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 macOS
    • Advanced Configuration
    • Uninstalling Python Versions
• Command Reference
• 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 root)/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 root)/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, including multiple
versions of Python2 or Python3 simultaneously. This allows for parallel usage of
Python2 and Python3, and is required with tools like tox. For example, to set
your path to first use your system Python and Python3 (set to 2.7.9 and 3.4.2
in this example), but also have Python 3.3.6, 3.2, and 2.5 available on your
PATH, one would first pyenv install the missing versions, then set pyenv global system 3.3.6 3.2 2.5. At this point, one should be able to find the full
executable path to each of these using pyenv which, e.g. pyenv which python2.5
(should display $(pyenv root)/versions/2.5/bin/python2.5), or pyenv which python3.4 (should display path to system Python3). You can also specify multiple
versions in a .python-version file, separated by newlines or any whitespace.

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 root)/versions.

For example, you might have these versions installed:

• $(pyenv root)/versions/2.7.8/
• $(pyenv root)/versions/3.4.2/
• $(pyenv root)/versions/pypy-2.4.0/

As far as pyenv is concerned, version names are simply the directories in
$(pyenv root)/versions.

Managing Virtual Environments

There is a pyenv plugin named pyenv-virtualenv which comes with various features to help pyenv users to manage virtual environments created by virtualenv or Anaconda.
Because the activate script of those virtual environments are relying on mutating $PATH variable of user's interactive shell, it will intercept pyenv's shim style command execution hooks.
We'd recommend to install pyenv-virtualenv as well if you have some plan to play with those virtual environments.

Installation

If you're on macOS, consider installing with Homebrew.

If you're on Windows, consider using @kirankotari's pyenv-win fork. (pyenv does not work on windows outside the Windows Subsystem for Linux)

The automatic installer

Visit my other project:
https://github.com/pyenv/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 https://github.com/pyenv/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 ~/.zshrc file instead of ~/.bash_profile.
   • Ubuntu and Fedora note: Modify your ~/.bashrc file instead of ~/.bash_profile.
   • Proxy note: If you use a proxy, export http_proxy and HTTPS_PROXY too.

3. Add pyenv init to your shell to enable shims and autocompletion.
   Please make sure eval "$(pyenv init -)" is placed toward the end of the shell
   configuration file since it manipulates PATH during the initialization.

   $ echo -e 'if command -v pyenv 1>/dev/null 2>&1; then\n  eval "$(pyenv init -)"\nfi' >> ~/.bash_profile
   

   • Zsh note: Modify your ~/.zshrc file instead of ~/.bash_profile.
   • fish note: Use pyenv init -, source instead of eval (pyenv init -).
   • Ubuntu and Fedora 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 above mentioned 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 build dependencies before attempting to install a new Python version. The
   pyenv wiki provides suggested installation packages
   and commands for various operating systems.

6. Install Python versions into $(pyenv root)/versions.
   For example, to download and install Python 2.7.8, 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 want to use proxy to download, please use http_proxy and https_proxy
   environment variable.

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

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 root)
$ git pull

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

$ cd $(pyenv root)
$ git fetch
$ git tag
v0.1.0
$ git checkout v0.1.0

Uninstalling pyenv

The simplicity of pyenv makes it easy to temporarily disable it, or
uninstall from the system.

1. To disable pyenv managing your Python versions, simply remove the
   pyenv init line from your shell startup configuration. This will
   remove pyenv shims directory from PATH, and future invocations like
   python will execute the system Python version, as before pyenv.

pyenv will still be accessible on the command line, but your Python
apps won't be affected by version switching.

2. To completely uninstall pyenv, perform step (1) and then remove
   its root directory. This will delete all Python versions that were
   installed under $(pyenv root)/versions/ directory:

   rm -rf $(pyenv root)
   

   If you've installed pyenv using a package manager, as a final step
   perform the pyenv package removal. For instance, for Homebrew:

    brew uninstall pyenv
   

Homebrew on macOS

You can also install pyenv using the Homebrew
package manager for macOS.

$ brew update
$ brew install pyenv

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

Then follow the rest of the post-installation steps under Basic GitHub Checkout above, starting with #3 ("Add pyenv init to your shell to enable shims and autocompletion").

Advanced 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 root)/shims to your $PATH.

2. Installs autocompletion. This is entirely optional but pretty
   useful. Sourcing $(pyenv root)/completions/pyenv.bash will set that
   up. There is also a $(pyenv root)/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 root)/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.

Command Reference

See COMMANDS.md.

Environment variables

You can affect how pyenv operates with the following settings:

name, default, description
-----, ---------, ------------
PYENV_VERSION, Specifies the Python version to be used.Also see pyenv shell
PYENV_ROOT, ~/.pyenv, Defines the directory under which Python versions and shims reside.Also see pyenv root
PYENV_DEBUG, Outputs debug information.Also as: pyenv --debug <subcommand>
PYENV_HOOK_PATH, see wiki, Colon-separated list of paths searched for pyenv hooks.
PYENV_DIR, $PWD, Directory to start searching for .python-version files.
PYTHON_BUILD_ARIA2_OPTS, Used to pass additional parameters to aria2.If the aria2c binary is available on PATH, pyenv uses aria2c instead of curl or wget to download the Python Source code. If you have an unstable internet connection, you can use this variable to instruct aria2 to accelerate the download.In most cases, you will only need to use -x 10 -k 1M as value to PYTHON_BUILD_ARIA2_OPTS environment variable

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.

Tests are executed using Bats:

$ bats test
$ bats/test/<file>.bats

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

Version History

See CHANGELOG.md.

License

The MIT License