Python 中关于 round 函数的小坑

遇到的小坑，round(10.5)得到的结果是10，很疑惑，查了一下资料，总结一下：

round的结果跟python版本有关

这是Python 2：

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$ python
Python 2.7.8 (default, Jun 18 2015, 18:54:19) 
[GCC 4.9.1] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> round(0.5)
1.0

这是Python3：

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$ python3
Python 3.4.3 (default, Oct 14 2015, 20:28:29) 
[GCC 4.8.4] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> round(0.5)
0

为什么？

看一下文档：

Values are rounded to the closest multiple of 10 to the power minus ndigits; if two multiples are equally close, rounding is done away from 0.

这是Python2.7的文档，可以看到这里是真正意义上的四舍六入。但到了Python3就变成了这样：

Values are rounded to the closest multiple of 10 to the power minus ndigits; if two multiples are equally close, rounding is done toward the even choice.

此时，如果距离两边一样远，会保留到偶数的一边。比如round(0.5)和round(-0.5)都会保留到0，而round(1.5)会保留到2。