random library

Official documentation

The random module generates pseudo-random numbers and makes random selections — rolling a dice, shuffling a deck, or picking a specimen to feature at random.

Install

random ships with Python's standard library — nothing to install.

Import

The whole module is used through the random. prefix, so a plain import is all you need.

import random

Function	Returns	Example
`random()`	A random float between `0.0` and `1.0`	`0.37454`
`randint(a, b)`	A random integer, `a` and `b` both included	`randint(1, 6)` → `4`
`choice(seq)`	One random item from a sequence	`choice(species)`
`shuffle(seq)`	Reorders a list in place, returns `None`	`shuffle(species)`
`sample(seq, k)`	`k` unique random items, as a new list	`sample(species, 2)`

Random numbers

random() and randint() are the two basic building blocks — a random fraction, or a random whole number within a range.

import random

print(random.random())
print(random.randint(1, 6))

Setting a seed

Locks the sequence of "random" numbers that follow it, so the same seed always produces the same results. Useful for a reproducible example or a test, where genuine randomness would make output impossible to predict or verify.

random.seed(0)
random.randint(1, 6)    # always 4, whenever the seed is 0

Run a random numbers example

All the examples above, combined into one script:

import random

print(random.random())
print(random.randint(1, 6))

import random

random.seed(0)
print(random.randint(1, 6))
print(random.randint(1, 6))

Random selections

choice() picks one item at random from an existing sequence — no need to generate a number and index into the list by hand.

import random

species = ["ball", "burmese", "boa", "blood"]

print(random.choice(species))

Shuffling a list

Reorders a list randomly, in place. shuffle() returns None, so the point is the side effect on species itself, not a return value to assign.

species = ["ball", "burmese", "boa", "blood"]
random.shuffle(species)
species    # e.g. ["boa", "ball", "blood", "burmese"] — order is randomized

Sampling without replacement

Picks several items at once, all guaranteed distinct. Unlike calling choice() in a loop, which could return the same item twice. The original list is left unchanged; sample() returns a new list.

species = ["ball", "burmese", "boa", "blood"]
random.sample(species, 2)    # e.g. ["burmese", "blood"] — 2 distinct items

Run a random selections example

All the examples above, combined into one script:

import random

species = ["ball", "burmese", "boa", "blood"]

print(random.choice(species))

import random

species = ["ball", "burmese", "boa", "blood"]
random.shuffle(species)
print(species)

import random

species = ["ball", "burmese", "boa", "blood"]
print(random.sample(species, 2))