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

Use variable parameters with default parameters

Python supports variable parameters. The easiest way is to use default parameters, for example:

def test_defargs(one, two = 2):
   print 'Required argument: ', one
   print 'Optional argument: ', two

test_defargs(1)
# result:
# Required argument: 1
# Optional argument: 2

test_defargs(1, 3)
# result:
# Required argument: 1
# Optional argument: 3

Use *args and **kwargs when defining the function

Of course, this article mainly Let’s talk about a way to achieve variable parameters (Variable Argument): use *args and **kwargs syntax. Among them, *args is a variable positional arguments list, and **kwargs is a variable keyword arguments list. Also, *args must come before **kwargs because positional arguments must come before keyword arguments.

First 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: ', v

test_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 list of required parameters and *args:

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 syntax can be used not only in function definitions, but also when calling functions. The difference is that if using *args and **kwargs at the location where the function is defined is a process of packing parameters, then when the function is called, it is a process of unpacking parameters. Let’s use an example to deepen our 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 *args
args = [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 **kwargs
kwargs = {
    '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

Using *args and **kwargs can be very convenient to define functions, and can also 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 this kind of function, follow the following rules:
Variable parameters must come after immutable parameters
*args is a variable positional arguments list, **kwargs is A variable list of keyword arguments. And, *args must be before **kwargs, because positional arguments must be before keyword arguments