Python2 中除法默认向下取整，因此 1/2 = 0，为整型。
For (plain or long) integer division, the result is an integer. The result is always rounded towards minus infinity: 1/2 is 0, (-1)/2 is -1, 1/(-2) is -1, and (-1)/(-2) is 0.
而 Python3 中的除法为正常除法，会保留小数位，因此 1/2 = 0.5，为浮点型。