Understanding *args and **kwargs of s variable parameters in Python

Using variable parameters with default parameters

Python is the simplest way to support mutable parameters, such as using default parameters, such as:

Def Test_defargs (one, 2):   print ' Required argument: ', one   print ' Optional argument: ', Twotest_defargs (1) # result:# Required argument:1# Optional Argument:2test_defargs (1, 3) # result:# Required argument:1# Optional argument:3

Use *args and **kwargs when defining a function

Of course, this article focuses on a method to achieve variable parameters (Variable Argument): Using *args and **kwargs syntax. Where *args is a mutable list of positional arguments, **kwargs is a variable keyword arguments list. Also, *args must precede **kwargs, because positional arguments must precede keyword arguments.

The basic usage of both is introduced first.

The following example uses *args and contains a required parameter:

Def Test_args (First, *args):   print ' Required argument: ', first for   v in args:      print ' Optional argument: ', VT Est_args (1, 2, 3, 4) # result:# Required argument:1# Optional argument: Optional argument  :  3# Optional argument :  4

The following example uses *kwargs and contains a list of required parameters and *args:

Def Test_kwargs (First, *args, **kwargs):   print ' Required argument: ', first for   v in args:      print ' Optional arg  Ument (*args): ', V for   K, V in Kwargs.items ():      print ' Optional argument%s (*kwargs):%s '% (k, v) Test_kwargs (1, 2, 3, 4, k1=5, k2=6) # results:# Required argument: #  Optional argument (*args): *  Optional argument (*args): c13/>3# Optional argument (*args):  4# Optional argument K2 (*kwargs): 6# Optional argument K1 (*kwargs): 5

When a function is called, use *args and **kwargs

The *args and **kwargs grammars can be used not only in function definitions, but also in function calls. The difference is that if you use *args and **kwargs at the location where the function is defined is a process that takes the parameter pack, then it is a procedure to unpack the parameters when the function is called. The following example is used to deepen understanding:

Def Test_args (first, second, third, fourth, fifth):    print ' first argument: ', first    print ' second argument: ', sec Ond    print ' third argument: ', third    print ' fourth argument: ', fourth    print ' fifth argument: ', fifth# use *ar Gsargs = [1, 2, 3, 4, 5]test_args (*args) # results:# first argument: #  Second argument: third  argument:  3# Fourth argument:  4# fifth argument:  AA Use **kwargskwargs = {    ' first ': 1,    ' second ': 2,    ' third ': 3 ,    ' fourth ': 4,    ' fifth ': 5}test_args (**kwargs) # results:# first argument: #  Second argument:  * Third argument:  3# fourth argument:  4# fifth argument:  5

Functions can be easily defined using *args and **kwargs, and extensibility can be enhanced for future code maintenance.

Example

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)

Attention

Note: When defining or invoking this function, the following rules apply:
The variable parameter must be after the immutable parameter
*args is a mutable list of positional arguments, **kwargs is a variable keyword arguments list. Also, *args must be located before **kwargs, because positional arguments must be located before keyword arguments

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