The random Module

1random() — a decimal between 0 and 1

import random

print(random.random())
print(random.random())
print(random.random())

0.09257646839864098
0.6501665598866379
0.5743197615487686

random() takes no arguments and always gives a float from 0 up to (but never reaching) 1. On its own that is rarely what you want — it is a building block. Multiply it to stretch it, add to shift it:

# 0-1 stretched into other ranges

import random

print(random.random() * 10)
print(random.random() * 5 + 20)

4.51715653708381
20.246290398300456

The first line lands somewhere in 0 to 10, the second somewhere in 20 to 25. For whole numbers there is a better tool, which is the next one.

2randint(a, b) — a whole number, both ends included

# six rolls of a die

import random

for i in range(6):
    print(random.randint(1, 6), end=' ')

5 3 2 3 4 2 

That is unusual for Python. range(1, 6) stops at 5, and so does slicing, and so does almost everything else in the language. randint is the odd one out — which is exactly why it needs remembering.

3randrange() — the same shape as range()

randrange() takes the same arguments as range() and obeys the same rule: the stop value is not included.

import random

print(random.randrange(5))          # 0, 1, 2, 3 or 4
print(random.randrange(1, 6))       # 1, 2, 3, 4 or 5 -- never 6
print(random.randrange(0, 101, 10)) # 0, 10, 20, ... 100

1
4
30

The third form takes a step, which randint cannot do at all. It is how you pick a random multiple of ten, or a random even number, without any arithmetic of your own.

4Watch the 6 go missing

The gap is easy to agree with and hard to believe until you see it. Draw from both a few dozen times:

The right-hand column never fills its last bar, however long you keep going. That is the entire difference between the two functions, and it is the reason a dice program written with randrange(1, 6) looks fine, runs fine, and rolls a six roughly never.

5Program 1 — two dice

import random

a = random.randint(1, 6)
b = random.randint(1, 6)

print('Dice:', a, 'and', b)
print('Total:', a + b)

Dice: 5 and 3
Total: 8

6Program 2 — ten coin tosses

# 1 is heads, 0 is tails

import random

heads = 0
tails = 0

for i in range(10):
    if random.randint(0, 1) == 1:
        heads = heads + 1
    else:
        tails = tails + 1

print('Heads:', heads)
print('Tails:', tails)

Heads: 8
Tails: 2

Eight heads out of ten, on this run. That is not a bug and not a bias — it is what ten tosses of a fair coin look like sometimes. Raise the 10 to 1000 and the two counts come out close to each other, which is the honest way to check that a random program is fair.

7Program 3 — pick one item at random

The trick is to pick a random position rather than a random item — and the last valid position is len(colours) - 1:

import random

colours = ('red', 'green', 'blue', 'yellow')

i = random.randint(0, len(colours) - 1)

print('Picked:', colours[i])

Picked: green

8Program 4 — a six-digit OTP

# build the code one digit at a time, as a string

import random

otp = ''

for i in range(6):
    otp = otp + str(random.randint(0, 9))

print('Your OTP is', otp)

Your OTP is 920537

Built as a string, not a number, so that a leading zero survives — as a number, 075129 would print as 75129 and be five digits long. This is why PINs, phone numbers and PIN codes are stored as text in real systems.

import random

otp = ''

for i in range(6):
    otp = otp + str(random.randint(0, 9))

print('Your OTP is', otp)

9Program 5 — three different lucky numbers

Three calls to randint can easily give the same number twice. The fix is a while loop that keeps drawing until it has enough, and throws away anything already picked:

# keep drawing until three different numbers are in

import random

picked = ()

while len(picked) < 3:
    n = random.randint(1, 20)

    if n not in picked:
        picked = picked + (n,)

print('Lucky numbers:', picked)

Lucky numbers: (1, 6, 11)

while len(picked) < 3: rather than for i in range(3): — because a round that draws a repeat makes no progress, and a for loop would count it anyway and finish with two numbers.

10Program 6 — guess the number

# the computer thinks of a number and gives hints

import random

secret = random.randint(1, 100)
tries = 0

while True:
    guess = int(input('Your guess: '))
    tries = tries + 1

    if guess == secret:
        print('Correct! You took', tries, 'tries.')
        break
    elif guess < secret:
        print('Too low')
    else:
        print('Too high')

Your guess: 50
Too high
Your guess: 25
Too low
Your guess: 37
Too low
Your guess: 43
Too high
Your guess: 40
Too low
Your guess: 41
Too low
Your guess: 42
Correct! You took 7 tries.

while True: with a break, because nobody knows in advance how many guesses it will take. Seven is not luck — halving the range each time finds any number from 1 to 100 in at most seven tries.

11Recap