Python actual Mastering eleventh speaking - the function of any number of acceptable parameters

Constructing a function of any number of acceptable parameters.

In order to allow a function that takes an arbitrary number of location parameters, a * parameter may be used. E.g:

def avg(first, *rest):
    return (first + sum(rest)) / (1 + len(rest))

# Sample use
avg(1, 2) # 1.5
avg(1, 2, 3, 4) # 2.5

In this example, rest tuple parameters of all the other positions thereof. Then we regard it as the code to perform a sequence of subsequent calculations.

In order to receive any number of keyword arguments, beginning with the use of a ** parameters. such as:

import html

def make_element(name, value, **attrs):
    keyvals = [' %s="%s"' % item for item in attrs.items()]
    attr_str = ''.join(keyvals)
    element = '<{name}{attrs}>{value}</{name}>'.format(
                name=name,
                attrs=attr_str,
                value=html.escape(value))
    return element

# Example
# Creates '<item size="large" quantity="6">Albatross</item>'
make_element('item', 'Albatross', size='large', quantity=6)

# Creates '<p>&lt;spam&gt;</p>'
make_element('p', '<spam>')

Here, attrs is a dictionary of all incoming coming keyword arguments included.

If you want a function can also accept an arbitrary number of positional parameters and keyword parameters, you can use * and ** the same time. such as:

def anyargs(*args, **kwargs):
    print(args) # A tuple
    print(kwargs) # A dict

When using this function, all the parameters will be placed in the position of the tuple args, all keyword arguments will be placed in the dictionary kwargs.

A * parameter can appear in the function definition a position behind the last parameter, and  ** parameters appear only in the last parameter. One thing to note is that in the * parameter is still behind other parameters can be defined.

def a(x, *args, y):
    pass

def b(x, *args, y, **kwargs):
    pass