random

The random module implements pseudo-random number generators for various distributions. It is useful for simulations, games, security (with caveats), and statistical sampling. Python uses the Mersenne Twister as the core generator.

Quick Reference

Function	Description
random()	Random float in [0.0, 1.0)
randint(a, b)	Random integer from a to b (inclusive)
randrange(start, stop, step)	Random integer from range
choice(seq)	Random element from sequence
shuffle(x)	Shuffle list in place
sample(population, k)	Sample k unique elements
seed(a, version)	Initialize generator
uniform(a, b)	Random float in [a, b]
triangular(low, high, mode)	Triangular distribution

Seeding the Generator

Set the seed for reproducible results.

import random

random.seed(42)
print(random.random())
# 0.6394267984578837

# Same seed = same sequence
random.seed(42)
print(random.random())
# 0.6394267984578837

Without seeding, Python seeds from the operating system entropy source.

Basic Random Numbers

random()

Returns a random float in the range [0.0, 1.0).

import random

for _ in range(5):
    print(random.random())
# 0.13334722086717508
# 0.8471536681046147
# 0.3708787306102069
# 0.7296064703100954
# 0.60140939580676593

randint(a, b)

Returns a random integer N where a <= N <= b. Both endpoints are inclusive.

import random

# Roll a six-sided die
die_roll = random.randint(1, 6)
print(die_roll)
# 4

# Generate a random password digit
digit = random.randint(0, 9)
print(digit)
# 7

randrange(start, stop, step)

Like range(), returns a random integer from the sequence. Does not include the stop value.

import random

# Random even number from 0 to 100
even = random.randrange(0, 101, 2)
print(even)
# 42

# Random month (1-12)
month = random.randrange(1, 13)
print(month)
# 8

Selecting from Sequences

choice(seq)

Returns a single random element from a non-empty sequence.

import random

colors = ["red", "green", "blue", "yellow"]
print(random.choice(colors))
# green

# Pick a random letter
import string
letter = random.choice(string.ascii_uppercase)
print(letter)
# M

sample(population, k)

Returns a list of k unique elements from population without replacement.

import random

# Deal 5 cards from a deck
deck = list(range(1, 53))
hand = random.sample(deck, 5)
print(hand)
# [17, 42, 8, 23, 51]

# Sample without replacement
items = ["a", "b", "c", "d", "e"]
selection = random.sample(items, 3)
print(selection)
# ['c', 'a', 'e']

shuffle(x)

Shuffles a list in place. Returns None.

import random

cards = ["A", "2", "3", "4", "5", "6", "7", "8", "9", "10", "J", "Q", "K"]
random.shuffle(cards)
print(cards)
# ['Q', '3', 'K', '5', 'J', '9', '7', '2', '8', '6', 'A', '4', '10']

# Fisher-Yates shuffle - works on any sequence
numbers = [1, 2, 3, 4, 5]
random.shuffle(numbers)
print(numbers)
# [3, 1, 5, 2, 4]

Floating-Point Distributions

uniform(a, b)

Returns a random float N where a <= N <= b.

import random

# Random temperature between 20 and 25 Celsius
temp = random.uniform(20, 25)
print(f"Temperature: {temp:.1f}C")
# Temperature: 22.7C

# Random delay between 0.5 and 2.0 seconds
delay = random.uniform(0.5, 2.0)
print(f"Delay: {delay:.3f}s")
# Delay: 1.234s

triangular(low, high, mode)

Returns a random float from a triangular distribution. The mode argument defaults to the midpoint.

import random

# Simulate wait times with known average
for _ in range(5):
    wait = random.triangular(5, 15, 10)  # min=5, max=15, mode=10
    print(f"Wait time: {wait:.1f} minutes")
# Wait time: 9.2 minutes
# Wait time: 7.8 minutes
# Wait time: 11.4 minutes
# Wait time: 10.1 minutes
# Wait time: 6.5 minutes

Common Patterns

Random password generation

import random
import string

def generate_password(length=16):
    chars = string.ascii_letters + string.digits + string.punctuation
    return ''.join(random.choice(chars) for _ in range(length))

print(generate_password())
# kR9#mP2$vL5@nQ8

Random item with weights

import random

# Weighted selection using random.choices
items = ["common", "rare", "epic", "legendary"]
weights = [70, 20, 8, 2]

for _ in range(10):
    loot = random.choices(items, weights=weights, k=1)[0]
    print(loot)
# common
# common
# rare
# common
# common
# common
# epic
# common
# common
# legendary

Shuffling a string

import random

def shuffle_string(s):
    chars = list(s)
    random.shuffle(chars)
    return ''.join(chars)

print(shuffle_string("hello"))
# lohel

Security Considerations

The Mersenne Twister is not cryptographically secure. For security-sensitive randomness (passwords, tokens, keys), use the secrets module instead:

import secrets

# Generate a secure random token
token = secrets.token_urlsafe(32)
print(token[:20] + "...")
# q8XKLm2pMzN5Rv...

# Secure random integer in range
secure_num = secrets.randbelow(1000000)
print(secure_num)
# 847293

See Also

• datetime
• calendar
• time