Asterisk in Python

The use of * in python is related to the way of passing parameters to a function. First, in python, we have three types of parameters, Positional-or-Keyword Arguments, Positional-Only Parameters and Keyword-Only Arguments. The way most frequently used is Positional-or-Keyword Arguments, so we can either specify the name of argument or not when passing arguments in order. * and ** are especially useful when the function has a varying/arbitrary number of parameters. The difference between * and ** is:

• *args = list of argument, positional arguments
• **kwargs = a dictionary of arguments, whose keys become separate keyword arguments and the values. become values of these arguments

use * and ** when defining function parameter

a) When defining a function, *args can pack an arbitrary number of positional arguments.

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def tuple_args(*args):
    ''' all arguments form a tuple '''
    for element in args:
        print(element)
    return '-'*10 + '{0}'.format(args[0]*5) + '-'*10

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tuple_args(1)

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'----------5----------'

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tuple_args(1, 2, 3)

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'----------5----------'

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tuple_args([1, 2, 3])

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[1, 2, 3]
'----------[1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3]----------'

b) **kwargs can pack an arbitrary number of keyword arguments.

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def dict_args(**kwargs):
    ''' all arguemnts in (keyword, value) pairs form a dictionary '''
    for (key, value) in kwargs.items():
        print(key, ':', value)
        
dict_args(one = 1, two = 2)

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one : 1
two : 2

c) mix the way of passing arguments

Both idioms can be mixed with normal arguments to allow a set of fixed and some variable arguments:

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def mix_args(*args, arg1, **kwargs):
    print('arg1 is', arg1)
    for element in args:
        print (element)
    for key, value in kwargs.items():
        print (key, ':', value)

mix_args(1, 2, arg1 = 3, five = 5, six = 6)

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arg1 is 3
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five : 5
six : 6

Note: arg1 must be explicitly specified so that we tell the function to pack the arguments after arg1 into kwargs and packge the arguments before arg1 into args. Otherwise, like the following call, it will cause an error.

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mix_args(1, 2, 3, five = 5, six = 6)

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---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-149-89543e4f8596> in <module>
----> 1 mix_args(1, 2, 3, five = 5, six = 6)

TypeError: mix_args() missing 1 required keyword-only argument: 'arg1'

But the more natural way to mix them is def mix_args(arg1, *args, **kwargs): so that the first argument can be passed by position.

use * and ** calling a function

When calling a function, the functionality of * and ** is opposite to that in defining a function; that is, * unpacks a list/tuple (any iterable data structure) and ** unpacks a dictionary into several arguments.

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def many_argments(arg1, arg2, arg3, arg4, arg5, arg6):
    print('arg1:', arg1, ', arg2:', arg2, ', arg3:', arg3, ', arg4:', arg4,  ', arg5:', arg5, ', arg6:', arg6)

a) * unpacks a list/tuple

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many_argments(1, *[2, 3, 4], arg5 = 5, arg6 = 6) # or *(2, 3, 4)

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arg1: 1 , arg2: 2 , arg3: 3 , arg4: 4 , arg5: 5 , arg6: 6

b) ** unpacks a dictionary

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many_argments(1, *(2, 3, 4), **{'arg5': 5, 'arg6': 6})
### or 
values = list(range(2, 7))
keys = ['arg'+str(i) for i in values]
dict_arg = dict(zip(keys, values))
many_argments(arg1 = 1, **dict_arg)

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arg1: 1 , arg2: 2 , arg3: 3 , arg4: 4 , arg5: 5 , arg6: 6

c) position arguments can NOT after keyword arguments

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many_argments(arg1 = 1, *(2, 3, 4), **{'arg5': 5, 'arg6': 6})

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---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-202-c1452a56ac4f> in <module>
----> 1 many_argments(arg1 = 1, *(2, 3, 4), **{'arg5': 5, 'arg6': 6})

TypeError: many_argments() got multiple values for argument 'arg1'

unpack a list

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first, *middle, last = list(range(1, 6))
print('first is', first, '-'*3,  'middle is', middle, '-'*3, 'last is', last)

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first is 1 --- middle is [2, 3, 4] --- last is 5

References

1. * and ** as parameters when defining a function
2. * in function call
3. Python document of * and **