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Updating variables

2 September, 2015 - 17:48

One of the most common forms of multiple assignment is an update, where the new value of the variable depends on the old.

x = x+1 

This means “get the current value of x, add one, 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+1NameError: 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.