Numbers and arithmetic

In Python 3, 7 / 2 gives 3.5, 7 // 2 gives 3 and 7 % 2 gives the remainder 1.

Four divisions to predict before reading on

Cover the output below and write down what you expect each line to print. Most people get the first one right and at least one of the others wrong, and that wrong guess is where this unit starts.

print(7 / 2)
print(8 / 2)
print(7 // 2)
print(7 % 2)

3.5
4.0
3
1

The second line surprises people. 8 / 2 is exactly 4, yet Python prints 4.0. The reason is that / always gives a float, a number with a decimal point, even when the division comes out even. If you want whole-number division, Python has a separate operator, //, called floor division. It divides and then rounds down to the whole number below. The partner operator % gives the remainder that is left over. Together they answer the primary-school question "7 divided by 2 is 3, remainder 1".

% turns up in more places than you would expect. A number is even when n % 2 is 0. The last digit of a number is n % 10. Turning 135 minutes into hours and minutes is 135 // 60 hours and 135 % 60 minutes.

minutes = 135
print(minutes // 60, "h", minutes % 60, "min")
print(2026 % 2, 2027 % 2)
print(4589 % 10)

2 h 15 min
0 1
9

The other operators, and the order Python uses

Python follows the same order as maths class: ** first, then *, /, // and %, then + and -, left to right within each group. Brackets come before everything, so when in doubt, add them. They cost nothing and make the intention plain to the next reader.

print(2 + 3 * 4)
print((2 + 3) * 4)
print(2 ** 10)
print(10_000 * 3)

14
20
1024
30000

The last line shows a small help: Python lets you write underscores inside long numbers, so 10_000 is easier to read than 10000. They are ignored when the number is worked out and do not appear in the output. Commas cannot be used that way, because a comma already means "next value".

Decimals are stored in binary, so some answers come out a hair off

Try 0.1 + 0.2. The Python tutorial devotes a whole chapter to why the answer is 0.30000000000000004 rather than 0.3. Floats are stored in binary, and 0.1 has no exact binary form, just as 1/3 has no exact decimal form. The error is tiny, and round() hides it when you print. Be aware that round() rounds a value that sits exactly halfway to the even neighbour: the built-in functions page gives round(0.5) as 0 and round(1.5) as 2. For money in whole won or whole cents, keep amounts as int and avoid the problem entirely.

print(0.1 + 0.2)
print(round(0.1 + 0.2, 2))
print(round(0.5), round(1.5), round(2.5))
print(int(3.99), float(7))

0.30000000000000004
0.3
0 2 2
3 7.0

input() always gives you a string

input() shows a prompt, waits for the user to type something and press Enter, and returns what they typed as a string. Even if they type 15, you get "15", text. To do arithmetic with it, convert it with int() or float(). int() on a float cuts off the decimal part rather than rounding, so int(3.99) is 3.

age = input("How old are you? ")
next_year = int(age) + 1
print("Next year you will be", next_year)

How old are you? 14
Next year you will be 15