The Last Printed Expression

In interactive mode, the last printed expression is assigned to the variable _. This means that when you are using Python as a desk calculator, it is somewhat easier to continue calculations, for example:

        >>> tax = 17.5 / 100
        >>> price = 3.50
        >>> price * tax
        0.6125
        >>> price + _
        4.1125
        >>> round(_, 2)
        4.11
        >>>

For reasons too embarrassing to explain, this variable is implemented as a built-in (living in the module __builtin__), so it should be treated as read-only by the user. I.e. don't explicitly assign a value to it --- you would create an independent local variable with the same name masking the built-in variable with its magic behavior.