pyenv/README.md

# 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](https://github.com/rbenv/rbenv) and
[ruby-build](https://github.com/rbenv/ruby-build), and modified for Python.

![Terminal output example](/terminal_output.png)


### 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](https://pypi.python.org/pypi/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](https://pypi.python.org/pypi/virtualenv)
    yourself, or [pyenv-virtualenv](https://github.com/pyenv/pyenv-virtualenv)
    to automate the process.


----


## Table of Contents

* **[How It Works](#how-it-works)**
  * [Understanding PATH](#understanding-path)
  * [Understanding Shims](#understanding-shims)
  * [Choosing the Python Version](#choosing-the-python-version)
  * [Locating the Python Installation](#locating-the-python-installation)
* **[Installation](#installation)**
  * [Basic GitHub Checkout](#basic-github-checkout)
    * [Upgrading](#upgrading)
    * [Homebrew on macOS](#homebrew-on-macos)
    * [Advanced Configuration](#advanced-configuration)
    * [Uninstalling Python Versions](#uninstalling-python-versions)
* **[Command Reference](#command-reference)**
* **[Development](#development)**
  * [Version History](#version-history)
  * [License](#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`](https://github.com/pyenv/pyenv/blob/master/COMMANDS.md#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`](https://github.com/pyenv/pyenv/blob/master/COMMANDS.md#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`](https://github.com/pyenv/pyenv/blob/master/COMMANDS.md#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.
Lines starting with a `#` are ignored.

### 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 directories under
`$(pyenv root)/versions`.

### Managing Virtual Environments

There is a pyenv plugin named [pyenv-virtualenv](https://github.com/pyenv/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

### Prerequisites:

For pyenv to install python correctly you should [**install the Python build dependencies**](https://github.com/pyenv/pyenv/wiki#suggested-build-environment).

### Homebrew on macOS

   1. Consider installing with [Homebrew](https://brew.sh):
      ```sh
      brew update
      brew install pyenv
      ```
   2. Then follow the rest of the post-installation steps under [Basic GitHub Checkout](https://github.com/pyenv/pyenv#basic-github-checkout), starting with #2 ("Configure your shell's environment for Pyenv").

If you're on Windows, consider using @kirankotari's [`pyenv-win`](https://github.com/pyenv-win/pyenv-win) fork. (Pyenv does not work in Windows outside the Windows Subsystem for Linux.)

### The automatic installer

Visit our 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

   Optionally, try to compile a dynamic Bash extension to speed up Pyenv. Don't
   worry if it fails; Pyenv will still work normally:

        cd ~/.pyenv && src/configure && make -C src

2. **Configure your shell's environment for Pyenv**

   **Note:** The below instructions for specific shells are designed for common shell setups.  
   If you have an uncommon setup and they don't work for you,
   use the guidance text and the [Advanced Configuration](#advanced-configuration)
   section below to figure out what you need to do in your specific case.
   
   1. **Adjust the session-wide environment for your account.** Define
   the `PYENV_ROOT` environment variable to point to the path where
   you cloned the Pyenv repo, add the `pyenv` command-line utility to your `PATH`,
   run the output of `pyenv init --path` to enable shims.
   
      These commands need to be added into your shell startup files in such a way
      that _they are executed only once per session, by its login shell._
      This typically means they need to be added into a per-user shell-specific
      `~/.*profile` file, _and_ into `~/.profile`, too, so that they are also
      run by GUI managers (which typically act as a `sh` login shell).

      **MacOS note:** If you installed Pyenv with Homebrew, you don't need
      to add the `PYENV_ROOT=` and `PATH=` lines.
      You also don't need to add commands into `~/.profile` if your shell doesn't use it.
   
      - For **bash**:

         ~~~ bash
         echo 'export PYENV_ROOT="$HOME/.pyenv"' >> ~/.profile
         echo 'export PATH="$PYENV_ROOT/bin:$PATH"' >> ~/.profile
         echo 'eval "$(pyenv init --path)"' >> ~/.profile
         ~~~

         - **If your `~/.profile` sources `~/.bashrc` (Debian, Ubuntu, Mint):**

            Put these lines into `~/.profile` _before_ the part that sources `~/.bashrc`:
            ~~~bash
            export PYENV_ROOT="$HOME/.pyenv"
            export PATH="$PYENV_ROOT/bin:$PATH"
            eval "$(pyenv init --path)"
            ~~~

            <!--This is an alternative option and needn't be replicated to `pyenv init`-->
            Alternatively, for an automated installation, you can run the following:
            ~~~ bash
            echo -e 'if shopt -q login_shell; then' \
                  '\n  export PYENV_ROOT="$HOME/.pyenv"' \
                  '\n  export PATH="$PYENV_ROOT/bin:$PATH"' \
                  '\n eval "$(pyenv init --path)"' \
                  '\nfi' >> ~/.bashrc
            echo -e 'if [ -z "$BASH_VERSION" ]; then'\
                  '\n  export PYENV_ROOT="$HOME/.pyenv"'\
                  '\n  export PATH="$PYENV_ROOT/bin:$PATH"'\
                  '\n  eval "$(pyenv init --path)"'\
                  '\nfi' >>~/.profile
            ~~~

         **Note:** If you have `~/.bash_profile`, make sure that it too executes the above-added commands,
         e.g. by copying them there or by `source`'ing `~/.profile`.

