Changing Python versions

Overview

Recent platforms (v6+) support multiple Python interpreters from the shell using pyenv. pyenv allows seamless switching between available Python versions, and manages version-specific package installations too.

Basics

All commands are done from the terminal.

Listing versions

To get a current list of available Python interpreters, use pyenv versions:

[myadmin@sol ~]$ pyenv versions

system

2.7.8

* 3.3.5 (set by /home/myadmin/.python-version)

3.4.1

Of importance is system, which is the default system interpreter. Without pyenv installed, system is the default interpreter used. 2.7.8, 3.3.5, and 3.4.1 are additional interpreters installed in addition to the system version under /.socket/python/versions.

Selecting a version

A version may be defined per-directory and inherited recursively within all child directories. Versions are defined for a directory, and its child directories, by pyenv local:

[myadmin@sol ~]$ pyenv version

3.3.5 (set by /home/myadmin/.python-version)

[myadmin@sol ~]$ cd /var/www/

[myadmin@sol www]$ pyenv local system

[myadmin@sol www]$ pyenv version

system (set by /var/www/.python-version)

[myadmin@sol www]$ cd ~/test

[myadmin@sol ~/test]$ pyenv version

3.3.5 (set by /home/myadmin/.python-version)

Two versions of Python are defined, one in /home/myadmin and another in /var/www. Versions are controlled by a file called .python-version and pyenv will ascend each parent directory until it reaches root (/) or .python-version is found. If no control file is found, the current Python interpreter will be used.

You can then delegate a single Python version to handle a collection of projects by locating projects under an additional directory and defining a Python version in its parent directory:

www       <-- NO .python-version
|-- proj3 <-- .python-version (3.3.5)
| |-- mydjango  (3.3.5)
| `-- flask     (3.3.5)
`-- proj2 <-- .python-version (2.7.5)
  |-- newdjango (2.7.5)
  `-- mezzanine (2.7.5)

Control files are located under /var/www/proj3 and /var/www/proj2 allowing projects located under these directories to use Python 3 or Python 2 respectively.

Package locations

Packages are installed using pip as normal. Packages are located under /usr/local/lib/python/<MAJOR>.<MINOR>.<PATCH> using semantic versioning to avoid conflict. Any binaries installed under /usr/local/bin, however, will conflict with each other if those binaries, bundled in different versions of the package, differ. pyenv-virtualenv + virtualenv is recommended to isolate packages of different versions that rely on the same Python interpreter.

Caveats

Shebangs

Shell or FastCGI scripts typically begin with #!/usr/bin/python. This is the system version that will bypass pyenv initialization. Change this line to #!/usr/bin/env python to use python found in the shell path.