.. blogpost::
    :title: Producing a version for Python 2.7 from Python 3
    :keywords: python 2.7, migration
    :date: 2015-04-17
    :categories: documentation, Python 2.7
    :lid: b-migration-py2py3

    I tried to make most of the unit tests run under Python 2.7.
    Most of the function deals with strings for the documentation
    and it is a real pain to think again about
    str, `unicode <https://docs.python.org/2/howto/unicode.html>`_, bytes.
    Some of the functions only works in Python 3 but the goal
    was more to see what needed to be done.

    The first issue came from exception::

        try:
            something
        except Exception as e:
            raise Exception("other message") from e

    This syntax is not available and to avoid losing it, I decided to
    have two separate versions of the same module.
    I created a function
    `py3to2_convert_tree <pyquickhelper.pycode.py3to2.py3to2_convert_tree>`
    which copies the source and deals with this case (it removes everything it can).

    I also had an issue with code like ``isinstance(v, long)`` as the long type
    does not exists. So I added the string ``int #long#``
    to be replaced by ``long`` by a function. The one
    I use the most is ``str  #unicode#`` replaced by ``unicode``.

    The second issue is the function open. I usually use the following trick::

        if sys.version_info[0]==2:
            from codecs import open

    It does not solve everything (strings become unicode) and will melt
    with strings because I do not use ``u"..."`` which I could implement in
    the function
    `py3to2_convert_tree <pyquickhelper.pycode.py3to2.py3to2_convert_tree>`.

    Running out of courage,
    I disabled some unit tests because they were not passing due to the encoding issues.
    I had to add in some files but maybe I should have added that everywhere::

        from __future__ import print_function

    The following is added everywhere::

        from __future__ import unicode_literals