Order of Operations and Shortcuts

What does Python calculate first?

Python follows the same rules as maths class (sometimes called BODMAS or PEMDAS):

• 1. Brackets ( )
• 2. Powers **
• 3. Multiply and divide * / // %
• 4. Add and subtract + -

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

Output

14
20

In the first line, 3 * 4 happens first. In the second line the brackets make 2 + 3 happen first.

Tip: When in doubt, add brackets. They make your intention clear to Python and to anyone reading your code.

A real example: the average

a = 70
b = 80
c = 90
wrong = a + b + c / 3
right = (a + b + c) / 3
print("Wrong:", wrong)
print("Right:", right)

Output

Wrong: 180.0
Right: 80.0

Shortcut operators

Changing a variable based on its old value is so common that Python has shortcuts:

lives = 3
lives -= 1
print("Lives left:", lives)

points = 100
points += 50
print("Points:", points)

Output

Lives left: 2
Points: 150