Pycon hallway session #2: thoughts for multiple versions in Python

We had an excellent brainstorming session today in the hall, with Toshio, Georg, Martin, Thomas, etc.. (sorry we were so many I don't have the full list) with some insights from Guido and Brett. We tried to think about a way to handle multiple versions of a same package.

Here's the two most important concepts :
- Unicity: There should be one and only one instance of a Python package at a given version on a system - Combination: One Python application combines several packages to run

About unicity

A Python package is a component that can be installed on a system. If you use the standard Distutils approach, it will end up in the Python site-packages directory and be importable by the interpreter. This package comes with a version number and is unique.

This unicity is important for security and maintainability. For instance, if there's a security hole in a package, the fix is applied in one place and the system maintainer knows it can't be present elsewhere on the system.

This is the system administrator point of view

About Combination

What defines a Python application is the fact that is selects a list of packages it needs to run. And this varies for every application. So two Python applications might need a different version of a given package and that is normal.

What's important is to have the right list of packages when an application loads. Tools like zc.buildout or virtualenv are perfect for this need : they create an isolated environment for your application to run with the right set of packages.

So the simplest way to release an application is to ship it with everything required, regardless the unicity.

This is the application developer point of view

The idea

zc.buildout and virtualenv are a blast for developers, and another thing system packagers might dislike. This is because they break the unicity by allowing developers to ship their applications as black boxes. Of course one may say that this is perfectly fine since what's inside an application is not the problem of the system packager. But since this application is made of packages that may be shared on the system by other applications, that is redondant.

Forget site-packages for a moment. And let's think about a new loading system for packages. This approach is similar in some ways to setuptools' multiple version system.

Storing multiple versions

First of all, let's store the packages in a directory, and for each version of the package, under a sub-directory which name is the version.

For example:
- SQLAlchemy
- 0.4
- package code is here - package egg-info here

  • 0.5

    • package code is here
      • package egg-info here
  • jsonlib

  • 1.2.6

    • package code is here
      • package egg-info here
  • 1.3.10

    • package code is here
      • package egg-info here

Given this structure, some mechanism can provide to the interpreter the latest available version of a package, as long as Distutils knows how to handle version comparisons correctly (which will be the case in a near future)

Choosing specific versions

Back to our application. Let's call it MyApp, version 1.0.

It needs specific versions for some packages. Forget zc.buildout and pip for a moment. And let's think about a different way to express the packages (and versions) its needs.

Let's make a Python package for this application and let's make a few assumptions on some features in Distutils:
- setuptools' install_requires has made it into Distutils, as part of metadata. - metadata are defined statically in a package, apart from

So basically, the application is mainly a static list of dependencies defined into install_requires. For example:
- SQLAlchemy > 0.4 - jsonlib == 1.2.6

When MyApp will get installed by Distutils, it will be added in the packages tree.

When it is used, it will need to load the versions of SQLAlchemy and jsonlib it needs. The ones that are inside its metadata.

To make it possible, the script that launches the application calls a built-in function called read_deps, that takes the metadata and reads them to know which versions fit:
# the script


import SQLAlchemy

import jsonlib


This call will load the right versions in the packages tree:
- SQLAlchemy
- 0.4 - 0.5

  • jsonlib
  • 1.2.6

    • 1.3.10
  • MyApp

  • 1.0

What's next

I just dropped here the rough idea, and a lot of details are missing. But I think it's a good thing to share this idea here in its early stage.

We have decided we would try to write a prototype using importlib and sys.meta_path. Maybe Georg will start it during the Pycon sprint, and start to digg into the details. He was working on this when I left the open session at Pycon.

Stay tuned