python 中参数*args, **kwargs

た入场券 2024-04-17 15:26 12阅读 0赞

## python 中参数\*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)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”。

\-------------------------------------------------------------------------------------------

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()

\--------------------------------------------------------------------------------------------

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" )`