Do not treat a walrus argument as a keyword argument (#242)

Closes #212

parso/python/errors.py

@@ -936,6 +936,9 @@ class _ArglistRule(SyntaxRule):
                             self.add_issue(argument, message=message)
                     else:
                         kw_unpacking_only = True
+                elif argument.children[1] == ':=':
+                    # f(a := 1) is a positional argument, not a keyword one.
+                    pass
                 else:  # Is a keyword argument.
                     kw_only = True
                     if first.type == 'name':

test/test_python_errors.py

@@ -366,6 +366,8 @@ def test_valid_fstrings(code):
         'a[(b:=0)]',
         'a[(b:=0, c:=0)]',
         'a[(b:=0):1:2]',
+        'f(a := 1, b)',
+        'f(a := 1, b, c := 2)',
     ]
 )
 def test_valid_namedexpr(code):