Understanding *args and **kwargs in Python

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

def test_defargs(one, two = 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

Of course, the topic in this article is not about default parameters, but another way 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: ‘, vtest_args(1, 2, 3, 4)# result:# Required argument: 1# Optional argument:  2# Optional argument:  3# Optional argument:  4下面一个例子使用*kwargs, 同时包含一个必须的参数和*args列表：def test_kwargs(first, *args, **kwargs):   print ‘Required argument: ‘, first   for v in args:      print ‘Optional argument (*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:  1# Optional argument (*args):  2# Optional argument (*args):  3# Optional argument (*args):  4# Optional argument k2 (*kwargs): 6# Optional argument k1 (*kwargs): 5

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: ‘, second    print ‘Third argument: ‘, third    print ‘Fourth argument: ‘, fourth    print ‘Fifth argument: ‘, fifth# Use *argsargs = [1, 2, 3, 4, 5]test_args(*args)# results:# First argument:  1# Second argument:  2# Third argument:  3# Fourth argument:  4# Fifth argument:  5# Use **kwargskwargs = {    ‘first‘: 1,    ‘second‘: 2,    ‘third‘: 3,    ‘fourth‘: 4,    ‘fifth‘: 5}test_args(**kwargs)# results:# First argument:  1# Second argument:  2# 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.

Understanding *args and **kwargs in Python