python *args and * 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)输出结果如下：

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，这两个是python中的可变参数。*args表示任何多个无名参数，它是一个tuple；**kwargs表示关键字参数，它是一个 dict。并且同时使用*args和**kwargs时，必须*args参数列要在**kwargs前，像foo(a=1, b='2', c=3, a', 1, None, )这样调用的话，会提示语法错误“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 as

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