Python — Numbers

Three numeric types

age = 25            # int   — whole numbers
price = 9.99        # float — decimals
c = 2 + 3j          # complex — engineering/math (rare in apps)

print(type(age))    # <class 'int'>

The operators

a, b = 10, 3

a + b     # 13
a - b     # 7
a * b     # 30
a / b     # 3.333…  ← division ALWAYS returns float
a // b    # 3       floor division — drops the decimal
a % b     # 1       remainder
a ** b    # 1000    power (10³)

/ vs // is the classic surprise: 10 / 5 gives 2.0, not 2. Use // when you truly want integer division.

Order of operations

Math rules apply; parentheses win:

2 + 3 * 4       # 14
(2 + 3) * 4     # 20
-3 ** 2         # -9  (power before minus!)
(-3) ** 2       # 9

Underscores for readability

Big numbers may include visual separators — Python ignores them:

population = 1_400_000_000
print(population)      # 1400000000

Float precision quirk

Floats are binary under the hood:

0.1 + 0.2          # 0.30000000000000004 😱
round(0.1 + 0.2, 2)   # 0.3  ✓ round for display/comparison

Same IEEE-float reality as every language. For money, compute in cents (int) or use the decimal module.

Useful built-ins

abs(-7)              # 7
round(3.14159, 2)    # 3.14
max(3, 7, 5)         # 7
min(3, 7, 5)         # 3
sum([1, 2, 3])       # 6
pow(2, 8)            # 256

The math module

import math

math.sqrt(81)        # 9.0
math.floor(4.9)      # 4   always down
math.ceil(4.1)       # 5   always up
math.pi              # 3.14159…
math.inf             # infinity

Converting between types

int(3.99)      # 3     truncates toward zero!
int("42")      # 42
float("3.5")   # 3.5
str(42)        # "42"

Gotchas: int("3.5") raises ValueError (parse with int(float("3.5"))); 0.1 + 0.2 != 0.3; / never gives an int.

Mini Practice

1. Predict then verify all seven operators on 17 and 5.
2. Show 10 / 5 vs 10 // 5 type difference.
3. Fix 0.1 + 0.2 == 0.3 using round().
4. Convert seconds to h:m:s with // and %.
5. Import math; compute hypotenuse via sqrt(a2 + b2).

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