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pyenv
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Simple Python version management

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Simple Python Version Management: pyenv

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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.

Terminal output example

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

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

Homebrew on macOS

  1. Consider installing with Homebrew
    sh
      brew update
      brew install pyenv
    
  2. Then follow the rest of the post-installation steps under Basic GitHub Checkout, starting with #3 ("Add
    pyenv init
    to your shell to enable shims and autocompletion").

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
    $PATH
    for access to the
    pyenv
    command-line utility.
  • For bash: ~~~ bash echo 'export PYENVROOT="$HOME/.pyenv"' >> ~/.bashprofile echo 'export PATH="$PYENVROOT/bin:$PATH"' >> ~/.bashprofile ~~~

  • For Ubuntu Desktop: ~~~ bash echo 'export PYENVROOT="$HOME/.pyenv"' >> ~/.bashrc echo 'export PATH="$PYENVROOT/bin:$PATH"' >> ~/.bashrc ~~~

  • For Zsh: ~~~ zsh echo 'export PYENVROOT="$HOME/.pyenv"' >> ~/.zshrc echo 'export PATH="$PYENVROOT/bin:$PATH"' >> ~/.zshrc ~~~

  • For Fish shell: ~~~ fish set -Ux PYENVROOT $HOME/.pyenv set -Ux fishuserpaths $PYENVROOT/bin $fishuserpaths ~~~

  • Proxy note: If you use a proxy, export

    http_proxy
    and
    HTTPS_PROXY
    too.
  1. Add

    pyenv init
    to your shell to enable shims and autocompletion. Please make sure

    eval "$(pyenv init -)"
    is placed toward the end of the shell configuration file since it manipulates
    PATH
    during the initialization.
    sh
    echo -e 'if command -v pyenv 1>/dev/null 2>&1; then\n  eval "$(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.
  2. Restart your shell so the path changes take effect. You can now begin using pyenv.

    sh
    exec "$SHELL"
    
  3. Install Python build dependencies before attempting to install a new Python version.

  4. Install Python versions into

    $(pyenv root)/versions
    . For example, to download and install Python 2.7.8, run:

    sh
    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 homebrew, upgrade using:

sh
brew upgrade pyenv

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
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.
  1. 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:
    sh
    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
    

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 
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/.bats

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

Version History

See CHANGELOG.md.

License

The MIT License

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