python-guide/docs/dev/virtualenvs.rst

Virtual Environments
====================

A Virtual Environment is a tool to keep the dependencies required by different
projects in separate places, by creating virtual Python environments for them.
It solves the "Project X depends on version 1.x but, Project Y needs 4.x"
dilemma, and keeps your global site-packages directory clean and manageable.

For example, you can work on a project which requires Django 1.3 while also
maintaining a project which requires Django 1.0.

virtualenv
----------

`virtualenv <http://pypi.python.org/pypi/virtualenv>`_ is a tool to create
isolated Python environments. virtualenv creates a folder which contains all the 
necessary executables to use the packages that a Python project would need. 

Install virtualenv via pip:

.. code-block:: console

  $ pip install virtualenv

Basic Usage
~~~~~~~~~~~

1. Create a virtual environment for a project:

.. code-block:: console

   $ cd my_project_folder
   $ virtualenv venv

``virtualenv venv`` will create a folder in the current directory which will
contain the Python executable files, and a copy of the ``pip`` library which you
can use to install other packages. The name of the virtual environment (in this
case, it was ``venv``) can be anything; omitting the name will place the files
in the current directory instead.

This creates a copy of Python in whichever directory you ran the command in,
placing it in a folder named :file:`venv`.

You can also use a Python interpreter of your choice.

.. code-block:: console

   $ virtualenv -p /usr/bin/python2.7 venv

This will use the Python interpreter in :file:`/usr/bin/python2.7`

2. To begin using the virtual environment, it needs to be activated:

.. code-block:: console

   $ source venv/bin/activate

The name of the current virtual environment will now appear on the left of 
the prompt (e.g. ``(venv)Your-Computer:your_project UserName$)`` to let you know 
that it's active. From now on, any package that you install using pip will be 
placed in the ``venv`` folder, isolated from the global Python installation.

Install packages as usual, for example:

.. code-block:: console

    $ pip install requests

3. If you are done working in the virtual environment for the moment, you can
   deactivate it:

.. code-block:: console

   $ deactivate

This puts you back to the system's default Python interpreter with all its
installed libraries.

To delete a virtual environment, just delete its folder. (In this case, 
it would be ``rm -rf venv``.)

After a while, though, you might end up with a lot of virtual environments
littered across your system, and its possible you'll forget their names or
where they were placed.

Other Notes
~~~~~~~~~~~

Running ``virtualenv`` with the option :option:`--no-site-packages` will not
include the packages that are installed globally. This can be useful
for keeping the package list clean in case it needs to be accessed later.
[This is the default behavior for ``virtualenv`` 1.7 and later.]

In order to keep your environment consistent, it's a good idea to "freeze"
the current state of the environment packages. To do this, run

.. code-block:: console

    $ pip freeze > requirements.txt

This will create a :file:`requirements.txt` file, which contains a simple
list of all the packages in the current environment, and their respective
versions. You can see the list of installed packages without the requirements 
format using "pip list". Later it will be easier for a different developer 
(or you, if you need to re-create the environment) to install the same packages
using the same versions:

.. code-block:: console

    $ pip install -r requirements.txt

This can help ensure consistency across installations, across deployments,
and across developers.

Lastly, remember to exclude the virtual environment folder from source
control by adding it to the ignore list.

.. _virtualenvwrapper-ref:

virtualenvwrapper
-----------------

`virtualenvwrapper <http://virtualenvwrapper.readthedocs.org/en/latest/index.html>`_
provides a set of commands which makes working with virtual environments much
more pleasant. It also places all your virtual environments in one place.

To install (make sure **virtualenv** is already installed):

.. code-block:: console

  $ pip install virtualenvwrapper
  $ export WORKON_HOME=~/Envs
  $ source /usr/local/bin/virtualenvwrapper.sh

(`Full virtualenvwrapper install instructions <http://virtualenvwrapper.readthedocs.org/en/latest/install.html>`_.)

For Windows, you can use the `virtualenvwrapper-win <https://github.com/davidmarble/virtualenvwrapper-win/>`_.

To install (make sure **virtualenv** is already installed):

.. code-block:: console

  $ pip install virtualenvwrapper-win
  
In Windows, the default path for WORKON_HOME is %USERPROFILE%\Envs

Basic Usage
~~~~~~~~~~~

1. Create a virtual environment:

.. code-block:: console

   $ mkvirtualenv venv

This creates the :file:`venv` folder inside :file:`~/Envs`.

2. Work on a virtual environment:

.. code-block:: console

   $ workon venv

Alternatively, you can make a project, which creates the virtual environment,
and also a project directory inside ``$PROJECT_HOME``, which is ``cd`` -ed into
when you ``workon myproject``.

.. code-block:: console

   $ mkproject myproject

**virtualenvwrapper** provides tab-completion on environment names. It really
helps when you have a lot of environments and have trouble remembering their
names.

``workon`` also deactivates whatever environment you are currently in, so you
can quickly switch between environments.

3. Deactivating is still the same:

.. code-block:: console

   $ deactivate

4. To delete:

.. code-block:: console

   $ rmvirtualenv venv

Other useful commands
~~~~~~~~~~~~~~~~~~~~~

``lsvirtualenv``
  List all of the environments.

``cdvirtualenv``
  Navigate into the directory of the currently activated virtual environment,
  so you can browse its :file:`site-packages`, for example.

``cdsitepackages``
  Like the above, but directly into :file:`site-packages` directory.

``lssitepackages``
  Shows contents of :file:`site-packages` directory.

`Full list of virtualenvwrapper commands <http://virtualenvwrapper.readthedocs.org/en/latest/command_ref.html>`_.

autoenv
-------
When you ``cd`` into a directory containing a :file:`.env`, `autoenv <https://github.com/kennethreitz/autoenv>`_
automagically activates the environment.

Install it on Mac OS X using ``brew``:

.. code-block:: console

   $ brew install autoenv

And on Linux:

.. code-block:: console

   $ git clone git://github.com/kennethreitz/autoenv.git ~/.autoenv
   $ echo 'source ~/.autoenv/activate.sh' >> ~/.bashrc