Updating variables is a very common operation in Python programs. Consider these updates:
x = 29
x = x + 1 # now 30
x = x - 5 # now 25
x = x * 3 # now 75
x = x / 4 # now 18.75
print(x)
When the variable on the left side of the assignment operator is the same as the variable on the right side, you can use a shortcut called compound assignment. Instead of saying x = x + 1, you can write x += 1. If you read x = x + 1 as “x becomes x + 1”, you can read x += 1 as “x plus-and-becomes 1.”
Table 2.16.1 gives a examples of the arithmetic operators and their compound assignment shortcuts.
Table2.16.1.Compound Assignments
Assignment
Compound Assignment
n = n + 3
n += 3
x = x - 4.2
x -= 4.2
y = y * 6
y *= 6
z = z / 5
z /= 5
x = x // 3
x //= 3
n = n % 7
n %= 7
We can, therefore, rewrite the example at the top of the page using compound assignment:
The update expression doesn’t have to be a simple variable: x = x + (y / z) can be shortened to x += (y / z). However, if the variable on the left-hand side is part of a parenthesized expression on the right, such as x = (x + 5) / 3, you cannot shorten it with compound assignment.
Checkpoint2.16.2.
What is printed when the following statements execute?
n = 12
n -= 3
n *= 8
n //= 2
n %= 5
print(n)
12
The value of n changes in the second statement.
2
Each statement changes the value of n, so 2 is not the final result.
1
Yes, starting with 12, subtract 3, multiply by 8, integer divide by 2, and then take remainder after dividing by 5.
Nothing. An error occurs because n cannot be used that many times in compound assignment statements.
Remember that variables in Python are different from variables in math in that they (temporarily) hold values, but can be reassigned.
