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

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>
>>>