5.1: Updating Variables

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A common pattern in assignment statements is an assignment statement that updates a variable, where the new value of the variable depends on the old.

x = x + 1

This means "get the current value of x, add 1, and then update x with the new value."

If you try to update a variable that doesn't exist, you get an error, because Python evaluates the right side before it assigns a value to x:

>>> x = x + 1
NameError: name 'x' is not defined

Before you can update a variable, you have to initialize it, usually with a simple assignment:

>>> x = 0
>>> x = x + 1

Updating a variable by adding 1 is called an increment; subtracting 1 is called a decrement.