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.