Numbers and arithmetic

Two kinds of number

Python has whole numbers (int) and decimals (float). You do not declare which is which — 7 is an int, 7.0 is a float.

The arithmetic operators are mostly what you would expect:

Operator	Meaning	Example
+ - *	add, subtract, multiply	3 * 4 → 12
/	divide — always gives a float	7 / 2 → 3.5
//	floor divide — throws away the remainder	7 // 2 → 3
%	modulo — only the remainder	7 % 2 → 1
**	power	2 ** 10 → 1024

/ returning a float catches everyone out once: 4 / 2 is 2.0, not 2.

% looks obscure but earns its keep: n % 2 is 0 for even numbers, and n % 5 == 0 tells you n is a multiple of five. You will use it constantly.

Your turn: with total = 47 and per_box = 10, work out how many full boxes you can fill and how many are left over.

total = 47
per_box = 10

# Set full_boxes and leftover.

Converting between types

Reading a number from somewhere usually hands you a string, and "5" + 1 is an error rather than 6 — Python refuses to guess whether you meant addition or joining.

Convert explicitly:

• int("5") → 5
• float("2.5") → 2.5
• str(5) → "5"

round() is the other one you will reach for: round(3.14159, 2) gives 3.14. The second argument is how many decimal places to keep.

Your turn: price_text holds a price as text. Turn it into a number, add 20% tax, and round the result to two decimal places in total_price.

price_text = "19.99"

# Convert, add 20%, round to 2 decimal places.