Re-land: Added virtualenv for depot_tools

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