We do not discourage authors to release software on Python 2. While this guide is mostly written with the assumption that software are going to stop Python 2 support, it does perfectly apply to a package that wish to not support Python 3, or is stopping support for any minor version.

This page gather information and links to resources allowing to release a library that stop supporting an older version of Python without causing too much disruption for users who haven’t upgraded to this new version.

Whether you are a user, or a developer, being aware of the issue listed here, at least the main points should ease lots of the pain.

Too long, did not read:

  • Help and encourage users to install pip 9.0+
  • Help and encourage users to install setuptools 24.3+
  • As maintainer use setup(..., python_requires='>=3.4') new option.
  • do use pip install [-e] . and do not invoke setup.py directly.
  • Fail early at install time if on Python 2.
  • We are giving a talk at PyCon 2017 (likely recorded), add link here.

The problem

Up until December 2016 it was hard to publish a new major version of library that changed requirements in Python version and mark it as such so that user system will not try to upgrade said library.

With the recent changes in Python packaging this is now possible.

As an example let’s look at the example of the fictitious library.

  • fictitious 1.1, 1.2, 1.3, 1.4 are compatible Python 2.7 and 3.3+
  • fictitious 2.0 has been released and is python 3.4+ only.

As a Python 2.7 user, if I don’t pay attention, or if the library is not correctly tagged, if I issue the following:

$ python -c 'import fictitious; print(fictitious.__version__)'
1.3.2
$ pip install fictitious --upgrade

Either my system will install 2.0, which will not work, on the worst case scenario, or fail to install, in which case I will not get the critical 1.4 upgrade.

As a user

Install Pip 9.0

If you are already a Python 3 user, you should not encounter a lot of disruption. Please still check that the libraries you use follow best practices not to break for Python 2 users. Python is a community regardless of which python version you have to (or decided to) run, making sure that everything works make the community strong.

Make sure you have Pip ≥ 9.0, this is especially important if you have Python 2 installations. Having pip 9.0+ is not a guaranty to flawless upgrade. But pip 9.0+ does have a number of safety check not available on previous versions.

Having a version of pip < 9.0 can lead your system to try to upgrade to non-compatible versions of Python packages even if these are marked as non-compatible.

Help as many other users as possible to install pip ≥ 9.0, for the transition, it is the slowest part of the ecosystem to update, and is the only piece that requires action of all Python users.

The simplest way to make sure all is up to date is to run the following for each installation of Python:

$ pip install --upgrade setuptools pip

This will install the latest version of pip and setuptools.

You can issue the following to see the version of pip:

$ pip --version
9.0.0

All good.

Setuptools

If you are on a system for which no wheel is available, pip will try to install a source distribution (aka sdist).

Installing an sdist will require setuptools make sure you have setuptools ≥ 24.2.0 or building Python 3 only libraries is likely to fail. In particular if library authors have taken time to mark their library as Python 3 only, the python_requires argument to setup() may not be recognized and installation will fail.

Use the following to check setuptools version :

$ python -c 'import setuptools; print(setuptools.__version__)'
24.2.0

Again make sure to upgrade pip and setuptools to make sure you have an up to date system:

$ pip install --upgrade setuptools pip

Local package index

If you are using a custom local package index, for example if you are working at a company with private packages, make sure it implement correctly pep-503 and let pip knows about the python_requires field. This mostly mean that the html you are exposing should get a data-python-requires data attribute with the (html escaped) version specifier.

The state of PyPI

Warehouse and Legacy PyPI have received various patches to insure they support this new functionality.

Preparing your library

As a library author one of the most important factor in a smooth transition is planning and communication, letting your user base know in advance that the transition is happening and what step to take is critical for a transition.

For your library code here the steps you need to take to ensure that installation will fail in the least number of case:

You need to release your new packages version with setuptools version 24.2.0 or above. You can also use one of the alternate package manager that can set the Requires-Python metadata field. Without this, pip 9.0 will try to install non-compatible version of your software on Python 2. This version of setuptools is recent (July 20, 2016) and this possible thank to the work of Xavier Fernandez

Add the following to your setup.py

setup(
   ...
   python_requires='>=3.3'
   ...
)

Change >=3.3 accordingly depending on what version your library decides to support. In particular you can use >=2.6 or >=3.5 ! Note that this also support the compatible with syntax: ~=2.5 (meaning, >=2.5 and <3).

This will make PyPI aware that your package is Python 3.3+ only, and allow pip to be made aware of this.

Thus as long as your user have recent enough versions of pip and setuptools they will get the right version of your library.

Unit Testing and documentation

It is recommended not to invoke setup.py directly either with install or develop subcommands. These may not correctly resolve dependencies, and can install incompatible versions of dependencies. Please recommend and use pip install . and pip install -e . for regular and developer install.

Check in scripts, and documentation that the correct installation command is used.

Recommended Mitigations

These are not mandatory but should make the transition seamless by warning your user early enough and providing useful error messages.

Runtime warning on master

Add a warning at runtime early on master (before switching to Python 3 only)

import warnings
import sys
if sys.version_info < (3,):
    warnings.warn('You are using master of `Frobulator` with Python 2. '
                  'Frobulator will soon be Python 3 only. '
                  'See this issue to know more.',
                  UserWarning)

Your Python 2 user have a chance to upgrade, or get off master, (for example on the LTS branch).

