path: root/testing/mozharness/external_tools/virtualenv/docs/userguide.rst

User Guide
==========


Usage
-----

Virtualenv has one basic command::

    $ virtualenv ENV

Where ``ENV`` is a directory to place the new virtual environment. It has
a number of usual effects (modifiable by many :ref:`options`):

 - :file:`ENV/lib/` and :file:`ENV/include/` are created, containing supporting
   library files for a new virtualenv python. Packages installed in this
   environment will live under :file:`ENV/lib/pythonX.X/site-packages/`.

 - :file:`ENV/bin` is created, where executables live - noticeably a new
   :command:`python`. Thus running a script with ``#! /path/to/ENV/bin/python``
   would run that script under this virtualenv's python.

 - The crucial packages pip_ and setuptools_ are installed, which allow other
   packages to be easily installed to the environment. This associated pip
   can be run from :file:`ENV/bin/pip`.

The python in your new virtualenv is effectively isolated from the python that
was used to create it.

.. _pip: https://pypi.python.org/pypi/pip
.. _setuptools: https://pypi.python.org/pypi/setuptools


.. _activate:

activate script
~~~~~~~~~~~~~~~

In a newly created virtualenv there will also be a :command:`activate` shell
script. For Windows systems, activation scripts are provided for
the Command Prompt and Powershell.

On Posix systems, this resides in :file:`/ENV/bin/`, so you can run::

    $ source bin/activate

For some shells (e.g. the original Bourne Shell) you may need to use the
:command:`.` command, when :command:`source` does not exist. There are also
separate activate files for some other shells, like csh and fish.
:file:`bin/activate` should work for bash/zsh/dash.

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.

The ``activate`` script will also modify your shell prompt to indicate
which environment is currently active. To disable this behaviour, see
:envvar:`VIRTUAL_ENV_DISABLE_PROMPT`.

To undo these changes to your path (and prompt), just run::

    $ deactivate

On Windows, the equivalent `activate` script is in the ``Scripts`` folder::

    > \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 can 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

Removing an Environment
~~~~~~~~~~~~~~~~~~~~~~~

Removing a virtual environment is simply done by deactivating it and deleting the
environment folder with all its contents::

    (ENV)$ deactivate
    $ rm -r /path/to/ENV

The :option:`--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.

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.


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

The :option:`--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.