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7.2: Updating variables

  • Page ID
    40893
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    A common kind of reassignment 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 + 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.


    7.2: Updating variables is shared under a CC BY-NC 3.0 license and was authored, remixed, and/or curated by Allen B. Downey.

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