Calculating

Calculating is what computers do fastest, and in Python a calculation looks almost the way it does on paper. A few operators only exist in programming, though, and you will need two of them, // and %, all the time.

The operators

print(7 + 3)
print(7 - 3)
print(7 * 3)
print(7 / 3)
print(7 // 3)
print(7 % 3)
print(7 ** 3)

// and % belong together: 7 divided by 3 is 2, remainder 1. With them you split an amount into whole pieces and what is left over, for example minutes into hours and minutes:

minutes = 135
hours = minutes // 60
rest = minutes % 60
print(f"{minutes} minutes are {hours} hours and {rest} minutes.")

% also answers the question whether one number divides another: then the remainder is zero. A number n is even, for example, when n % 2 is zero.

Precedence

The same precedence rules apply as in mathematics: ** first, then *, /, // and %, and + and - last. Round brackets let you decide yourself what is calculated first.

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

Brackets do no harm where they would not be needed. If you are not sure when reading a calculation, put them in.

Rounding

A division with / often gives more decimal places than anyone wants to read. round(value, places) rounds to the number of places you want:

price = 10 / 3
print(price)
print(round(price, 2))
print(f"{price:.2f} euros")

The last line shows a second way: in an f-string, :.2f after the name rounds the number to two places only for the output, while the value in the variable stays as it is. This is handy for amounts of money because it also writes 5.10 with two places instead of 5.1.

A small surprise

print(0.1 + 0.2)

The result is not exactly 0.3. This is not a bug in Python: computers store decimal numbers as bits, and many decimal fractions such as 0.1 can only be approximated very closely that way, not represented exactly, just as 1/3 never comes out even in the decimal system. For most calculations the tiny difference does not matter; you should just know about it so that it does not confuse you, and round results for output.

Calculating with variables

Python of course calculates with variables too, and a calculation that changes a variable can be shortened: total += 5 means the same as total = total + 5, and there are also -=, *= and /=.

total = 0
total += 4.50
total += 2.20
total += 3.80
print(f"Total: {total:.2f} euros")

Takeaways

• +, -, *, / and ** calculate as usual; / always returns a float.
• // divides without remainder, % returns the remainder. Together they split an amount into whole units and what is left over.
• Brackets decide the order; otherwise multiplication comes before addition.
• round(x, 2) rounds the value, f"{x:.2f}" only the output.
• Decimal numbers are approximations in a computer; round them for output.