20201228-1 数字

Numbers
2-1
>>> 2 + 2
4
>>> 50 - 5*6
20
>>> (50 - 5*6) / 4
5.0
>>> 8 / 5  # division always returns a floating point number
1.6

2-2
>>> 17 / 3  # classic division returns a float
5.666666666666667
>>>
>>> 17 // 3  # floor division discards the fractional part
5
>>> 17 % 3  # the % operator returns the remainder of the division
2
>>> 5 * 3 + 2  # result * divisor + remainder
17

2-3
>>> 5 ** 2  # 5 squared
25
>>> 2 ** 7  # 2 to the power of 7
128

3-1
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 = 12.5 / 100
>>> price = 100.50
>>> price * tax
12.5625
>>> price + _
113.0625
>>> round(_, 2)
113.06

This variable should be treated as read-only by the user. 
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.