Nick, thanks for validating my pain. :-| In the meantime, I found out Python already knows more finely what kind of expression something is, and uses that knowledge in the error messages. Look: >>> a + b = 4 SyntaxError: can't assign to operator >>> a(b) = 4 SyntaxError: can't assign to function call >>> 'a' = 4 SyntaxError: can't assign to literal So, how about "keyword argument name cannot be an operator / function call / literal"? (Of course, if you ask me, "operator" is also unfortunate, but at least there is a precedent.)