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 mathematicalresult 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.
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