How to use the Random Module in Python

Overview

In this post, I would like to describe the usage of the random module in Python. The random module provides access to functions that support many operations. Perhaps the most important thing is that it allows you to generate random numbers.

When to use it?

We want the computer to pick a random number in a given range Pick a random element from a list, pick a random card from a deck, flip a coin etc. When making your password database more secure or powering a random page feature of your website.

Random functions

The Random module contains some very useful functions.

Randint

If we wanted a random integer, we can use the randint function Randint accepts two parameters: a lowest and a highest number. Generate integers between 1,5. The first value should be less than the second.

import random
print random.randint(0, 5)

This will output either 1, 2, 3, 4 or 5.

Random

If you want a larger number, you can multiply it.

For example, a random number between 0 and 100:

import random
random.random() * 100

Choice

Generate a random value from the sequence sequence.

random.choice( ['red', 'black', 'green'] ).

The choice function can often be used for choosing a random element from a list.

import random
myList = [2, 109, False, 10, "Lorem", 482, "Ipsum"]
random.choice(myList)

Shuffle

The shuffle function, shuffles the elements in list in place, so they are in a random order.

random.shuffle(list) Example taken from this post on Stackoverflow

from random import shuffle
x = [[i] for i in range(10)]
shuffle(x)

Output:
# print x  gives  [[9], [2], [7], [0], [4], [5], [3], [1], [8], [6]]
# of course your results will vary

Randrange

Generate a randomly selected element from range(start, stop, step)

random.randrange(start, stop[, step])

import random
for i in range(3):
    print random.randrange(0, 101, 5)

Code Example

Let’s see this example (copied from Doug Hellmann PYMOTW)

import random
import itertools

outcomes = { 'heads':0,
             'tails':0,
             }
sides = outcomes.keys()

for i in range(10000):
    outcomes[ random.choice(sides) ] += 1

print 'Heads:', outcomes['heads']
print 'Tails:', outcomes['tails']

There are only two outcomes allowed, so rather than use numbers and convert them, the words “heads” and “tails” are used with choice().

The results are tabulated in a dictionary using the outcome names as keys.

$ python random_choice.py

Heads: 4984
Tails: 5016

More information

http://en.wikibooks.org/wiki/Choose_Your_Own_Pyventure/Random_and_PRNGs
http://docs.python.org/2/library/random.html
http://www.cs.swarthmore.edu/~adanner/cs21/f09/randomlib.php