[Python-Dev] Language Summit Follow-Up

[Brett Cannon]

On Wed May 28 2014 at 10:14:39 PM, Glyph Lefkowitz <glyph at twistedmatrix.com>
wrote:

> At the language summit, Alex and I volunteered to put together some
> recommendations on what changes could be made to Python (the language) in
> order to facilitate a smoother transition from Python 2 to Python 3.  One
> of the things that motivated this was the (surprising, to us) consideration
> that features like ensurepip might be added to the future versions of the
> 2.7 installers from python.org.
>
> The specific motivations for writing this are:
>
>
>    1. Library maintainers have a rapidly expanding matrix that requires
>    an increasing number of branches to satisfy.
>    2. People with large corporate codebases absolutely cannot port all at
>    once.
>
>
> If you don't have perfect test coverage then you can't make any progress.
>  So these changes are intended to make porting from python 2 to python 3
> more guided and incremental.  We believe that these attributes are
> necessary.
>
> We would like to stress that we don't believe anything on this list is as
> important as the continuing efforts that everyone in the broader ecosystem
> is making.  If you just want to ease the transition by working on anything
> at all, the best use of your time right now is porting
> https://warehouse.python.org/project/MySQL-python/ to Python 3. :)
>
> Nevertheless there are some things that the language and CPython could do.
>
> Unfortunately we had to reject any proposal that involved new __future__
> imports, since unknown __future__ imports are un-catchable SyntaxErrors.
>
> Here are some ideas for Python 2.7+.
>
[SNIP]

>
>    1. Get rid of 2to3. Particularly, of any discussion of using 2to3 in
>    the documentation.  More than one very experienced, well-known Python
>    developer in this discussion has told me that they thought 2to3 was the
>    blessed way to port their code, and it's no surprise that they think so,
>    given that the first technique <
>    https://docs.python.org/3/howto/pyporting.html> mentions is still
>    2to3.  We should replace 2to3 with something like <
>    https://github.com/mitsuhiko/python-modernize>. 2to3 breaks your code
>    on python 2, and doesn't necessarily get it running on python 3.  A more
>    conservative approach that *reduced* the amount of work to get your
>    code 2/3 compatible but was careful to leave everything working would be a
>    lot more effective.
>
>
Two things. One is the HOWTO mentions 2to3 *only* when you are dropping
Python 3 support and since that makes your life the easiest it's listed as
the first option. You will notice point 2 in the TL;DR is be source
compatible if keeping Python 2 compatibility. The main part of the doc is
also all about source compatibility and using 2to3 while maintaining
compatibility is relegated to the Alternatives section at the end (heck,
2to3 is mentioned only 3 times in that entire document, one of which is in
a code comment).

Two, is how do you see python-future fit into this (I know others have
brought it up but we have two different approaches now to keeping source
compatibility that vary in magic and thus smoothing out edges)?
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