In Python 2, floor division is the default for integer arguments and exact (to within machine precision) division the default for floating point arguments, but this caused a lot of unnecessary confusion: new users were surprised that 3.0/2.0 == 1.5, but 3/2 == 1.
Thus for Python 3 the new // operator was added for explicit floor division (which is often quite useful in its own right), leaving the / operator to always return an exact (to machine precision) floating point result, even with integer arguments.
