future-import-boilerplate
=========================

Most Python files should include the following boilerplate at the top of the file, right after the
comment header:

.. code-block:: python

    from __future__ import (absolute_import, division, print_function)

This uses Python 3 semantics for absolute versus relative imports, division, and print.  By doing this,
we can write code which is portable between Python 2 and Python 3 by following the Python 3 semantics.


absolute_import
---------------

When Python 2 encounters an import of a name in a file like ``import copy`` it attempts to load
``copy.py`` from the same directory as the file is in.  This can cause problems if there is a python
file of that name in the directory and also a python module in ``sys.path`` with that same name.  In
that case, Python 2 would load the one in the same directory and there would be no way to load the
one on ``sys.path``.  Python 3 fixes this by making imports absolute by default.  ``import copy``
will find ``copy.py`` from ``sys.path``.  If you want to import ``copy.py`` from the same directory,
the code needs to be changed to perform a relative import: ``from . import copy``.

.. seealso::

    * `Absolute and relative imports <https://www.python.org/dev/peps/pep-0328>`_

division
--------

In Python 2, the division operator (``/``) returns integer values when used with integers.  If there
was a remainder, this part would be left off (aka, `floor division`).  In Python 3, the division
operator (``/``) always returns a floating point number.  Code that needs to calculate the integer
portion of the quotient needs to switch to using the floor division operator (`//`) instead.

.. seealso::

    * `Changing the division operator <https://www.python.org/dev/peps/pep-0238>`_

print_function
--------------

In Python 2, :func:`python:print` is a keyword.  In Python 3, :func:`python3:print` is a function with different
parameters.  Using this ``__future__`` allows using the Python 3 print semantics everywhere.

.. seealso::

    * `Make print a function <https://www.python.org/dev/peps/pep-3105>`_

