python from __future__ import division

from __future__ import division

导入python未来支持的语言特征division(精确除法)，当我们没有在程序中导入该特征时，"/"操作符执行的是截断除法(Truncating Division),当我们导入精确除法之后，"/"执行的是精确除法，如下所示：

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>>> 3/4
0
>>> from __future__ import division
>>> 3/4

0.75

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导入精确除法后，若要执行截断除法，可以使用"//"操作符：

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>>> 3//4
0
>>> 

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一些将来特征如下：

feature	optional in	mandatory in	effect
nested_scopes	2.1.0b1	2.2	PEP 227: Statically Nested Scopes
generators	2.2.0a1	2.3	PEP 255: Simple Generators
division	2.2.0a2	3.0	PEP 238: Changing the Division Operator
absolute_import	2.5.0a1	2.7	PEP 328: Imports: Multi-Line and Absolute/Relative
with_statement	2.5.0a1	2.6	PEP 343: The “with” Statement
print_function	2.6.0a2	3.0	PEP 3105: Make print a function
unicode_literals	2.6.0a2	3.0	PEP 3112: Bytes literals in Python 3000

PEP：Python Enhancement Proposals

可以在这个地方找到很多PEP：http://www.python.org/dev/peps/ 里面还能看到许多提议的动机

比如有division：

The current division (/) operator has an ambiguous meaning for numerical arguments: 

it returns the floor of the mathematical  result of division if the arguments are ints or longs, but it returns a reasonable approximation of the division result if the arguments are floats or complex.  

This makes expressions expecting float or complex results error-prone when integers are not expected but possible as inputs.