      - For **Zsh**:

        Same as for Bash above, but add the commands into both `~/.profile`
        and `~/.zprofile`.
        
      - For **Fish shell**:

        Execute this interactively:
        ~~~ fish
        set -Ux PYENV_ROOT $HOME/.pyenv
        set -U fish_user_paths $PYENV_ROOT/bin $fish_user_paths
        ~~~

        And add this to `~/.config/fish/config.fish`:
        ~~~ fish
        status is-login; and pyenv init --path | source
        ~~~

        If Fish is not your login shell, also follow the Bash/Zsh instructions to add to `~/.profile`.

      **Proxy note**: If you use a proxy, export `http_proxy` and `https_proxy`, too.

   2. **Add `pyenv` into your shell** by running the output of `pyenv init -`
     to enable autocompletion and all subcommands.
   
      This command needs to run at startup of any interactive shell instance.
      In an interactive login shell, it needs to run _after_ the commands
      from the previous step.

      - For **bash**:
        ~~~ bash
        echo 'eval "$(pyenv init -)"' >> ~/.bashrc
        ~~~
        
        - **If your `/etc/profile` sources `~/.bashrc` (SUSE):**
        
          ~~~bash
          echo 'if command -v pyenv >/dev/null; then eval "$(pyenv init -)"; done' >> ~/.bashrc 
          ~~~

      - For **Zsh**:
        ~~~ zsh
        echo 'eval "$(pyenv init -)"' >> ~/.zshrc
        ~~~

      - For **Fish shell**:
        Add this to `~/.config/fish/config.fish`:
        ~~~ fish
        pyenv init - | source
        ~~~

      **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](https://github.com/pyenv/pyenv/issues/264) for details.

4. **Restart your login session for the changes to take effect.**
   E.g. if you're in a GUI session, you need to fully log out and log back in.
   
   In MacOS, restarting terminal windows is enough (because MacOS runs shells
   in them as login shells by default).

5. [**Install Python build dependencies**](https://github.com/pyenv/pyenv/wiki#suggested-build-environment) before attempting to install a new Python version.

6. **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 a `configure` option to a build, please use the
   ```CONFIGURE_OPTS``` environment variable.

   **NOTE:** If you want to use proxy to download, please set the `http_proxy` and `https_proxy`
   environment variables.

   **NOTE:** If you are having trouble installing a Python version,
   please visit the wiki page about
   [Common Build Problems](https://github.com/pyenv/pyenv/wiki/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`:

```sh
cd $(pyenv root)
git pull
```

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

```sh
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` invocations 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 it was 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, remove _all_ configuration lines for it
   from your shell startup configuration, 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.
Step 1 is done by `eval "$(pyenv init --path)"`, the others are done by
`eval "$(pyenv init -)"`.


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 -`
or `pyenv init --path`.

If you don't want to use `pyenv init` and shims, you can still benefit
from pyenv's ability to install Python versions for you. Just run
`pyenv install` and you will find versions installed in
`$(pyenv root)/versions`, which you can manually execute or symlink
as required.

### 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](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.<br>Also see [`pyenv shell`](https://github.com/pyenv/pyenv/blob/master/COMMANDS.md#pyenv-shell)
`PYENV_ROOT` | `~/.pyenv` | Defines the directory under which Python versions and shims reside.<br>Also see `pyenv root`
`PYENV_DEBUG` | | Outputs debug information.<br>Also as: `pyenv --debug <subcommand>`
`PYENV_HOOK_PATH` | [_see wiki_][hooks] | 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`](https://aria2.github.io/).<br>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.<br>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](https://github.com/pyenv/pyenv).  It's clean, modular,
and easy to understand, even if you're not a shell hacker.

Tests are executed using [Bats](https://github.com/bats-core/bats-core):

    bats test
    bats/test/<file>.bats

Please feel free to submit pull requests and file bugs on the [issue
tracker](https://github.com/pyenv/pyenv/issues).


  [pyenv-virtualenv]: https://github.com/pyenv/pyenv-virtualenv#readme
  [hooks]: https://github.com/pyenv/pyenv/wiki/Authoring-plugins#pyenv-hooks

### Version History

See [CHANGELOG.md](CHANGELOG.md).

### License

[The MIT License](LICENSE)