背景

在Python2.x代码中经常能看到使用__future__模块。那么__future__到底是做什么的呢？

1. To avoid confusing existing tools that analyze import statements and expect to find the modules they’re importing.
2. To ensure that future statements run under releases prior to 2.1 at least yield runtime exceptions (the import of future will fail, because there was no module of that name prior to 2.1).
3. To document when incompatible changes were introduced, and when they will be — or were — made mandatory. This is a form of executable documentation, and can be inspected programmatically via importing future and examining its contents.

• 避免和现有分析import工具混淆，并得到你期望的模块
• 确保2.1之前的版本导入__future__产生运行时异常，因为2.1之前没有这个模块
• 文档化不兼容的改变，通常这些改变会在新版中强制执行。这类文档以可执行的形式组织，通过导入__future__进行可编程式的检查。

Python2.x可用Python3.x的feature

• absolute_import： 绝对导入
• print_function：打印函数
• division：精确除法
• unicode_literals：unicode字面量

divsion例子

• 2.x里面的除法/是地板除法，整数相除，结果仍是整数，余数会被扔掉
• 而在Python 3.x中，所有的除法都是精确除法，地板除用//表示

# output: 3，地板除
print(10/3)

vs

from __future__ import division

# output: 3.333333333
print(10/3)

# output: 3，地板除
print(10//3)

unicode例子

• 2.x里的字符串用’xxx’表示str，Unicode字符串用u’xxx’表示unicode，而在3.x中，所有字符串都被视为unicode，因此，写u’xxx’和’xxx’是完全一致的
• 而在2.x中以’xxx’表示的str就必须写成b’xxx’，以此表示“二进制字符串”。

# False
print '\'xxx\' is unicode?', isinstance('xxx', unicode)
# True
print 'u\'xxx\' is unicode?', isinstance(u'xxx', unicode)
# True
print '\'xxx\' is str?', isinstance('xxx', str)
# True
print 'b\'xxx\' is str?', isinstance(b'xxx', str)

vs

from __future__ import unicode_literals

# True
print '\'xxx\' is unicode?', isinstance('xxx', unicode)
# True
print 'u\'xxx\' is unicode?', isinstance(u'xxx', unicode)
# False
print '\'xxx\' is str?', isinstance('xxx', str)
# True
print 'b\'xxx\' is str?', isinstance(b'xxx', str)

代码下载

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