Simple Python Version Management: pyenv

pyenv lets you easily switch between multiple versions of Python. It's simple, unobtrusive, and follows the UNIX tradition of single-purpose tools that do one thing well.

This project was forked from rbenv and ruby-build, and modified for Python.

pyenv does...

• Let you change the global Python version on a per-user basis.
• Provide support for per-project Python versions.
• Allow you to override the Python version with an environment variable.
• Search commands from multiple versions of Python at a time. This may be helpful to test across Python versions with tox.

In contrast with pythonbrew and pythonz, pyenv does not...

• Depend on Python itself. pyenv was made from pure shell scripts. There is no bootstrap problem of Python.
• Need to be loaded into your shell. Instead, pyenv's shim approach works by adding a directory to your $PATH.
• Manage virtualenv. Of course, you can create virtualenv yourself, or pyenv-virtualenv to automate the process.

Table of Contents

• How It Works
  • Understanding PATH
  • Understanding Shims
  • Choosing the Python Version
  • Locating the Python Installation
• Installation
  • Basic GitHub Checkout
    • Upgrading
    • Homebrew on Mac OS X
    • Neckbeard Configuration
    • Uninstalling Python Versions
• Command Reference
  • pyenv local
  • pyenv global
  • pyenv shell
  • pyenv versions
  • pyenv version
  • pyenv rehash
  • pyenv which
  • pyenv whence
• Development
  • Version History
  • License

How It Works

At a high level, pyenv intercepts Python commands using shim executables injected into your PATH, determines which Python version has been specified by your application, and passes your commands along to the correct Python installation.

Understanding PATH

When you run a command like python or pip, your operating system searches through a list of directories to find an executable file with that name. This list of directories lives in an environment variable called PATH, with each directory in the list separated by a colon:

/usr/local/bin:/usr/bin:/bin

Directories in PATH are searched from left to right, so a matching executable in a directory at the beginning of the list takes precedence over another one at the end. In this example, the /usr/local/bin directory will be searched first, then /usr/bin, then /bin.

Understanding Shims

pyenv works by inserting a directory of shims at the front of your PATH:

~/.pyenv/shims:/usr/local/bin:/usr/bin:/bin

Through a process called rehashing, pyenv maintains shims in that directory to match every Python command across every installed version of Python—python, pip, and so on.

Shims are lightweight executables that simply pass your command along to pyenv. So with pyenv installed, when you run, say, pip, your operating system will do the following:

• Search your PATH for an executable file named pip
• Find the pyenv shim named pip at the beginning of your PATH
• Run the shim named pip, which in turn passes the command along to pyenv

Choosing the Python Version

When you execute a shim, pyenv determines which Python version to use by reading it from the following sources, in this order:

1. The PYENV_VERSION environment variable, if specified. You can use the pyenv shell command to set this environment variable in your current shell session.

2. The application-specific .python-version file in the current directory, if present. You can modify the current directory's .python-version file with the pyenv local command.

3. The first .python-version file found by searching each parent directory until reaching the root of your filesystem, if any.

4. The global ~/.pyenv/version file. You can modify this file using the pyenv global command. If the global version file is not present, pyenv assumes you want to use the "system" Python—i.e. whatever version would be run if pyenv weren't in your path.

Locating the Python Installation

Once pyenv has determined which version of Python your application has specified, it passes the command along to the corresponding Python installation.

Each Python version is installed into its own directory under ~/.pyenv/versions. For example, you might have these versions installed:

• ~/.pyenv/versions/2.7.5/
• ~/.pyenv/versions/3.3.2/
• ~/.pyenv/versions/pypy-1.9/

Version names to pyenv are simply the names of the directories in ~/.pyenv/versions.

Installation

If you're on Mac OS X, consider installing with Homebrew.

Basic GitHub Checkout

This will get you going with the latest version of pyenv and make it easy to fork and contribute any changes back upstream.

1. Check out pyenv where you want it installed. A good place to choose is $HOME/.pyenv but you may install it somewhere else.

