Assignment and Compound Operators

15 min · Beginner

Programs constantly change a variable using its own current value. Compound operators write that update in half the space.

Compound assignment

Compound assignment is an operator such as += that does a calculation on a variable and stores the result back in the same variable. x += 5 is a shorter way to write x = x + 5.

Writing the variable twice is noisy. Join the operator and the equals sign instead.

Python

1score = 502score += 103print(score)4score *= 25print(score)6score //= 77print(score)8score **= 29print(score)

Example 1 · Wallet balance

Python

1wallet = 1002wallet += 503wallet -= 304wallet *= 25
6print(wallet)

Output

What this code does

wallet starts at 100.
+= 50 gives 150, -= 30 gives 120.
*= 2 doubles it to 240.

score += 10 means score = score + 10. Every arithmetic operator has one: -=, *=, /=, //=, %= and **=.

Why Python has no ++ or --

Other languages add one with x++. Python asks you to write x += 1 instead.

Python

counter = 0counter += 1print(counter)print(++counter)

++counter is not an increment. Python reads it as plus-plus-counter, two unary plus signs, so the value stays 1. Writing counter++ is a SyntaxError.

Multiple assignment and swapping

Multiple assignment is a single statement that stores several values in several names at once, such as x, y = 10, 20. The right side is fully worked out before any name on the left changes.

To swap two values you normally need a third variable. Python lets you write both sides at once.

Python

x, y = 10, 20x, y = y, xprint(x)print(y)

Example 2 · Swapping two readings

Python

1first_score = 722second_score = 883
4first_score, second_score = second_score, first_score5
6print(first_score, second_score)

Output

88 72

What this code does

The right side (88, 72) is built first.
Then it is stored into the two names on the left.

Example 3 · One name or two?

Python

a = b = 0a += 5
print(a, b)

Output

5 0

What this code does

a = b = 0 gives both names the value 0.
a += 5 makes a new int 5 for a only.
Ints cannot change in place, so b stays 0.

Python first builds the pair (y, x) from the old values, then stores it in x and y. That is why no temporary variable is needed.

Compound operators on text

The same operators work on strings: + joins text and * repeats it.

Python

1report = "Orders"2report += ": 12"3print(report)4divider = "-"5divider *= 106print(divider)

Example 4 · Building a report line

Python

1report = "Orders"2report += ": 12"3print(report)4
5divider = "-"6divider *= 107print(divider)

Output

Orders: 12
----------

What this code does

+= joins text onto the end.
*= 10 repeats the dash ten times.

A string cannot be changed in place, so report += ": 12" builds a new string and points report at it.

Next step

Check your understanding

9 quick questions

Assignment and Compound Operators

15 min · Beginner

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Programs constantly change a variable using its own current value. Compound operators write that update in half the space.

Compound assignment

Compound assignment is an operator such as += that does a calculation on a variable and stores the result back in the same variable. x += 5 is a shorter way to write x = x + 5.

Writing the variable twice is noisy. Join the operator and the equals sign instead.

Python

1score = 502score += 103print(score)4score *= 25print(score)6score //= 77print(score)8score **= 29print(score)

Example 1 · Wallet balance

Python

1wallet = 1002wallet += 503wallet -= 304wallet *= 25
6print(wallet)

Output

What this code does

wallet starts at 100.
+= 50 gives 150, -= 30 gives 120.
*= 2 doubles it to 240.

score += 10 means score = score + 10. Every arithmetic operator has one: -=, *=, /=, //=, %= and **=.

Why Python has no ++ or --

Other languages add one with x++. Python asks you to write x += 1 instead.

Python

counter = 0counter += 1print(counter)print(++counter)

++counter is not an increment. Python reads it as plus-plus-counter, two unary plus signs, so the value stays 1. Writing counter++ is a SyntaxError.

Multiple assignment and swapping

Multiple assignment is a single statement that stores several values in several names at once, such as x, y = 10, 20. The right side is fully worked out before any name on the left changes.

To swap two values you normally need a third variable. Python lets you write both sides at once.

Python

x, y = 10, 20x, y = y, xprint(x)print(y)

Example 2 · Swapping two readings

Python

1first_score = 722second_score = 883
4first_score, second_score = second_score, first_score5
6print(first_score, second_score)

Output

88 72

What this code does

The right side (88, 72) is built first.
Then it is stored into the two names on the left.

Example 3 · One name or two?

Python

a = b = 0a += 5
print(a, b)

Output

5 0

What this code does

a = b = 0 gives both names the value 0.
a += 5 makes a new int 5 for a only.
Ints cannot change in place, so b stays 0.

Python first builds the pair (y, x) from the old values, then stores it in x and y. That is why no temporary variable is needed.

Compound operators on text

The same operators work on strings: + joins text and * repeats it.

Python

1report = "Orders"2report += ": 12"3print(report)4divider = "-"5divider *= 106print(divider)

Example 4 · Building a report line

Python

1report = "Orders"2report += ": 12"3print(report)4
5divider = "-"6divider *= 107print(divider)

Output

Orders: 12
----------

What this code does

+= joins text onto the end.
*= 10 repeats the dash ten times.

A string cannot be changed in place, so report += ": 12" builds a new string and points report at it.

Next step

Check your understanding

9 quick questions