k not in a Equivalent to not k in a 字典中不存在键k则为返回true,反之返回False (2) a.has_key(k) Equivalent to k in a, use that form in new code 等价于k in a a.items() a copy of a's list of (key, value) pairs 得到一个键,值的list (3) a.keys() a copy of a's list of keys 得到