    $ cd
    $ git clone git://github.com/yyuu/pyenv.git .pyenv
   

2. Define environment variable PYENV_ROOT to point to the path where pyenv repo is cloned and add $PYENV_ROOT/bin to your $PATH for access to the pyenv command-line utility.

    $ echo 'export PYENV_ROOT="$HOME/.pyenv"' >> ~/.bash_profile
    $ echo 'export PATH="$PYENV_ROOT/bin:$PATH"' >> ~/.bash_profile
   

   Zsh note: Modify your ~/.zshenv file instead of ~/.bash_profile. Ubuntu note: Modify your ~/.bashrc file instead of ~/.bash_profile.

3. Add pyenv init to your shell to enable shims and autocompletion.

    $ echo 'eval "$(pyenv init -)"' >> ~/.bash_profile
   

   Zsh note: Modify your ~/.zshenv file instead of ~/.bash_profile. Ubuntu note: Modify your ~/.bashrc file instead of ~/.bash_profile.

4. Restart your shell so the path changes take effect. You can now begin using pyenv.

    $ exec $SHELL
   

5. Install Python versions into $PYENV_ROOT/versions. For example, to install Python 2.7.5, download and unpack the source, then run:

    $ pyenv install 2.7.5
   

   NOTE If you need to pass configure option to build, please use CONFIGURE_OPTS environment variable.

6. Rebuild the shim binaries. You should do this any time you install a new Python binary (for example, when installing a new Python version, or when installing a package that provides a binary).

    $ pyenv rehash
   

Upgrading

If you've installed pyenv using the instructions above, you can upgrade your installation at any time using git.

To upgrade to the latest development version of pyenv, use git pull:

$ cd ~/.pyenv
$ git pull

To upgrade to a specific release of pyenv, check out the corresponding tag:

$ cd ~/.pyenv
$ git fetch
$ git tag
v0.1.0
$ git checkout v0.1.0

Homebrew on Mac OS X

You can also install pyenv using the Homebrew package manager on Mac OS X.

$ brew update
$ brew install pyenv

To later update these installs, use upgrade instead of install.

Afterwards you'll still need to add eval "$(pyenv init -)" to your profile as stated in the caveats. You'll only ever have to do this once.

Neckbeard Configuration

Skip this section unless you must know what every line in your shell profile is doing.

pyenv init is the only command that crosses the line of loading extra commands into your shell. Coming from rvm, some of you might be opposed to this idea. Here's what pyenv init actually does:

1. Sets up your shims path. This is the only requirement for pyenv to function properly. You can do this by hand by prepending ~/.pyenv/shims to your $PATH.

2. Installs autocompletion. This is entirely optional but pretty useful. Sourcing ~/.pyenv/completions/pyenv.bash will set that up. There is also a ~/.pyenv/completions/pyenv.zsh for Zsh users.

3. Rehashes shims. From time to time you'll need to rebuild your shim files. Doing this on init makes sure everything is up to date. You can always run pyenv rehash manually.

4. Installs the sh dispatcher. This bit is also optional, but allows pyenv and plugins to change variables in your current shell, making commands like pyenv shell possible. The sh dispatcher doesn't do anything crazy like override cd or hack your shell prompt, but if for some reason you need pyenv to be a real script rather than a shell function, you can safely skip it.

Run pyenv init - for yourself to see exactly what happens under the hood.

Uninstalling Python Versions

As time goes on, Python versions you install will accumulate in your ~/.pyenv/versions directory.

To remove old Python versions, pyenv uninstall command to automate the removal process.

Or, simply rm -rf the directory of the version you want to remove. You can find the directory of a particular Python version with the pyenv prefix command, e.g. pyenv prefix 2.6.8.

Command Reference

Like git, the pyenv command delegates to subcommands based on its first argument. The most common subcommands are:

pyenv local

Sets a local application-specific Python version by writing the version name to a .python-version file in the current directory. This version overrides the global version, and can be overridden itself by setting the PYENV_VERSION environment variable or with the pyenv shell command.

