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

Python supports variable parameters. The simplest method is to use default parameters. This document describes the use of * args and ** kwargs.

Variable parameters with default parameters

Python supports variable parameters. The simplest method is to use default parameters. for example:

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

When defining a function, use * args and ** kwargs

Of course, this article mainly describes a method to achieve Variable Argument: use the * args and ** kwargs syntax. Here, * args is the variable positional arguments list, and ** kwargs is the variable keyword arguments list. In addition, * args must be located before ** kwargs, because positional arguments must be located before keyword arguments.

First, we will introduce the basic usage of the two.

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

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

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

When calling a function, use * args and ** kwargs

* Args and ** kwargs syntaxes can be used not only in function definitions, but also in function calls. The difference is that if * args and ** kwargs are used in the function definition position to pack the parameter, it is a process of unpack the parameter 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

You can use * args and ** kwargs to easily define functions and enhance scalability 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)

Note:

Note: When defining or calling such a function, follow the following rules:
Variable parameters must follow the immutable parameters
* Args is a variable positional arguments list, and ** kwargs is a variable keyword arguments list. In addition, * args must be located before ** kwargs, because positional arguments must be located before keyword arguments