python from __future__ import division

xuol001

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：

---------------------------------------------------------------------------------------------
>>> 3/4

0

>>> from __future__ import division

>>> 3/4

　　0.75
　　--------------------------------------------------------------------------------------------


导入精确除法后，若要执行截断除法，可以使用"//"操作符：

--------------------------------------------------------------------------------------------

>>> 3//4

0

>>>



--------------------------------------------------------------------------------------------

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.