Python Cookbook, 3rd Edition

Chapter 7. Functions

Defining functions using the def statement is a cornerstone of all programs. The goal of this chapter is to present some more advanced and unusual function definition and usage patterns. Topics include default arguments, functions that take any number of arguments, keyword-only arguments, annotations, and closures. In addition, some tricky control flow and data passing problems involving callback functions are addressed.

7.1. Writing Functions That Accept Any Number of Arguments

Problem

You want to write a function that accepts any number of input arguments.

Solution

To write a function that accepts any number of positional arguments, use a * argument. For example:

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 is a tuple of all the extra positional arguments passed. The code treats it as a sequence in performing subsequent calculations.

To accept any number of keyword arguments, use an argument that starts with **. For example:

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>')

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