Unit 12.1A · Term 1

Random Library

Python's random module generates pseudo-random numbers — numbers that look random but are produced by a deterministic algorithm called a seed.

Learning Objectives

12.6.1.1 Use the functions of the library Random to get a pseudo-random number
12.6.1.3 Identify code snippets that use random sequence generation

Lesson Presentation

12.1A-random-library.pdf · Slides for classroom use

View Download

Conceptual Anchor

The Loaded Dice Analogy

A pseudo-random number is like rolling dice — the result looks random, but if you knew the exact position, spin, and force, you could predict it. The seed is that starting position. Same seed → same "random" sequence every time.

Rules & Theory

Key random Functions

Function	Description	Example
`random.random()`	Float in [0.0, 1.0)	`0.37444887175646646`
`random.randint(a, b)`	Integer in [a, b] (inclusive)	`random.randint(1, 6)` → 4
`random.randrange(start, stop, step)`	Random from range (exclusive stop)	`random.randrange(0, 100, 5)` → 35
`random.uniform(a, b)`	Float in [a, b]	`random.uniform(1.5, 9.5)` → 6.28
`random.choice(seq)`	Random element from sequence	`random.choice(["a","b","c"])` → "b"
`random.shuffle(list)`	Shuffle list in-place	`random.shuffle(cards)`
`random.sample(seq, k)`	k unique elements from sequence	`random.sample(range(100), 5)`
`random.seed(n)`	Set seed for reproducible results	`random.seed(42)`

Import Methods

import random              # Use as: random.randint(1, 6)
from random import randint  # Use as: randint(1, 6)
from random import *        # All functions without prefix (not recommended)

Worked Examples

1 Simulating a Dice Roll

import random

# Roll a standard 6-sided die
roll = random.randint(1, 6)
print(f"You rolled: {roll}")

# Roll two dice
dice1 = random.randint(1, 6)
dice2 = random.randint(1, 6)
total = dice1 + dice2
print(f"Dice: {dice1} + {dice2} = {total}")

2 Generating a Random Password

import random
import string

# All possible characters
chars = string.ascii_letters + string.digits + "!@#$%"

# Generate 12-character password
password = ''.join(random.choice(chars) for _ in range(12))
print(f"Password: {password}")
# Example output: Password: kR7$mPx2Qn!v

3 Reproducible Results with seed()

import random

random.seed(42)
print(random.randint(1, 100))  # Always: 82
print(random.randint(1, 100))  # Always: 15
print(random.randint(1, 100))  # Always: 4

# Reset seed → same sequence!
random.seed(42)
print(random.randint(1, 100))  # Again: 82

4 Identifying Random Code Snippets

# Which snippets generate random sequences?

# ✓ YES — generates random sequence of 5 numbers
nums = [random.randint(1, 50) for _ in range(5)]

# ✓ YES — randomly shuffles a list
deck = list(range(1, 53))
random.shuffle(deck)

# ✗ NO — deterministic, not random
nums = [i * 2 for i in range(5)]  # [0, 2, 4, 6, 8]

# ✓ YES — samples 3 unique random items
winners = random.sample(["Alice","Bob","Clara","Dan"], 3)

Common Pitfalls

randint vs randrange

randint(1, 10) includes BOTH 1 and 10. randrange(1, 10) includes 1 but NOT 10 (just like range()). This is the most common source of off-by-one errors.

shuffle() Returns None

random.shuffle() modifies the list in-place and returns None. Don't write x = random.shuffle(mylist) — x will be None!

Tasks

Remember

List 5 functions from the random module and explain what each returns.

Apply

Write a program that simulates flipping a coin 100 times and counts heads/tails.

Apply

Write a number guessing game: the computer picks a random number 1–100, the user guesses, and gets "higher/lower" hints.

Analyze

Given a code snippet, identify which lines use random generation and explain the possible outputs.

Self-Check Quiz

Q1: What does random.randint(1, 6) return?

A random integer between 1 and 6 (inclusive), like rolling a die.

Q2: What is a pseudo-random number?

A number generated by a deterministic algorithm (seed-based) that appears random but is predictable if you know the seed.

Q3: What is the difference between choice() and sample()?

choice() picks ONE random element. sample(seq, k) picks k UNIQUE elements without replacement.