The Tor Browser / file revision

 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).

 Installation

 ------------

 .. warning::

     We advise installing virtualenv-1.9 or greater. Prior to version 1.9, the

     pip included in virtualenv did not not download from PyPI over SSL.

 .. warning::

     When using pip to install virtualenv, we advise using pip 1.3 or greater.

     Prior to version 1.3, pip did not not download from PyPI over SSL.

 .. warning::

     We advise against using easy_install to install virtualenv when using

     setuptools < 0.9.7, because easy_install didn't download from PyPI over SSL

     and was broken in some subtle ways.

 To install globally with `pip` (if you have pip 1.3 or greater installed globally):

 ::

  $ [sudo] pip install virtualenv

 Or to get the latest unreleased dev version:

 ::

  $ [sudo] pip install https://github.com/pypa/virtualenv/tarball/develop

 To install globally from source:

 ::

  $ curl -O https://pypi.python.org/packages/source/v/virtualenv/virtualenv-X.X.tar.gz

  $ tar xvfz virtualenv-X.X.tar.gz

  $ cd virtualenv-X.X

  $ [sudo] python setup.py install

 To *use* locally from source:

 ::

  $ curl -O https://pypi.python.org/packages/source/v/virtualenv/virtualenv-X.X.tar.gz

  $ tar xvfz virtualenv-X.X.tar.gz

  $ cd virtualenv-X.X

  $ python virtualenv.py myVE

 .. note::

     The ``virtualenv.py`` script is *not* supported if run without the

     necessary pip/setuptools/virtualenv distributions available locally. All

     of the installation methods above include a ``virtualenv_support``

     directory alongside ``virtualenv.py`` which contains a complete set of

     pip and setuptools distributions, and so are fully supported.

 Usage

 -----

 The basic usage is::

     $ virtualenv ENV

 This creates ``ENV/lib/pythonX.X/site-packages``, where any libraries you

 install will go. It also creates ``ENV/bin/python``, which is a Python

 interpreter that uses this environment. Anytime you use that interpreter

 (including when a script has ``#!/path/to/ENV/bin/python`` in it) the libraries

 in that environment will be used.

 It also installs `Setuptools

 <http://peak.telecommunity.com/DevCenter/setuptools>`_ into the environment.

 .. note::

   Virtualenv (<1.10) used to provide a ``--distribute`` option to use the

   setuptools fork Distribute_. Since Distribute has been merged back into

   setuptools this option is now no-op, it will always use the improved

   setuptools releases.

 A new virtualenv also includes the `pip <http://pypi.python.org/pypi/pip>`_

 installer, so you can use ``ENV/bin/pip`` to install additional packages into

 the environment.