$ pyenv local 2.7.5

When run without a version number, pyenv local reports the currently configured local version. You can also unset the local version:

$ pyenv local --unset

Previous versions of pyenv stored local version specifications in a file named .pyenv-version. For backwards compatibility, pyenv will read a local version specified in an .pyenv-version file, but a .python-version file in the same directory will take precedence.

pyenv feature

You can specify multiple versions as local Python. Commands within these Python versions are searched by specified order.

$ pyenv local 2.7.5 3.2.5
$ pyenv local
2.7.5
3.2.5
$ pyenv which python2.7
/home/yyuu/.pyenv/versions/2.7.5/bin/python2.7
$ pyenv which python3.2
/home/yyuu/.pyenv/versions/3.2.5/bin/python3.2
$ pyenv which python
/home/yyuu/.pyenv/versions/2.7.5/bin/python

pyenv global

Sets the global version of Python to be used in all shells by writing the version name to the ~/.pyenv/version file. This version can be overridden by an application-specific .python-version file, or by setting the PYENV_VERSION environment variable.

$ pyenv global 2.7.5

The special version name system tells pyenv to use the system Python (detected by searching your $PATH).

When run without a version number, pyenv global reports the currently configured global version.

pyenv feature

You can specify multiple versions as global Python. Commands within these Python versions are searched by specified order.

$ pyenv global 2.7.5 3.2.5
$ pyenv global
2.7.5
3.2.5
$ pyenv which python2.7
/home/yyuu/.pyenv/versions/2.7.5/bin/python2.7
$ pyenv which python3.2
/home/yyuu/.pyenv/versions/3.2.5/bin/python3.2
$ pyenv which python
/home/yyuu/.pyenv/versions/2.7.5/bin/python

pyenv shell

Sets a shell-specific Python version by setting the PYENV_VERSION environment variable in your shell. This version overrides application-specific versions and the global version.

$ pyenv shell pypy-1.9

When run without a version number, pyenv shell reports the current value of PYENV_VERSION. You can also unset the shell version:

$ pyenv shell --unset

Note that you'll need pyenv's shell integration enabled (step 3 of the installation instructions) in order to use this command. If you prefer not to use shell integration, you may simply set the PYENV_VERSION variable yourself:

$ export PYENV_VERSION=pypy-1.9

pyenv feature

You can specify multiple versions via PYENV_VERSION environment variable in your shell.

$ pyenv shell pypy-1.9 2.7.5
$ echo $PYENV_VERSION
pypy-1.9:2.7.5
$ pyenv version
pypy-1.9 (set by PYENV_VERSION environment variable)
2.7.5 (set by PYENV_VERSION environment variable)

pyenv versions

Lists all Python versions known to pyenv, and shows an asterisk next to the currently active version.

$ pyenv versions
  2.5.6
  2.6.8
* 2.7.5 (set by /home/yyuu/.pyenv/version)
  3.2.5
  jython-2.5.3
  pypy-1.9

pyenv version

Displays the currently active Python version, along with information on how it was set.

$ pyenv version
2.7.5 (set by /home/yyuu/.pyenv/version)

pyenv rehash

Installs shims for all Python binaries known to pyenv (i.e., ~/.pyenv/versions/*/bin/*). Run this command after you install a new version of Python, or install a package that provides binaries.

$ pyenv rehash

pyenv which

Displays the full path to the executable that pyenv will invoke when you run the given command.

$ pyenv which python3.2
/home/yyuu/.pyenv/versions/3.2.5/bin/python3.2

pyenv whence

Lists all Python versions with the given command installed.

$ pyenv whence 2to3
2.6.8
2.7.5
3.2.5

Development

The pyenv source code is hosted on GitHub. It's clean, modular, and easy to understand, even if you're not a shell hacker.

Please feel free to submit pull requests and file bugs on the issue tracker.

Version History

See CHANGELOG.md.

License

(The MIT license)

• Copyright (c) 2013 Yamashita, Yuu
• Copyright (c) 2013 Sam Stephenson

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.