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.

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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
• Development
  • Version History
  • License

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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 (if any) by searching each parent directory, until reaching the root of your filesystem.

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. (In other words, whatever version would run if pyenv weren't in your PATH.)

NOTE: You can activate multiple versions at the same time, e.g. pyenv global 3.4.2 2.7.8. This allows for parallel usage of python2 and python3, and is required with tools like tox.

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/2.7.8/
• ~/.pyenv/versions/3.4.2/
• ~/.pyenv/versions/pypy-2.4.0/

As far as pyenv is concerned, version names are simply the directories in ~/.pyenv/versions.

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Installation

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

The automatic installer

Visit my other project: https://github.com/yyuu/pyenv-installer

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 can install it somewhere else).

    $ 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.

   General warning: There are some systems, where the BASH_ENV variable is configured to point to .bashrc. On such systems you should almost certainly put the abovementioned line eval "$(pyenv init -) into .bash_profile, and not into .bashrc. Otherwise you may observe strange behaviour, such as pyenv getting into an infinite loop. See #264 for details.

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 2.7.8, download and unpack the source, then run:

    $ pyenv install 2.7.8
   

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

   NOTE: If you are having trouble installing a python version, please visit the wiki page about Common Build Problems

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

    $ pyenv rehash
   

   This can be automated for pip using pyenv-pip-rehash, which invokes pyenv rehash after (un)installing packages using pip.

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 for Mac OS X.

$ brew update
$ brew install pyenv

To upgrade pyenv in the future, just use upgrade instead of install.

After installation, 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.

To see exactly what happens under the hood for yourself, run pyenv init -.

Uninstalling Python Versions

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

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

Alternatively, 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.

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Command Reference

See COMMANDS.md.

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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 report 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.