 .. _Distribute: https://pypi.python.org/pypi/distribute

 activate script

 ~~~~~~~~~~~~~~~

 In a newly created virtualenv there will be a ``bin/activate`` shell

 script. For Windows systems, activation scripts are provided for CMD.exe

 and Powershell.

 On Posix systems you can do::

     $ source bin/activate

 This will change your ``$PATH`` so its first entry is the virtualenv's

 ``bin/`` directory. (You have to use ``source`` because it changes your

 shell environment in-place.) This is all it does; it's purely a

 convenience. If you directly run a script or the python interpreter

 from the virtualenv's ``bin/`` directory (e.g. ``path/to/env/bin/pip``

 or ``/path/to/env/bin/python script.py``) there's no need for

 activation.

 After activating an environment you can use the function ``deactivate`` to

 undo the changes to your ``$PATH``.

 The ``activate`` script will also modify your shell prompt to indicate

 which environment is currently active. You can disable this behavior,

 which can be useful if you have your own custom prompt that already

 displays the active environment name. To do so, set the

 ``VIRTUAL_ENV_DISABLE_PROMPT`` environment variable to any non-empty

 value before running the ``activate`` script.

 On Windows you just do::

     > \path\to\env\Scripts\activate

 And type `deactivate` to undo the changes.

 Based on your active shell (CMD.exe or Powershell.exe), Windows will use

 either activate.bat or activate.ps1 (as appropriate) to activate the

 virtual environment. If using Powershell, see the notes about code signing

 below.

 .. note::

     If using Powershell, the ``activate`` script is subject to the

     `execution policies`_ on the system. By default on Windows 7, the system's

     excution policy is set to ``Restricted``, meaning no scripts like the

     ``activate`` script are allowed to be executed. But that can't stop us

     from changing that slightly to allow it to be executed.

     In order to use the script, you have to relax your system's execution

     policy to ``AllSigned``, meaning all scripts on the system must be

     digitally signed to be executed. Since the virtualenv activation

     script is signed by one of the authors (Jannis Leidel) this level of

     the execution policy suffices. As an administrator run::

         PS C:\> Set-ExecutionPolicy AllSigned

     Then you'll be asked to trust the signer, when executing the script.

     You will be prompted with the following::

         PS C:\> virtualenv .\foo

         New python executable in C:\foo\Scripts\python.exe

         Installing setuptools................done.

         Installing pip...................done.

         PS C:\> .\foo\scripts\activate

         Do you want to run software from this untrusted publisher?

         File C:\foo\scripts\activate.ps1 is published by E=jannis@leidel.info,

         CN=Jannis Leidel, L=Berlin, S=Berlin, C=DE, Description=581796-Gh7xfJxkxQSIO4E0

         and is not trusted on your system. Only run scripts from trusted publishers.

         [V] Never run  [D] Do not run  [R] Run once  [A] Always run  [?] Help

         (default is "D"):A

         (foo) PS C:\>

     If you select ``[A] Always Run``, the certificate will be added to the

     Trusted Publishers of your user account, and will be trusted in this

     user's context henceforth. If you select ``[R] Run Once``, the script will

     be run, but you will be prometed on a subsequent invocation. Advanced users

     can add the signer's certificate to the Trusted Publishers of the Computer

     account to apply to all users (though this technique is out of scope of this

     document).

     Alternatively, you may relax the system execution policy to allow running

     of local scripts without verifying the code signature using the following::

         PS C:\> Set-ExecutionPolicy RemoteSigned

     Since the ``activate.ps1`` script is generated locally for each virtualenv,

     it is not considered a remote script and can then be executed.

 .. _`execution policies`: http://technet.microsoft.com/en-us/library/dd347641.aspx

 The ``--system-site-packages`` Option

 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 If you build with ``virtualenv --system-site-packages ENV``, your virtual

 environment will inherit packages from ``/usr/lib/python2.7/site-packages``

 (or wherever your global site-packages directory is).

 This can be used if you have control over the global site-packages directory,

 and you want to depend on the packages there. If you want isolation from the

 global system, do not use this flag.

 Environment variables and configuration files

 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 virtualenv can not only be configured by passing command line options such as

 ``--python`` but also by two other means:

 - Environment variables

   Each command line option is automatically used to look for environment

   variables with the name format ``VIRTUALENV_<UPPER_NAME>``. That means

   the name of the command line options are capitalized and have dashes

   (``'-'``) replaced with underscores (``'_'``).

   For example, to automatically use a custom Python binary instead of the

   one virtualenv is run with you can also set an environment variable::

       $ export VIRTUALENV_PYTHON=/opt/python-3.3/bin/python

       $ virtualenv ENV

   It's the same as passing the option to virtualenv directly::

       $ virtualenv --python=/opt/python-3.3/bin/python ENV

   This also works for appending command line options, like ``--find-links``.

   Just leave an empty space between the passsed values, e.g.::

       $ export VIRTUALENV_EXTRA_SEARCH_DIR="/path/to/dists /path/to/other/dists"

       $ virtualenv ENV

   is the same as calling::

       $ virtualenv --extra-search-dir=/path/to/dists --extra-search-dir=/path/to/other/dists ENV

 - Config files

   virtualenv also looks for a standard ini config file. On Unix and Mac OS X

   that's ``$HOME/.virtualenv/virtualenv.ini`` and on Windows, it's

   ``%APPDATA%\virtualenv\virtualenv.ini``.

   The names of the settings are derived from the long command line option,

   e.g. the option ``--python`` would look like this::

       [virtualenv]

       python = /opt/python-3.3/bin/python

   Appending options like ``--extra-search-dir`` can be written on multiple

   lines::

       [virtualenv]

       extra-search-dir =

           /path/to/dists

           /path/to/other/dists

 Please have a look at the output of ``virtualenv --help`` for a full list

 of supported options.

 Windows Notes

 ~~~~~~~~~~~~~

 Some paths within the virtualenv are slightly different on Windows: scripts and

 executables on Windows go in ``ENV\Scripts\`` instead of ``ENV/bin/`` and

 libraries go in ``ENV\Lib\`` rather than ``ENV/lib/``.

 To create a virtualenv under a path with spaces in it on Windows, you'll need

 the `win32api <http://sourceforge.net/projects/pywin32/>`_ library installed.

