Revert of Removed virtualenv from depot_tools

bootstrap/virtualenv/docs/index.rst

+Virtualenv
+==========
+
+`Mailing list <http://groups.google.com/group/python-virtualenv>`_ |
+`Issues <https://github.com/pypa/virtualenv/issues>`_ |
+`Github <https://github.com/pypa/virtualenv>`_ |
+`PyPI <https://pypi.python.org/pypi/virtualenv/>`_ |
+User IRC: #pypa
+Dev IRC: #pypa-dev
+
+Introduction
+------------
+
+``virtualenv`` is a tool to create isolated Python environments.
+
+The basic problem being addressed is one of dependencies and versions,
+and indirectly permissions. Imagine you have an application that
+needs version 1 of LibFoo, but another application requires version
+2. How can you use both these applications?  If you install
+everything into ``/usr/lib/python2.7/site-packages`` (or whatever your
+platform's standard location is), it's easy to end up in a situation
+where you unintentionally upgrade an application that shouldn't be
+upgraded.
+
+Or more generally, what if you want to install an application *and
+leave it be*?  If an application works, any change in its libraries or
+the versions of those libraries can break the application.
+
+Also, what if you can't install packages into the global
+``site-packages`` directory?  For instance, on a shared host.
+
+In all these cases, ``virtualenv`` can help you. It creates an
+environment that has its own installation directories, that doesn't
+share libraries with other virtualenv environments (and optionally
+doesn't access the globally installed libraries either).
+
+.. comment: split here
+
+.. toctree::
+   :maxdepth: 2
+
+   installation
+   userguide
+   reference
+   development
+   changes
+
+.. warning::
+
+   Python bugfix releases 2.6.8, 2.7.3, 3.1.5 and 3.2.3 include a change that
+   will cause "import random" to fail with "cannot import name urandom" on any
+   virtualenv created on a Unix host with an earlier release of Python
+   2.6/2.7/3.1/3.2, if the underlying system Python is upgraded. This is due to
+   the fact that a virtualenv uses the system Python's standard library but
+   contains its own copy of the Python interpreter, so an upgrade to the system
+   Python results in a mismatch between the version of the Python interpreter
+   and the version of the standard library. It can be fixed by removing
+   ``$ENV/bin/python`` and re-running virtualenv on the same target directory
+   with the upgraded Python.
+
+Other Documentation and Links
+-----------------------------
+
+* `Blog announcement of virtualenv`__.
+
+  .. __: http://blog.ianbicking.org/2007/10/10/workingenv-is-dead-long-live-virtualenv/
+
+* James Gardner has written a tutorial on using `virtualenv with
+  Pylons
+  <http://wiki.pylonshq.com/display/pylonscookbook/Using+a+Virtualenv+Sandbox>`_.
+
+* Chris Perkins created a `showmedo video including virtualenv
+  <http://showmedo.com/videos/video?name=2910000&fromSeriesID=291>`_.
+
+* Doug Hellmann's `virtualenvwrapper`_ is a useful set of scripts to make
+  your workflow with many virtualenvs even easier. `His initial blog post on it`__.
+  He also wrote `an example of using virtualenv to try IPython`__.
+
+  .. _virtualenvwrapper: https://pypi.python.org/pypi/virtualenvwrapper/
+  .. __: http://www.doughellmann.com/articles/CompletelyDifferent-2008-05-virtualenvwrapper/index.html
+  .. __: http://www.doughellmann.com/articles/CompletelyDifferent-2008-02-ipython-and-virtualenv/index.html
+
+* `Pew`_ is another wrapper for virtualenv that makes use of a different
+  activation technique.
+
+  .. _Pew: https://pypi.python.org/pypi/pew/
+
+* `Using virtualenv with mod_wsgi
+  <http://code.google.com/p/modwsgi/wiki/VirtualEnvironments>`_.
+
+* `virtualenv commands
+  <https://github.com/thisismedium/virtualenv-commands>`_ for some more
+  workflow-related tools around virtualenv.
+
+* PyCon US 2011 talk: `Reverse-engineering Ian Bicking's brain: inside pip and virtualenv
+  <http://pyvideo.org/video/568/reverse-engineering-ian-bicking--39-s-brain--insi>`_.
+  By the end of the talk, you'll have a good idea exactly how pip
+  and virtualenv do their magic, and where to go looking in the source
+  for particular behaviors or bug fixes.
+
+Compare & Contrast with Alternatives
+------------------------------------
+
+There are several alternatives that create isolated environments:
+
+* ``workingenv`` (which I do not suggest you use anymore) is the
+  predecessor to this library. It used the main Python interpreter,
+  but relied on setting ``$PYTHONPATH`` to activate the environment.
+  This causes problems when running Python scripts that aren't part of
+  the environment (e.g., a globally installed ``hg`` or ``bzr``). It
+  also conflicted a lot with Setuptools.
+
+* `virtual-python
+  <http://peak.telecommunity.com/DevCenter/EasyInstall#creating-a-virtual-python>`_
+  is also a predecessor to this library. It uses only symlinks, so it
+  couldn't work on Windows. It also symlinks over the *entire*
+  standard library and global ``site-packages``. As a result, it
+  won't see new additions to the global ``site-packages``.
+
+  This script only symlinks a small portion of the standard library
+  into the environment, and so on Windows it is feasible to simply
+  copy these files over. Also, it creates a new/empty
+  ``site-packages`` and also adds the global ``site-packages`` to the
+  path, so updates are tracked separately. This script also installs
+  Setuptools automatically, saving a step and avoiding the need for
+  network access.
+
+* `zc.buildout <http://pypi.python.org/pypi/zc.buildout>`_ doesn't
+  create an isolated Python environment in the same style, but
+  achieves similar results through a declarative config file that sets
+  up scripts with very particular packages. As a declarative system,
+  it is somewhat easier to repeat and manage, but more difficult to
+  experiment with. ``zc.buildout`` includes the ability to setup
+  non-Python systems (e.g., a database server or an Apache instance).
+
+I *strongly* recommend anyone doing application development or
+deployment use one of these tools.