Python 2 中“//”和“/”的区别

Difference between "//" and "/" in Python 2

提问by Ryan Haining

Python 2 has two division operators: /and //. The following output:

Python 2 有两个除法运算符：/和//。以下输出：

========================================= RESTART: Shell =========================================
>>> for x in range(10):
        for y in range(1, 10):
                print x,'//',y,'=',x//y
                print x,'/',y,'=',x/y
                print


0 // 1 = 0
0 / 1 = 0

0 // 2 = 0
0 / 2 = 0

0 // 3 = 0
0 / 3 = 0

0 // 4 = 0
0 / 4 = 0

0 // 5 = 0
0 / 5 = 0

0 // 6 = 0
0 / 6 = 0

0 // 7 = 0
0 / 7 = 0

0 // 8 = 0
0 / 8 = 0

0 // 9 = 0
0 / 9 = 0

1 // 1 = 1
1 / 1 = 1

1 // 2 = 0
1 / 2 = 0

1 // 3 = 0
1 / 3 = 0

1 // 4 = 0
1 / 4 = 0

1 // 5 = 0
1 / 5 = 0

1 // 6 = 0
1 / 6 = 0

1 // 7 = 0
1 / 7 = 0

1 // 8 = 0
1 / 8 = 0

1 // 9 = 0
1 / 9 = 0

2 // 1 = 2
2 / 1 = 2

2 // 2 = 1
2 / 2 = 1

2 // 3 = 0
2 / 3 = 0

2 // 4 = 0
2 / 4 = 0

2 // 5 = 0
2 / 5 = 0

2 // 6 = 0
2 / 6 = 0

2 // 7 = 0
2 / 7 = 0

2 // 8 = 0
2 / 8 = 0

2 // 9 = 0
2 / 9 = 0

3 // 1 = 3
3 / 1 = 3

3 // 2 = 1
3 / 2 = 1

3 // 3 = 1
3 / 3 = 1

3 // 4 = 0
3 / 4 = 0

3 // 5 = 0
3 / 5 = 0

3 // 6 = 0
3 / 6 = 0

3 // 7 = 0
3 / 7 = 0

3 // 8 = 0
3 / 8 = 0

3 // 9 = 0
3 / 9 = 0

4 // 1 = 4
4 / 1 = 4

4 // 2 = 2
4 / 2 = 2

4 // 3 = 1
4 / 3 = 1

4 // 4 = 1
4 / 4 = 1

4 // 5 = 0
4 / 5 = 0

4 // 6 = 0
4 / 6 = 0

4 // 7 = 0
4 / 7 = 0

4 // 8 = 0
4 / 8 = 0

4 // 9 = 0
4 / 9 = 0

5 // 1 = 5
5 / 1 = 5

5 // 2 = 2
5 / 2 = 2

5 // 3 = 1
5 / 3 = 1

5 // 4 = 1
5 / 4 = 1

5 // 5 = 1
5 / 5 = 1

5 // 6 = 0
5 / 6 = 0

5 // 7 = 0
5 / 7 = 0

5 // 8 = 0
5 / 8 = 0

5 // 9 = 0
5 / 9 = 0

6 // 1 = 6
6 / 1 = 6

6 // 2 = 3
6 / 2 = 3

6 // 3 = 2
6 / 3 = 2

6 // 4 = 1
6 / 4 = 1

6 // 5 = 1
6 / 5 = 1

6 // 6 = 1
6 / 6 = 1

6 // 7 = 0
6 / 7 = 0

6 // 8 = 0
6 / 8 = 0

6 // 9 = 0
6 / 9 = 0

7 // 1 = 7
7 / 1 = 7

7 // 2 = 3
7 / 2 = 3

7 // 3 = 2
7 / 3 = 2

7 // 4 = 1
7 / 4 = 1

7 // 5 = 1
7 / 5 = 1

7 // 6 = 1
7 / 6 = 1

7 // 7 = 1
7 / 7 = 1

7 // 8 = 0
7 / 8 = 0

7 // 9 = 0
7 / 9 = 0

8 // 1 = 8
8 / 1 = 8

8 // 2 = 4
8 / 2 = 4

8 // 3 = 2
8 / 3 = 2

8 // 4 = 2
8 / 4 = 2

8 // 5 = 1
8 / 5 = 1

8 // 6 = 1
8 / 6 = 1

8 // 7 = 1
8 / 7 = 1

8 // 8 = 1
8 / 8 = 1

8 // 9 = 0
8 / 9 = 0

9 // 1 = 9
9 / 1 = 9

9 // 2 = 4
9 / 2 = 4

9 // 3 = 3
9 / 3 = 3

9 // 4 = 2
9 / 4 = 2

9 // 5 = 1
9 / 5 = 1

9 // 6 = 1
9 / 6 = 1

9 // 7 = 1
9 / 7 = 1

9 // 8 = 1
9 / 8 = 1

9 // 9 = 1
9 / 9 = 1

>>> 

proves (almost?) all the time a/bequals a//b. Is there any time that it isn't? If not, why did Python 2 include two operators that do the same thing?

证明（几乎？）一直a/b等于a//b。有什么时候不是吗？如果不是，为什么 Python 2 包含两个做同样事情的运算符？

回答by Ryan Haining

//is the floored-division operator in Python. The result will be easily noticeable if you try dividing floating point values

//是 Python 中的地板除法运算符。如果您尝试对浮点值进行除法，结果将很容易被注意到

# Python 2
>>> 10.0 / 3
3.3333333333333335
>>> 10.0 // 3
3.0

In Python2, dividing two ints uses integer division, which ends up getting you the same thing as floored division. However, you can still use //to get a floored result of floating point division.

在 Python2 中，将两个整数相除使用整数除法，最终得到与地板除法相同的结果。但是，您仍然可以使用//来获得浮点除法的地板结果。

In Python3, dividing two ints results in a float, but using //acts as integer division.

在Python3 中，将两个整数相除会产生一个浮点数，但 using//充当整数除法。

# Python3
>>> 10 / 3
3.3333333333333335
>>> 10 // 3
3

If you are (still) working in Python2, but want to someday convert to Python3, you should always use //when dividing two ints, or use from __future__ import divisionto get the Python3 behavior in Python2

如果您（仍然）在 Python2 中工作，但想有一天转换为 Python3，则应始终//在除以两个整数时使用，或用于from __future__ import division在 Python2 中获取 Python3 行为

回答by MrGeek

In python 2.7, to do real division you'll need to import division from a module named future:

在 python 2.7 中，要进行真正的除法，您需要从名为 future 的模块导入除法：

from __future__ import division

Then, the /will be the real (floating) division, i.e.:

然后，/将是真正的（浮动）除法，即：

15 / 4 => 3.75

And the //will be the integer division (the integer part of the real division), i.e.:

并且//将是整数除法（实数除法的整数部分），即：

15 // 4 => 3