valemobil/python-future: Easy, clean, reliable Python 2/3 compatibility

docs/porting.rst
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Ed Schofield
Update porting doc
27 фев 2014, 09:21
27 фев 2014, 09:21170a0d4
.. _porting:

Python 2 to 2&3 porting cheat-sheet
===================================

Instructions and notes on porting code from Python 2 to both Python 3 and 2 using ``future``:

.. _porting-setup:

Step 0: setup
-------------

Step 0 goal: set up and see the tests passing on Python 2 and failing on Python 3.

a. Clone the package from github/bitbucket. Optionally rename your repo to ``package-future``. Examples: ``reportlab-future``, ``paramiko-future``, ``mezzanine-future``.
b. Create and activate a Python 2 conda environment or virtualenv. Install the package with ``python setup.py install`` and run its test suite on Py2.7 or Py2.6 (e.g. ``python setup.py test`` or ``py.test`` or ``nosetests``)
c. Optionally: if there’s a ``.travis.yml`` file, add Python version 3.3 and remove any versions < 2.6.
d. Install Python 3.3 with e.g. ``sudo apt-get install python3``. On other platforms, an easy way is to use Miniconda3. See `Miniconda3 <http://repo.continuum.io/miniconda/index.html>`_. Then e.g.::
    
    conda create -n py33 python=3.3

.. _porting-step1:

Step 1: modern Py2 code
-----------------------

The goal for this step is to modernize the Python 2 code without introducing any dependencies (on ``future`` or e.g. ``six``) at this stage.

1a. Install ``future`` into the virtualenv using::
      
          pip install future
  
1b. Run ``futurize --stage1 -w *.py subdir1/*.py subdir2/*.py``

    Note that with ``zsh``, you can apply stage1 to all Python source files
    recursively with::

        futurize --stage1 -w **/*.py

1c. Commit all changes

1d. Re-run the test suite on Py2 and fix any errors.

See :ref:`forwards-conversion-stage1` for more info.


Example error
~~~~~~~~~~~~~

One relatively common error after conversion is::

    Traceback (most recent call last):
      ... 
      File "/home/user/Install/BleedingEdge/reportlab/tests/test_encrypt.py", line 19, in <module>
        from .test_pdfencryption import parsedoc
    ValueError: Attempted relative import in non-package

If you get this error, try adding an empty ``__init__.py`` file in the package
directory. (In this example, in the tests/ directory.) If this doesn’t help,
and if this message appears for all tests, they must be invoked differently
(from the cmd line or e.g. ``setup.py``). The way to run a module inside a
package on Python 3, or on Python 2 with ``absolute_import`` in effect, is::

    python -m tests.test_platypus_xref

(For more info, see `PEP 328 <http://www.python.org/dev/peps/pep-0328/>`_ and
the `PEP 8 <http://www.python.org/dev/peps/pep-0008/>`_ section on absolute
imports.)


.. _porting-step2:

Step 2: working Py3 code that still supports Py2
------------------------------------------------

The goal for this step is to get the tests passing first on Py3 and then on Py2
again with the help of the ``future`` package.

2a. Run::

        futurize —-stage2 myfolder1/*.py myfolder2/*.py

    Alternatively, with ``zsh``, you can view the stage 2 changes to all Python source files
    recursively with::

        futurize --stage2 **/*.py

    To apply the changes, add the ``-w`` argument.

    This stage makes further conversions needed to support both Python 2 and 3.
    These will likely require imports from ``future``, such as::

        from future import standard_library
        standard_library.install_hooks()
        from future.builtins import bytes
        from future.builtins import open

Optionally, you can use the ``--unicode-literals`` flag to adds this further
import to the top of each module::

    from __future__ import unicode_literals

All strings would then be unicode (on Py2 as on Py3) unless explicitly marked
with a ``b''`` prefix.

If you would like ``futurize`` to import all the changed builtins to have their
Python 3 semantics on Python 2, invoke it like this::

    futurize --stage2 --all-imports myfolder/*.py

   
2b. Re-run your tests on Py3 now. Make changes until your tests pass on Python 3.

2c. Commit your changes! :)

2d. Now run your tests on Python 2 and notice the errors. Add wrappers from ``future`` to re-enable Python 2 compatibility:

    - :func:`utils.reraise()` function for raising exceptions compatibly
    - ``bytes(b'blah')`` instead of ``b'blah'``
    - ``str('my string')`` instead of ``'my string'`` if you need to enforce Py3’s strict type-checking on Py2
    - ``int(1234)`` instead of ``1234`` if you want to enforce a Py3-like long integer
    - :func:`@utils.implements_iterator` decorator for any custom iterator class with a ``.__next__()`` method (which used to be ``.next()``)
    - :func:`@utils.python_2_unicode_compatible` decorator for any class with a ``__str__`` method (which used to be ``__unicode__``).
    - :func:`utils.with_metaclass` to define any metaclasses.

See :ref:`what-else` for more info.

After each change, re-run the tests on Py3 and Py2 to ensure they pass on both.

2e. You’re done! Celebrate! Push your code and announce to the world! Hashtags
#python3 #python-future.