Multi-version Installs

easy_install allows simultaneous installation of different versions of the same project into a single environment shared by multiple programs which must require the appropriate version of the project at run time (using pkg_resources).

For many use cases, virtual environments address this need without the complication of the require directive. However, the advantage of parallel installations within the same environment is that it works for an environment shared by multiple applications, such as the system Python in a Linux distribution.

The major limitation of pkg_resources based parallel installation is that as soon as you import pkg_resources it locks in the default version of everything which is already available on sys.path. This can cause problems, since setuptools created command line scripts use pkg_resources to find the entry point to execute. This means that, for example, you can’t use require tests invoked through nose or a WSGI application invoked through gunicorn if your application needs a non-default version of anything that is available on the standard sys.path - the script wrapper for the main application will lock in the version that is available by default, so the subsequent require call in your own code fails with a spurious version conflict.

This can be worked around by setting all dependencies in __main__.__requires__ before importing pkg_resources for the first time, but that approach does mean that standard command line invocations of the affected tools can’t be used - it’s necessary to write a custom wrapper script or use python -c '<commmand>' to invoke the application’s main entry point directly.

Refer to the pkg_resources documentation for more details.