python字典合并哪种方式效率最高?

http://www.dewen.org/q/2002/python%E5%AD%97%E5%85%B8%E5%90%88%E5%B9%B6%E5%93%AA%E7%A7%8D%E6%96%B9%E5%BC%8F%E6%95%88%E7%8E%87%E6%9C%80%E9%AB%98%3F
>>> x = {'a':1, 'b': 2}

>>> y = {'b':10, 'c': 11}

>>> z = dict(x.items()+y.items())

>>> z

{'a': 1, 'c': 11, 'b': 10}

>>> z2 = dict(z,**y)

>>> z2

{'a': 1, 'c': 11, 'b': 10}

>>> z3 = x.copy()

>>> z3.update(y)

>>> z3

{'a': 1, 'c': 11, 'b': 10}

>>> z4=(lambda a, b: (lambda a_copy: a_copy.update(b) or a_copy)(a.copy()))(x, y)

>>> z4

{'a': 1, 'c': 11, 'b': 10}

>>>

方法四的效率最高，方法二和方法三差不多，较方法四效率次之，方法一效率最差。

参考： http://stackoverflow.com/questions/38987/how-can-i-merge-union-two-python-dictionaries-in-a-single-expression http://www.cnblogs.com/dkblog/archive/2012/02/02/2336089.html