Python Numbers

Python is a favourite language for anything involving numbers, from shopping carts to scientific research. This lesson covers Python’s number types, arithmetic, rounding, and the modules that handle maths, randomness and money.

int and float#

students = 32          # int
average = 78.5         # float
big = 1_000_000        # underscores make big numbers readable
print(students, average, big)
print(type(students), type(average))

• int: whole numbers of any size. 2 ** 200 works fine.
• float: decimal numbers with about 15–17 digits of precision.
• An operation with a float gives a float: 3 + 2.0 is 5.0.

Arithmetic operators#

a, b = 17, 5
print(a + b)    # 22
print(a - b)    # 12
print(a * b)    # 85
print(a / b)    # 3.4: true division, always a float
print(a // b)   # 3: floor division
print(a % b)    # 2: remainder
print(a ** b)   # 1419857: power
print(-a // b)  # -4: floor division rounds DOWN, towards minus infinity

// and % are very useful together:

total_seconds = 3725
hours = total_seconds // 3600
minutes = (total_seconds % 3600) // 60
seconds = total_seconds % 60
print(f"{hours}h {minutes}m {seconds}s")   # 1h 2m 5s

print(divmod(17, 5))   # (3, 2): quotient and remainder in one step

Operator precedence follows maths rules: ** first, then * / // %, then + -. Use brackets when in doubt.

Float precision#

print(0.1 + 0.2)              # 0.30000000000000004
print(0.1 + 0.2 == 0.3)       # False

import math
print(math.isclose(0.1 + 0.2, 0.3))  # True

Like in every language, binary floats can’t store some decimals exactly. Compare floats with math.isclose(), round when displaying, and use Decimal for money (below).

Rounding#

print(round(3.14159, 2))   # 3.14
print(round(7.5))          # 8
print(round(2.5))          # 2 (!)
print(round(3.5))          # 4

import math
print(math.floor(4.7))     # 4: always down
print(math.ceil(4.2))      # 5: always up
print(math.trunc(-4.7))    # -4: drop the decimals

round() uses banker’s rounding: exact halves round to the nearest even number, so round(2.5) is 2. This avoids bias in large statistics, but surprises people. For “school” rounding of money, use Decimal.

math.ceil is ideal for counting pages or boxes: 47 items at 10 per page need math.ceil(47 / 10) = 5 pages.

Useful built-in functions#

numbers = [12, 7, 25, 3]
print(abs(-9))          # 9
print(min(numbers))     # 3
print(max(numbers))     # 25
print(sum(numbers))     # 47
print(sum(numbers) / len(numbers))  # 11.75: average
print(pow(2, 8))        # 256

The math module#

import math

print(math.sqrt(144))         # 12.0
print(math.pi)                # 3.141592653589793
print(math.factorial(5))      # 120
print(math.gcd(24, 36))       # 12
print(math.log10(1000))       # 3.0
print(round(math.sin(math.pi / 2), 2))  # 1.0

radius = 7
area = math.pi * radius ** 2
print(f"Circle area: {area:.2f}")       # 153.94

import math loads the module; you’ll learn more about modules in lesson 14.

Random numbers#

import random

print(random.randint(1, 6))          # a dice roll: 1 to 6 inclusive
print(random.random())               # float from 0.0 up to 1.0
print(random.choice(["rock", "paper", "scissors"]))

cards = ["A", "K", "Q", "J"]
random.shuffle(cards)
print(cards)
print(random.sample(range(1, 50), 6))  # 6 unique lottery numbers

For passwords, tokens or anything security-related, use the secrets module instead: secrets.token_hex(16).

Exact decimals for money: Decimal#

from decimal import Decimal, ROUND_HALF_UP

price = Decimal("19.99")
qty = 3
total = price * qty
print(total)                         # 59.97 exactly

print(Decimal("0.1") + Decimal("0.2"))   # 0.3 exactly

tax = (total * Decimal("0.18")).quantize(Decimal("0.01"), rounding=ROUND_HALF_UP)
print(tax)                           # 10.79
print(Decimal("2.5").quantize(Decimal("1"), rounding=ROUND_HALF_UP))  # 3

• Create Decimals from strings (Decimal("19.99")), not floats, or the float’s imprecision comes along.
• quantize rounds to a fixed number of places with the rounding rule you choose.

Banks, shops and accounting software use Decimal (or store amounts as whole paise/cents in ints).

Converting and validating numbers#

print(int("42"), float("3.5"), int(3.99))   # 42 3.5 3
print(int("ff", 16))                        # 255: from hexadecimal
print(bin(10), hex(255))                    # 0b1010 0xff

text = "12a"
if text.isdigit():
    print(int(text))
else:
    print(f"'{text}' is not a whole number")

Formatting numbers#

amount = 1234567.891
print(f"{amount:,.2f}")     # 1,234,567.89
print(f"{0.256:.1%}")       # 25.6%
print(f"{42:05d}")          # 00042
print(f"{1500000:.2e}")     # 1.50e+06

For Indian-style grouping (12,34,567) or currency symbols for any country, the third-party babel package handles locale-aware formatting.

Common mistakes#

• **Comparing floats with ==.** Use math.isclose.
• **Expecting round(2.5) to be 3.**
• Floats for money in serious applications. Use Decimal or integer paise.
• **Decimal(0.1)** instead of Decimal("0.1").
• Integer vs true division confusion: / always returns a float in Python 3.

Exercise#

1. Convert 500 minutes into hours and minutes using // and %.
2. Calculate the area and circumference of a circle with radius 10, rounded to 2 decimals.
3. Simulate rolling two dice 5 times and print each total.
4. Calculate 3 items at ₹19.99 plus 18% tax using Decimal, rounded to 2 places.
5. Find how many pages you need for 123 search results at 20 per page.

Summary#

• int for whole numbers, float for decimals; / gives floats, // floors, % gives remainders.
• Floats can be imprecise: compare with math.isclose, use Decimal for money.
• round() uses banker’s rounding; math.floor/ceil always go down/up.
• The math, random, decimal and secrets modules extend Python’s number skills.

Next, you’ll store collections of values in lists.