

@@ -2266,6 +2266,22 @@ def test_plain(self):
         self.assertEqualCallArgs(f, '2, **collections.UserDict(b=3)')
         self.assertEqualCallArgs(f, 'b=2, **collections.UserDict(a=3)')
 
+    def test_positional_only(self):
+        # gh-107831: positional-only parameters cannot be filled by keyword,
+        # so getcallargs must match a real call (raise rather than bind).
+        f = self.makeCallable('a, b, /, c, d=1, *, e')
+        self.assertEqualCallArgs(f, '1, 2, 3, e=4')
+        self.assertEqualCallArgs(f, '1, 2, c=3, e=4')
+        self.assertEqualCallArgs(f, '1, 2, 3, 4, e=5')
+        self.assertEqualException(f, 'a=1, b=2, c=3, e=4')
+        self.assertEqualException(f, '1, b=2, c=3, e=4')
+        # With **kwargs the positional-only name is absorbed into it and the
+        # parameter itself is left unfilled (a missing-argument error).
+        g = self.makeCallable('a, /, **kw')
+        self.assertEqualCallArgs(g, '1')
+        self.assertEqualCallArgs(g, '1, x=2')
+        self.assertEqualException(g, 'a=1')
+
     def test_varargs(self):
         f = self.makeCallable('a, b=1, *c')
         self.assertEqualCallArgs(f, '2')

Misc/NEWS.d/next/Library/2026-06-22-15-30-00.gh-issue-107831.Np4kZc.rst

@@ -0,0 +1,3 @@
+:func:`inspect.getcallargs` now raises :exc:`TypeError` when a positional-only
+parameter is passed by keyword, consistent with calling the function directly
+and with :meth:`inspect.Signature.bind`.