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Supporting both Python 2 and Python 3 with Setuptools
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Starting with Distribute version 0.6.2 and Setuptools 0.7, the Setuptools
project supported Python 3. Installing and
using setuptools for Python 3 code works exactly the same as for Python 2
code, but Setuptools also helps you to support Python 2 and Python 3 from
the same source code by letting you run 2to3 on the code as a part of the
build process, by setting the keyword parameter ``use_2to3`` to True.

Setuptools as help during porting
=================================

Setuptools can make the porting process much easier by automatically running
2to3 as a part of the test running. To do this you need to configure the
setup.py so that you can run the unit tests with ``python setup.py test``.

See :ref:`test` for more information on this.

Once you have the tests running under Python 2, you can add the use_2to3
keyword parameters to setup(), and start running the tests under Python 3.
The test command will now first run the build command during which the code
will be converted with 2to3, and the tests will then be run from the build
directory, as opposed from the source directory as is normally done.

Setuptools will convert all Python files, and also all doctests in Python
files. However, if you have doctests located in separate text files, these
will not automatically be converted. By adding them to the
``convert_2to3_doctests`` keyword parameter Setuptools will convert them as
well.

By default, the conversion uses all fixers in the ``lib2to3.fixers`` package.
To use additional fixers, the parameter ``use_2to3_fixers`` can be set
to a list of names of packages containing fixers. To exclude fixers, the
parameter ``use_2to3_exclude_fixers`` can be set to fixer names to be
skipped.

A typical setup.py can look something like this::

from setuptools import setup

setup(
        name='your.module',
        version = '1.0',
        description='This is your awesome module',
        author='You',
        author_email='your@email',
        package_dir = {'': 'src'},
        packages = ['your', 'you.module'],
        test_suite = 'your.module.tests',
        use_2to3 = True,
        convert_2to3_doctests = ['src/your/module/README.txt'],
        use_2to3_fixers = ['your.fixers'],
        use_2to3_exclude_fixers = ['lib2to3.fixes.fix_import'],
    )

Differential conversion
-----------------------

Note that a file will only be copied and converted during the build process
if the source file has been changed. If you add a file to the doctests
that should be converted, it will not be converted the next time you run
the tests, since it hasn't been modified. You need to remove it from the
build directory. Also if you run the build, install or test commands before
adding the use_2to3 parameter, you will have to remove the build directory
before you run the test command, as the files otherwise will seem updated,
and no conversion will happen.

In general, if code doesn't seem to be converted, deleting the build directory
and trying again is a good saferguard against the build directory getting
"out of sync" with the source directory.

Distributing Python 3 modules
=============================

You can distribute your modules with Python 3 support in different ways. A
normal source distribution will work, but can be slow in installing, as the
2to3 process will be run during the install. But you can also distribute
the module in binary format, such as a binary egg. That egg will contain the
already converted code, and hence no 2to3 conversion is needed during install.

Advanced features
=================

If you don't want to run the 2to3 conversion on the doctests in Python files,
you can turn that off by setting ``setuptools.use_2to3_on_doctests = False``.

Note on compatibility with older versions of setuptools
=======================================================

Setuptools earlier than 0.7 does not know about the new keyword parameters to
support Python 3.
As a result it will warn about the unknown keyword parameters if you use
those versions of setuptools instead of Distribute under Python 2. This output
is not an error, and
install process will continue as normal, but if you want to get rid of that
error this is easy. Simply conditionally add the new parameters into an extra
dict and pass that dict into setup()::

from setuptools import setup
    import sys

extra = {}
    if sys.version_info >= (3,):
        extra['use_2to3'] = True
        extra['convert_2to3_doctests'] = ['src/your/module/README.txt']
        extra['use_2to3_fixers'] = ['your.fixers']

This way the parameters will only be used under Python 3, where Distribute or
Setuptools 0.7 or later is required.

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