Fail early at import time

Add an error early at import at runtime with a clear error message, leave the early import compatible Python 2 for users to not be welcomed with a useless SyntaxError. Don’t hesitate to use multi-line strings in error messages.

Error at import time will happen on system with old version of pip and setuptools. Keep in mind that saying the package is Python 3 only is not a lot more helpful than a Syntax error. The most reasonable reason would be out of data pip and setuptools:

import sys

if sys.version_info < (3,):
    raise ImportError(
    """You are running Frobulator 6.0 on Python 2

Unfortunately Frobulator 6.0 and above are not compatible with Python 2
anymore, and you still ended up with this version installed on your system.
That's a bummer. Sorry about that. It should not have happened. Make sure you
have pip ≥ 9.0 to avoid this kind of issues, as well as setuptools ≥ 24.2:

 $ pip install pip setuptools --upgrade

You have various other choices

- install an older version of Frobulator:

 $ pip install 'frobulator<6.0'

- Upgrade your system to use Python 3.

It would be great if you can figure out how this version ended up being
installed, and try to check how to prevent that for future users.

See the following url for more up to date informations:

https://i.am.an/url

""")

Watch out for beta releases

Make sure your version number match pep 440 or you will get surprises during beta in particular as the sdist and wheel will appear as being different versions, in particular sdist (during beta/rc/post) can appear with a greater version number than wheels. Pip thus try to install the sdist instead of the wheel, which have more chance of failing, in particular with pre 24.2 versions of setuptools.

The regular expression to check for validity of pep440 can be find below:

^
([1-9]\\d*!)?
(0|[1-9]\\d*)
(\\.(0|[1-9]\\d*))*
((a|b|rc)(0|[1-9]\\d*))?
(\\.post(0|[1-9]\\d*))?
(\\.dev(0|[1-9]\\d*))?

fail early in setup.py

Leave setup.py python 2 compatible and fail early. If you detect Python 2 raise a clear error message and ask user to make sure they have pip > 9.0 (or migrate to Python 3). You can (try to) conditionally import pip and check for its version but this might not be the same pip. Failing early is important to make sure the Python installation does not install an incompatible version. Otherwise user code can fail at runtime arbitrary later in the future, which can be a difficult to debug and fix. Get inspiration from the message of failure at runtime, and adapt for installation time.

Fix dependant libraries

If you control dependant packages, Make sure to include conditional dependencies depending on the version of Python.

Non recommended mitigations

This is a collection of “mitigation” or “solutions” you will find on the web and that you will hear about. This is an attempt to acknowledge them, and explain why they can’t work and what are their drawbacks before you attempt to implement them.

Use a meta-package.

It is possible to release a meta-package that has virtually no code and rely on conditional dependency to install its actual core code on the user system. For example, Frob-6.0 could be a meta-package which depends on Frob-real-py2 on Python < 3.0, and Frob-real-py3 on Python ≥ 3.4. While this approach is doable this can make imports confusing.

Depend on setuptools

You can mark your library as dependent on setuptools greater than 24.3 this will insure that during the next upgrade (when the packages drop python 2 support) will have the right version of setuptools.

Of course regardless of all the care you will take for your library to no break and to install only on python 2, you will likely have cases where it still end up being installed on incompatible versions of Python. Simply because users upgrades rarely and only an old version of pip or setuptools is enough to make the all update process broken.

Plus setuptools is rarely an actual dependency of your project but a requirement to build wheels.

Multiple Sdist.

Pip (used to) support a “feature” where a sdist ending in -pyX.Y.tar.gz would only be seen as compatible on Python X.Y, thus it used to be possible to publish multiple sdist of a package targeting various python version.

Though it is not possible anymore to upload multiple sdist on PyPI. This solution is thus not possible.

Wheel only ?

Releasing a package only using wheel for a given python version is doable, but this will break downstream packages that may require the original source to reproduce the build.

Why all that ?

You might wonder why all this, it’s 2016 already, so how come all these issues ? Python 3 has been out for 8+ years now !

Well there are many reasons to this, first of all, this issue mostly affect libraries that are currently python 2 and Python 3 compatible at the same time. Many libraries have transitioned from Python 2-only to Python 2 + 3. And the issue of transitioning to Python 3 only is relatively recent. Technically it can also apply to libraries that are only stopping support for 2.6, or even are already Python 3 only, but are starting to stop support for earlier versions of Python. For example a library releasing a Python 3.4+ only version.

Python 3.3 was release at the end of 2012, and was the first version to support (again) u as a prefix for Unicode string. It was one of the first minor version of Python 3 that saw a majority of single-source project working both on Python 2 and Python 3. These are the Project that will likely be affected by this issue.

The introduction of Python 3 was chaotic, there are still strong argument both in Python 2 and Python 3 camps. In the one suffering the most from this are users. Starting with the fact that inevitably some libraries will stop support for Python 2 and release Python 3 only library. And that inevitably some system will will not be upgraded to Python 3 how can we ensure that users get the least breakage as possible ? And what are the best practices to follow.