
virtualenv
**********

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What It Does
============

"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 either 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 installer, so you can use
"ENV/bin/pip" to install additional packages into the environment.


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.


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 library installed.


PyPy Support
------------

Beginning with virtualenv version 1.5 PyPy 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.


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 or mod_wsgi
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
===============================

Note: Currently, this feature only partially works for pip, and not at all
  for setuptools. For details, see Issue #327

This option allows you to provide your own versions of setuptools
and/or pip on the filesystem, and tell virtualenv to use those
distributions instead of the ones in "virtualenv_support".

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 distributions. Setuptools distributions
must be ".egg" files; pip distributions should be *.tar.gz* source
distributions.

If no satisfactory local distributions are found, virtualenv will
fail. Virtualenv will never download packages.

The distribution lookup is done in the following locations, with the
most recent version found used:

1. The current directory.

2. The directory where virtualenv.py is located.

3. A "virtualenv_support" directory relative to the directory where
   virtualenv.py is located.

4. If the file being executed is not named virtualenv.py (i.e. is a
   boot script), a "virtualenv_support" directory relative to wherever
   virtualenv.py is actually installed.


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 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 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 contributing to pip 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.


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.

* Blog announcement.

* Doug Hellmann wrote a description of his command-line work flow
  using virtualenv (virtualenvwrapper) including some handy scripts to
  make working with multiple environments easier. He also wrote an
  example of using virtualenv to try IPython.

* Chris Perkins created a showmedo video including virtualenv.

* Using virtualenv with mod_wsgi.

* virtualenv commands for some more workflow-related tools around
  virtualenv.


Status and License
==================

"virtualenv" is a successor to workingenv, and an extension of
virtual-python.

It was written by Ian Bicking, sponsored by the Open Planning Project
and is now maintained by a group of developers. It is licensed under
an MIT-style permissive license.
