Issue #14328: Add keyword-only parameters to PyArg_ParseTupleAndKeywords.

They're optional-only for now (unlike in pure Python) but that's all
I needed.  The syntax can easily be relaxed if we want to support
required keyword-only arguments for extension types in the future.

Lib/test/test_getargs2.py

@@ -1,6 +1,6 @@
 import unittest
 from test import support
-from _testcapi import getargs_keywords
+from _testcapi import getargs_keywords, getargs_keyword_only
 
 """
 > How about the following counterproposal. This also changes some of
@@ -293,6 +293,77 @@ class Keywords_TestCase(unittest.TestCase):
         else:
             self.fail('TypeError should have been raised')
 
+class KeywordOnly_TestCase(unittest.TestCase):
+    def test_positional_args(self):
+        # using all possible positional args
+        self.assertEqual(
+            getargs_keyword_only(1, 2),
+            (1, 2, -1)
+            )
+
+    def test_mixed_args(self):
+        # positional and keyword args
+        self.assertEqual(
+            getargs_keyword_only(1, 2, keyword_only=3),
+            (1, 2, 3)
+            )
+
+    def test_keyword_args(self):
+        # all keywords
+        self.assertEqual(
+            getargs_keyword_only(required=1, optional=2, keyword_only=3),
+            (1, 2, 3)
+            )
+
+    def test_optional_args(self):
+        # missing optional keyword args, skipping tuples
+        self.assertEqual(
+            getargs_keyword_only(required=1, optional=2),
+            (1, 2, -1)
+            )
+        self.assertEqual(
+            getargs_keyword_only(required=1, keyword_only=3),
+            (1, -1, 3)
+            )
+
+    def test_required_args(self):
+        self.assertEqual(
+            getargs_keyword_only(1),
+            (1, -1, -1)
+            )
+        self.assertEqual(
+            getargs_keyword_only(required=1),
+            (1, -1, -1)
+            )
+        # required arg missing
+        with self.assertRaisesRegex(TypeError,
+            "Required argument 'required' \(pos 1\) not found"):
+            getargs_keyword_only(optional=2)
+
+        with self.assertRaisesRegex(TypeError,
+            "Required argument 'required' \(pos 1\) not found"):
+            getargs_keyword_only(keyword_only=3)
+
+    def test_too_many_args(self):
+        with self.assertRaisesRegex(TypeError,
+            "Function takes at most 2 positional arguments \(3 given\)"):
+            getargs_keyword_only(1, 2, 3)
+
+        with self.assertRaisesRegex(TypeError,
+            "function takes at most 3 arguments \(4 given\)"):
+            getargs_keyword_only(1, 2, 3, keyword_only=5)
+
+    def test_invalid_keyword(self):
+        # extraneous keyword arg
+        with self.assertRaisesRegex(TypeError,
+            "'monster' is an invalid keyword argument for this function"):
+            getargs_keyword_only(1, 2, monster=666)
+
+    def test_surrogate_keyword(self):
+        with self.assertRaisesRegex(TypeError,
+            "'\udc80' is an invalid keyword argument for this function"):
+            getargs_keyword_only(1, 2, **{'\uDC80': 10})
+
 class Bytes_TestCase(unittest.TestCase):
     def test_c(self):
         from _testcapi import getargs_c
@@ -441,6 +512,7 @@ def test_main():
         Unsigned_TestCase,
         Tuple_TestCase,
         Keywords_TestCase,
+        KeywordOnly_TestCase,
         Bytes_TestCase,
         Unicode_TestCase,
     ]