 PyPy Support

 ~~~~~~~~~~~~

 Beginning with virtualenv version 1.5 `PyPy <http://pypy.org>`_ is

 supported. To use PyPy 1.4 or 1.4.1, you need a version of virtualenv >= 1.5.

 To use PyPy 1.5, you need a version of virtualenv >= 1.6.1.

 Creating Your Own Bootstrap Scripts

 -----------------------------------

 While this creates an environment, it doesn't put anything into the

 environment. Developers may find it useful to distribute a script

 that sets up a particular environment, for example a script that

 installs a particular web application.

 To create a script like this, call

 ``virtualenv.create_bootstrap_script(extra_text)``, and write the

 result to your new bootstrapping script. Here's the documentation

 from the docstring:

 Creates a bootstrap script, which is like this script but with

 extend_parser, adjust_options, and after_install hooks.

 This returns a string that (written to disk of course) can be used

 as a bootstrap script with your own customizations. The script

 will be the standard virtualenv.py script, with your extra text

 added (your extra text should be Python code).

 If you include these functions, they will be called:

 ``extend_parser(optparse_parser)``:

     You can add or remove options from the parser here.

 ``adjust_options(options, args)``:

     You can change options here, or change the args (if you accept

     different kinds of arguments, be sure you modify ``args`` so it is

     only ``[DEST_DIR]``).

 ``after_install(options, home_dir)``:

     After everything is installed, this function is called. This

     is probably the function you are most likely to use. An

     example would be::

         def after_install(options, home_dir):

             if sys.platform == 'win32':

                 bin = 'Scripts'

             else:

                 bin = 'bin'

             subprocess.call([join(home_dir, bin, 'easy_install'),

                              'MyPackage'])

             subprocess.call([join(home_dir, bin, 'my-package-script'),

                              'setup', home_dir])

     This example immediately installs a package, and runs a setup

     script from that package.

 Bootstrap Example

 ~~~~~~~~~~~~~~~~~

 Here's a more concrete example of how you could use this::

     import virtualenv, textwrap

     output = virtualenv.create_bootstrap_script(textwrap.dedent("""

     import os, subprocess

     def after_install(options, home_dir):

         etc = join(home_dir, 'etc')

         if not os.path.exists(etc):

             os.makedirs(etc)

         subprocess.call([join(home_dir, 'bin', 'easy_install'),

                          'BlogApplication'])

         subprocess.call([join(home_dir, 'bin', 'paster'),

                          'make-config', 'BlogApplication',

                          join(etc, 'blog.ini')])

         subprocess.call([join(home_dir, 'bin', 'paster'),

                          'setup-app', join(etc, 'blog.ini')])

     """))

     f = open('blog-bootstrap.py', 'w').write(output)

 Another example is available `here

 <https://github.com/socialplanning/fassembler/blob/master/fassembler/create-venv-script.py>`_.

 Using Virtualenv without ``bin/python``

 ---------------------------------------

 Sometimes you can't or don't want to use the Python interpreter

 created by the virtualenv. For instance, in a `mod_python

 <http://www.modpython.org/>`_ or `mod_wsgi <http://www.modwsgi.org/>`_

 environment, there is only one interpreter.

 Luckily, it's easy. You must use the custom Python interpreter to

 *install* libraries. But to *use* libraries, you just have to be sure

 the path is correct. A script is available to correct the path. You

 can setup the environment like::

     activate_this = '/path/to/env/bin/activate_this.py'

     execfile(activate_this, dict(__file__=activate_this))

 This will change ``sys.path`` and even change ``sys.prefix``, but also allow

 you to use an existing interpreter. Items in your environment will show up

 first on ``sys.path``, before global items. However, global items will

 always be accessible (as if the ``--system-site-packages`` flag had been used

 in creating the environment, whether it was or not). Also, this cannot undo

 the activation of other environments, or modules that have been imported.

 You shouldn't try to, for instance, activate an environment before a web

 request; you should activate *one* environment as early as possible, and not

 do it again in that process.

 Making Environments Relocatable

 -------------------------------

 Note: this option is somewhat experimental, and there are probably

 caveats that have not yet been identified.

 .. warning::

     The ``--relocatable`` option currently has a number of issues,

     and is not guaranteed to work in all circumstances. It is possible

     that the option will be deprecated in a future version of ``virtualenv``.

 Normally environments are tied to a specific path. That means that

 you cannot move an environment around or copy it to another computer.

 You can fix up an environment to make it relocatable with the

 command::

     $ virtualenv --relocatable ENV

 This will make some of the files created by setuptools use relative paths,

 and will change all the scripts to use ``activate_this.py`` instead of using

 the location of the Python interpreter to select the environment.

 **Note:** scripts which have been made relocatable will only work if

 the virtualenv is activated, specifically the python executable from

 the virtualenv must be the first one on the system PATH. Also note that

 the activate scripts are not currently made relocatable by

 ``virtualenv --relocatable``.

