任意參數*和**的使用

當函數的參數可能爲任意個時，參數列表使用*或者**代替，如

其中*代表列表，**代表dictionary，訪問的方式同列表和dictionary

def write_multiple_items(file, separator, *args):
    file.write(separator.join(args))

除了作爲任意參數解析，*和**還有一個作用，就是將數組或列表轉換爲位置參數，或者關鍵字參數，如：

>>> range(3, 6)             # normal call with separate arguments
[3, 4, 5]
>>> args = [3, 6]
>>> range(*args)            # call with arguments unpacked from a list
[3, 4, 5]

>>> def parrot(voltage, state='a stiff', action='voom'):
...     print "-- This parrot wouldn't", action,
...     print "if you put", voltage, "volts through it.",
...     print "E's", state, "!"
...
>>> d = {"voltage": "four million", "state": "bleedin' demised", "action": "VOOM"}
>>> parrot(**d)
-- This parrot wouldn't VOOM if you put four million volts through it. E's bleedin' demised !

使用*和**有用的實踐

1、子類複用父類的方法，而不用管父類如何實現，即使父類發生變化，也不影響到子類

class Foo(object):
    def __init__(self, value1, value2):
        # do something with the values
        print value1, value2

class MyFoo(Foo):
    def __init__(self, *args, **kwargs):
        # do something else, don't care about the args
        print 'myfoo'
        super(MyFoo, self).__init__(*args, **kwargs)

參考：

http://docs.python.org/2/tutorial/controlflow.html#arbitrary-argument-lists

http://stackoverflow.com/questions/3394835/args-and-kwargs