Planning an Early Death for Python 2

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On September 18 and 19, 2015, the Data Structure for Data Science workshop gathered at UC Berkeley's BIDS [Berkeley Institute for Data Science]. It was a productive two days of presentation, discussion and working groups — a collaborative effort aimed at expanding what data science can do.

Despite having mostly Python developers, the workshop reached out and included members from many other programming communities (e.g., C, C++, Julia, R, etc.) as the workshop's explicit goal was to improve cross language operability. In particular, the goal was to enable python's scientific computing tools (numpy, scipy, pandas, etc.) to have a consensus backbone data-structure that would enable easier interaction with other programming languages.

Out of the discussion arose a topic that has long plagued the python community at large: code that requires legacy Python 2.7 is holding back the development of data-science toolsets and – by extension – the progress of data science as a whole. Python 2.7 was an important part of the history of scientific computing, but now it should be left as part of that history. Thus, we convened a small working group to plan a early death for Legacy Python.

Move over Legacy Python once and for all.

In collaboration with many developers among whom @jiffyclub, @tacasswell, @kbarbary, @teoliphant, @pzwang, @ogrisel, we discussed different options to push Legacy Python more or less gently through the door. We understand that some people are still requiring the use of Legacy Python in their code base, or the use some libraries which are still only available on Legacy Python and we don't blame them. We understand that Legacy Python was a great language and that it's hard to move over it. Though the retirement of Legacy Python is 2020, you will ave to make the transition then, and it will be even harder to transition at that point.

So what are the step we can do to push the transition forward.

Choose your words.

The choice of words you make on the internet and in real life will influence the vision people have for Legacy Python vs Python. Assume that Python 3 is just Python, and refer to Python 2 as legacy python. IDEs and TwitterSphere is starting to do that, join the movement.

Refer to Legacy Python in the past tense. It will reinforce the old and deprecated state of Legacy Python. I still don't understand why people would like to stay with a language which that many defects:

  • it did not protect you from mixing Unicode and bytes,
  • tripped you with integer division
  • did not allow you to replace the printing function
  • had a range object which is not memory efficient
  • did not permit to re-raise exception
  • had a bad asynchronous support, without yield from
  • forced you to repeat the current class in super() call.
  • let you mix tab and space.
  • did not support function annotations

Legacy Python was missing many other feature which are now part of Python.

Do not state what's better in Python 3, state that it was missing/broken in Legacy Python. Like the missing matrix multiplication operator was missing multiplication operator. Legacy Python was preventing people to use efficient numeric library which are relying on the numerical operator.

Don't respond to "and on Python 2"

Personally during talks I plan to not pay attention to question regarding legacy Python, and will treat questions such questions as someone asking whether I support windows Vista. Next question please. The less you talk about Legacy Python the more you imply Legacy Python is not a thing anymore.

Drop support for Legacy Python (at least on paper)

If you a library author, you have probably had to deal with user trying your software on Legacy Python, and spend lot of time making your codebase compatible with both Python 3 and legacy Python. There are a few step you can take to push user toward Python 3.

If a user is not willing to update to a new version on Python, and decide to stay on legacy Python, they can most likely pin the version on your library to versions which support Legacy Python.

Make your examples/documentation Python 3 only

Or at least do not make effort to have examples that work using Legacy Python. Sprinkle with function annotation, and async/await keyword can help with communicating your example are Python 3 only.

You can even avoid mention of Legacy Python in your documentation and assume your users are using Python 3, this will make writing documentation much easier, and increase the chances to get examples right.

Ask user to reproduce but on up-to-date Python version.

Have you ever had a bug report where you ask users to upgrade your libraries dependencies ? Do the same with Python. If a user make a bug report with Python 2.7 ask them if they can reproduce with an up-to date version of Python, even if the bug is obviously from your side. If they really can't upgrade they will know, if they do and can reproduce, then you'll have at least converted one user from Legacy Python (and in the meantime you might have already corrected the bug).

Defer 2.7 support to companies like Continuum

This is already what Nick Coghlan recommands for Python 2.6, and that's what you can do for Legacy Python fix. If you have a sufficient number of user which are asking for 2.7 support, accept the bug report, but as an open source maintainer do not work on it. You can partner with companies like Continuum or Enthought, from which user would "buy" 2.7 support for your libraries, in exchange of which the Companies could spend some of their developer time fixing your Legacy Python bugs.

After a quick discussion with Peter Wang, it would be possible, but details need to be worked on.

Make Python 3 attractive

Create new features

Plan you new features explicitly for Python 3, even if the feature would be simple to make Legacy Python compatible, disable it on old platforms, and issue a warning indicating that the feature is not available on Legacy Python install.

You will be able to use all the shiny Python features which are lacking on Legacy Python like Unicode characters !

Create new Python packages

Make new packages Python 3 only, and make all the design decision you didn't do on your previous package. Pure python libraries are much easier to create and build once you are not hold back by legacy Python.

Helping Other project

Despite all the good will in the world the Migration path from Legacy Python can be hard. There are still a lot of things that can be done to help current and new project to push forward the adoption of Python.


Make sure that all the project you care about have continuous integration on Python 3, if possible even the documentation building done with Python 3, help to make Python 3 the default.

With continuous integration, check that your favorites projects are tested on Python Nightly, most CI provider allow the tests to be ran on nightly, but do not make the status of the project turn red if the test are failing. See allow_faillure on Travis-CI for example.

Porting C-extensions, move to Cython

The path to migrate C-extension is not well documented, the preferred approach is to use CFFI, but there is still alack of well written centralised, document on how to integrate with Python 3. IF you are knowledgeable on this domain, your help is welcomed.

The things we will (probably) not do.

Make a twitter account that shame people that use Legacy Python, though we might do a Parody account which say funny things, and push people toward Python 3.

Slow code on purpose and obviously on Legacy Python:

import sys
if sys.version_info[0] < 3:
    import time

Though it would be fun.

Ask user at IDE/CLI startup time if they want to upgrade to Python3:

$ python2
Python 2.7.10 (default, Jul 13 2015, 12:05:58)
Type "help", "copyright", "credits" or "license" for more information.

Warning you are starting a Legacy Python interpreter. Aren't you sure you don't
want not to upgrade to a newer version ? [y]:_

Delay Legacy Python packages releases by a few weeks to incentive people to migrate, or should we actually consider the people on Python 2 as guinea pig and release nightly ?

End word

Remember, Legacy Python is responsible for global warming, encourage people to stay with IE6, and is voting for Donald Trump.

If you have any ideas, please send me a Pull Request, I'll be happy to discuss.

As usual my English is far from perfect, so Pull Request welcomed on this blog post. Thanks to @michaelpacer who already did some rereading/rephrasing of first draft.