 **Note:** you must run this after you've installed *any* packages into

 the environment. If you make an environment relocatable, then

 install a new package, you must run ``virtualenv --relocatable``

 again.

 Also, this **does not make your packages cross-platform**. You can

 move the directory around, but it can only be used on other similar

 computers. Some known environmental differences that can cause

 incompatibilities: a different version of Python, when one platform

 uses UCS2 for its internal unicode representation and another uses

 UCS4 (a compile-time option), obvious platform changes like Windows

 vs. Linux, or Intel vs. ARM, and if you have libraries that bind to C

 libraries on the system, if those C libraries are located somewhere

 different (either different versions, or a different filesystem

 layout).

 If you use this flag to create an environment, currently, the

 ``--system-site-packages`` option will be implied.

 The ``--extra-search-dir`` option

 ---------------------------------

 This option allows you to provide your own versions of setuptools and/or

 pip to use instead of the embedded versions that come with virtualenv.

 To use this feature, pass one or more ``--extra-search-dir`` options to

 virtualenv like this::

     $ virtualenv --extra-search-dir=/path/to/distributions ENV

 The ``/path/to/distributions`` path should point to a directory that contains

 setuptools and/or pip wheels.

 virtualenv will look for wheels in the specified directories, but will use

 pip's standard algorithm for selecting the wheel to install, which looks for

 the latest compatible wheel.

 As well as the extra directories, the search order includes:

 #. The ``virtualenv_support`` directory relative to virtualenv.py

 #. The directory where virtualenv.py is located.

 #. The current directory.

 If no satisfactory local distributions are found, virtualenv will

 fail. Virtualenv will never download packages.

 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.

 Contributing

 ------------

 Refer to the `pip development`_ documentation - it applies equally to

 virtualenv, except that virtualenv issues should filed on the `virtualenv

 repo`_ at GitHub.

 Virtualenv's release schedule is tied to pip's -- each time there's a new pip

 release, there will be a new virtualenv release that bundles the new version of

 pip.

 Files in the `virtualenv_embedded/` subdirectory are embedded into

 `virtualenv.py` itself as base64-encoded strings (in order to support

 single-file use of `virtualenv.py` without installing it). If your patch

 changes any file in `virtualenv_embedded/`, run `bin/rebuild-script.py` to

 update the embedded version of that file in `virtualenv.py`; commit that and

 submit it as part of your patch / pull request.

 .. _pip development: http://www.pip-installer.org/en/latest/development.html

 .. _virtualenv repo: https://github.com/pypa/virtualenv/

 Running the tests

 ~~~~~~~~~~~~~~~~~

 Virtualenv's test suite is small and not yet at all comprehensive, but we aim

 to grow it.

 The easy way to run tests (handles test dependencies automatically)::

     $ python setup.py test

 If you want to run only a selection of the tests, you'll need to run them

 directly with nose instead. Create a virtualenv, and install required

 packages::

     $ pip install nose mock

 Run nosetests::

     $ nosetests

 Or select just a single test file to run::

     $ nosetests tests.test_virtualenv

 Other Documentation and Links

 -----------------------------

 * James Gardner has written a tutorial on using `virtualenv with

   Pylons

   <http://wiki.pylonshq.com/display/pylonscookbook/Using+a+Virtualenv+Sandbox>`_.

 * `Blog announcement

   <http://blog.ianbicking.org/2007/10/10/workingenv-is-dead-long-live-virtualenv/>`_.

 * Doug Hellmann wrote a description of his `command-line work flow

   using virtualenv (virtualenvwrapper)

   <http://www.doughellmann.com/articles/CompletelyDifferent-2008-05-virtualenvwrapper/index.html>`_

   including some handy scripts to make working with multiple

   environments easier. He also wrote `an example of using virtualenv

   to try IPython

   <http://www.doughellmann.com/articles/CompletelyDifferent-2008-02-ipython-and-virtualenv/index.html>`_.

 * Chris Perkins created a `showmedo video including virtualenv

   <http://showmedo.com/videos/video?name=2910000&fromSeriesID=291>`_.

 * `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.

 Status and License

 ------------------

 ``virtualenv`` is a successor to `workingenv

 <http://cheeseshop.python.org/pypi/workingenv.py>`_, and an extension

 of `virtual-python

 <http://peak.telecommunity.com/DevCenter/EasyInstall#creating-a-virtual-python>`_.

 It was written by Ian Bicking, sponsored by the `Open Planning

 Project <http://openplans.org>`_ and is now maintained by a

 `group of developers <https://github.com/pypa/virtualenv/raw/master/AUTHORS.txt>`_.

 It is licensed under an

 `MIT-style permissive license <https://github.com/pypa/virtualenv/raw/master/LICENSE.txt>`_.