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52 lines
2.1 KiB
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52 lines
2.1 KiB
ReStructuredText
future-import-boilerplate
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=========================
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Most Python files should include the following boilerplate at the top of the file, right after the
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comment header:
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.. code-block:: python
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from __future__ import (absolute_import, division, print_function)
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This uses Python 3 semantics for absolute vs relative imports, division, and print. By doing this,
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we can write code which is portable between Python 2 and Python 3 by following the Python 3 semantics.
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absolute_import
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---------------
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When Python 2 encounters an import of a name in a file like ``import copy`` it attempts to load
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``copy.py`` from the same directory as the file is in. This can cause problems if there is a python
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file of that name in the directory and also a python module in ``sys.path`` with that same name. In
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that case, Python 2 would load the one in the same directory and there would be no way to load the
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one on ``sys.path``. Python 3 fixes this by making imports absolute by default. ``import copy``
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will find ``copy.py`` from ``sys.path``. If you want to import ``copy.py`` from the same directory,
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the code needs to be changed to perform a relative import: ``from . import copy``.
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.. seealso::
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* `Absolute and relative imports <https://www.python.org/dev/peps/pep-0328>`_
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division
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--------
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In Python 2, the division operator (``/``) returns integer values when used with integers. If there
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was a remainder, this part would be left off (aka, `floor division`). In Python 3, the division
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operator (``/``) always returns a floating point number. Code that needs to calculate the integer
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portion of the quotient needs to switch to using the floor division operator (`//`) instead.
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.. seealso::
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* `Changing the division operator <https://www.python.org/dev/peps/pep-0238>`_
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print_function
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--------------
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In Python 2, :func:`python:print` is a keyword. In Python 3, :func:`python3:print` is a function with different
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parameters. Using this ``__future__`` allows using the Python 3 print semantics everywhere.
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.. seealso::
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* `Make print a function <https://www.python.org/dev/peps/pep-3105>`_
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