python 中參數*args, **kwargs

def foo(*args, **kwargs):
print 'args?= ',?args
print 'kwargs?= ',?kwargs
print '---------------------------------------'if __name__ == '__main__':
foo(1,2,3,4)
foo(a=1,b=2,c=3)
foo(1,2,3,4, a=1,b=2,c=3)
foo('a', 1, None, a=1, b='2', c=3)輸出結果如下：args?=? (1, 2, 3, 4)?
kwargs?=? {}?args?=? ()?
kwargs?=? {'a': 1, 'c': 3, 'b': 2}?args?=? (1, 2, 3, 4)?
kwargs?=? {'a': 1, 'c': 3, 'b': 2}?args?=? ('a', 1, None)?
kwargs?=? {'a': 1, 'c': 3, 'b': '2'}

可以看到，這兩個是python中的可變參數。*args表示任何多個無名參數，它是一個tuple；**kwargs表示關鍵字參數，它是一個 dict。并且同時使用*args和**kwargs時，必須*args參數列要在**kwargs前，像foo(a=1, b='2', c=3, a', 1, None, )這樣調用的話，會提示語法錯誤“SyntaxError: non-keyword arg after keyword arg”。

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class testClass:
def?__init__(self, **kwargs):
self.kwargs?=?kwargs
def print_kwargs(self):
i = 1
for key in?self.kwargs.keys():
print "Kwarg %s (%s: %s)" % (i, key,?self.kwargs[key])
i += 1
tc = testClass(hello=True, anumber=5, thisisgreat='Yes it is!')
tc.print_kwargs()

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Using **kwargs?and default values is easy. Sometimes, however, you shouldn't be using **kwargs?in the first place.

In this case, we're not really making best use of **kwargs.

class ExampleClass( object ):def?__init__(self, **kwargs):self.val =?kwargs.get('val',"default1")self.val2 =?kwargs.get('val2',"default2")The above is a "why bother?" declaration. It is the same asclass ExampleClass( object ):def?__init__(self, val="default1", val2="default2"):self.val = valself.val2 = val2

When you're using **kwargs, you mean that a keyword is not just optional, but conditional. There are more complex rules than simple default values.

When you're using **kwargs, you usually mean something more like the following, where simple defaults don't apply.

class ExampleClass( object ):def?__init__(self, **kwargs):self.val = "default1"self.val2 = "default2"if "val" in?kwargs:self.val =?kwargs["val"]self.val2 = 2*self.valelif "val2" in?kwargs:self.val2 =?kwargs["val2"]self.val =?self.val2 / 2else:raise TypeError( "must provide val= or val2= parameter values" )

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