Python – __future__ in Python used for and how/when to use it, and how it works

pythonpython-2.x

__future__ frequently appears in Python modules. I do not understand what __future__ is for and how/when to use it even after reading the Python's __future__ doc.

Can anyone explain with examples?

A few answers regarding the basic usage of __future__ I've received seemed correct.

However, I need to understand one more thing regarding how __future__ works:

The most confusing concept for me is how a current python release includes features for future releases, and how a program using a feature from a future release can be be compiled successfully in the current version of Python.

I am guessing that the current release is packaged with potential features for the future. However, the features are available only by using __future__ because they are not the current standard. Let me know if I am right.

Best Solution

With __future__ module's inclusion, you can slowly be accustomed to incompatible changes or to such ones introducing new keywords.

E.g., for using context managers, you had to do from __future__ import with_statement in 2.5, as the with keyword was new and shouldn't be used as variable names any longer. In order to use with as a Python keyword in Python 2.5 or older, you will need to use the import from above.

Another example is

from __future__ import division
print 8/7  # prints 1.1428571428571428
print 8//7 # prints 1

Without the __future__ stuff, both print statements would print 1.

The internal difference is that without that import, / is mapped to the __div__() method, while with it, __truediv__() is used. (In any case, // calls __floordiv__().)

Apropos print: print becomes a function in 3.x, losing its special property as a keyword. So it is the other way round.

>>> print

>>> from __future__ import print_function
>>> print
<built-in function print>
>>>