
``venv`` --- Creation of virtual environments
*********************************************

New in version 3.3.

**Source code:** Lib/venv.py

======================================================================

The ``venv`` module provides support for creating lightweight "virtual
environments" with their own site directories, optionally isolated
from system site directories.  Each virtual environment has its own
Python binary (allowing creation of environments with various Python
versions) and can have its own independent set of installed Python
packages in its site directories.


Creating virtual environments
=============================

Creation of *virtual environments* is done by executing the ``pyvenv``
script:

   pyvenv /path/to/new/virtual/environment

Running this command creates the target directory (creating any parent
directories that don't exist already) and places a ``pyvenv.cfg`` file
in it with a ``home`` key pointing to the Python installation the
command was run from.  It also creates a ``bin`` (or ``Scripts`` on
Windows) subdirectory containing a copy of the ``python`` binary (or
binaries, in the case of Windows).  It also creates an (initially
empty) ``lib/pythonX.Y/site-packages`` subdirectory (on Windows, this
is ``Lib\site-packages``).

On Windows, you may have to invoke the ``pyvenv`` script as follows,
if you don't have the relevant PATH and PATHEXT settings:

   c:\Temp>c:\Python33\python c:\Python33\Tools\Scripts\pyvenv.py myenv

or equivalently:

   c:\Temp>c:\Python33\python -m venv myenv

The command, if run with ``-h``, will show the available options:

   usage: pyvenv [-h] [--system-site-packages] [--symlinks] [--clear]
                 [--upgrade] ENV_DIR [ENV_DIR ...]

   Creates virtual Python environments in one or more target directories.

   positional arguments:
     ENV_DIR             A directory to create the environment in.

   optional arguments:
     -h, --help             show this help message and exit
     --system-site-packages Give access to the global site-packages dir to the
                            virtual environment.
     --symlinks             Try to use symlinks rather than copies, when symlinks
                            are not the default for the platform.
     --clear                Delete the environment directory if it already exists.
                            If not specified and the directory exists, an error is
                            raised.
     --upgrade              Upgrade the environment directory to use this version
                            of Python, assuming Python has been upgraded in-place.

If the target directory already exists an error will be raised, unless
the ``--clear`` or ``--upgrade`` option was provided.

The created ``pyvenv.cfg`` file also includes the
``include-system-site-packages`` key, set to ``true`` if ``venv`` is
run with the ``--system-site-packages`` option, ``false`` otherwise.

Multiple paths can be given to ``pyvenv``, in which case an identical
virtualenv will be created, according to the given options, at each
provided path.

Once a venv has been created, it can be "activated" using a script in
the venv's binary directory. The invocation of the script is platform-
specific: on a Posix platform, you would typically do:

   $ source <venv>/bin/activate

whereas on Windows, you might do:

   C:\> <venv>/Scripts/activate

if you are using the ``cmd.exe`` shell, or perhaps:

   PS C:\> <venv>/Scripts/Activate.ps1

if you use PowerShell.

You don't specifically *need* to activate an environment; activation
just prepends the venv's binary directory to your path, so that
"python" invokes the venv's Python interpreter and you can run
installed scripts without having to use their full path. However, all
scripts installed in a venv should be runnable without activating it,
and run with the venv's Python automatically.

You can deactivate a venv by typing "deactivate" in your shell. The
exact mechanism is platform-specific: for example, the Bash activation
script defines a "deactivate" function, whereas on Windows there are
separate scripts called ``deactivate.bat`` and ``Deactivate.ps1``
which are installed when the venv is created.

Note: A virtual environment (also called a ``venv``) is a Python
  environment such that the Python interpreter, libraries and scripts
  installed into it are isolated from those installed in other virtual
  environments, and (by default) any libraries installed in a "system"
  Python, i.e. one which is installed as part of your operating
  system.A venv is a directory tree which contains Python executable
  files and other files which indicate that it is a venv.Common
  installation tools such as ``Distribute`` and ``pip`` work as
  expected with venvs - i.e. when a venv is active, they install
  Python packages into the venv without needing to be told to do so
  explicitly. Of course, you need to install them into the venv first:
  this could be done by running ``distribute_setup.py`` with the venv
  activated, followed by running ``easy_install pip``. Alternatively,
  you could download the source tarballs and run ``python setup.py
  install`` after unpacking, with the venv activated.When a venv is
  active (i.e. the venv's Python interpreter is running), the
  attributes ``sys.prefix`` and ``sys.exec_prefix`` point to the base
  directory of the venv, whereas ``sys.base_prefix`` and
  ``sys.base_exec_prefix`` point to the non-venv Python installation
  which was used to create the venv. If a venv is not active, then
  ``sys.prefix`` is the same as ``sys.base_prefix`` and
  ``sys.exec_prefix`` is the same as ``sys.base_exec_prefix`` (they
  all point to a non-venv Python installation).


API
===

The high-level method described above makes use of a simple API which
provides mechanisms for third-party virtual environment creators to
customize environment creation according to their needs, the
``EnvBuilder`` class.

class class venv.EnvBuilder(system_site_packages=False, clear=False, symlinks=False, upgrade=False)

   The ``EnvBuilder`` class accepts the following keyword arguments on
   instantiation:

   * ``system_site_packages`` -- a Boolean value indicating that the
     system Python site-packages should be available to the
     environment (defaults to ``False``).

   * ``clear`` -- a Boolean value which, if True, will delete any
     existing target directory instead of raising an exception
     (defaults to ``False``).

   * ``symlinks`` -- a Boolean value indicating whether to attempt to
     symlink the Python binary (and any necessary DLLs or other
     binaries, e.g. ``pythonw.exe``), rather than copying. Defaults to
     ``True`` on Linux and Unix systems, but ``False`` on Windows.

   * ``upgrade`` -- a Boolean value which, if True, will upgrade an
     existing environment with the running Python - for use when that
     Python has been upgraded in-place (defaults to ``False``).

   Creators of third-party virtual environment tools will be free to
   use the provided ``EnvBuilder`` class as a base class.

   The returned env-builder is an object which has a method,
   ``create``:

   create(env_dir)

      This method takes as required argument the path (absolute or
      relative to the current directory) of the target directory which
      is to contain the virtual environment.  The ``create`` method
      will either create the environment in the specified directory,
      or raise an appropriate exception.

      The ``create`` method of the ``EnvBuilder`` class illustrates
      the hooks available for subclass customization:

         def create(self, env_dir):
             """
             Create a virtualized Python environment in a directory.
             env_dir is the target directory to create an environment in.
             """
             env_dir = os.path.abspath(env_dir)
             context = self.create_directories(env_dir)
             self.create_configuration(context)
             self.setup_python(context)
             self.setup_scripts(context)
             self.post_setup(context)

      Each of the methods ``create_directories()``,
      ``create_configuration()``, ``setup_python()``,
      ``setup_scripts()`` and ``post_setup()`` can be overridden.

   create_directories(env_dir)

      Creates the environment directory and all necessary directories,
      and returns a context object.  This is just a holder for
      attributes (such as paths), for use by the other methods.

   create_configuration(context)

      Creates the ``pyvenv.cfg`` configuration file in the
      environment.

   setup_python(context)

      Creates a copy of the Python executable (and, under Windows,
      DLLs) in the environment.

   setup_scripts(context)

      Installs activation scripts appropriate to the platform into the
      virtual environment.

   post_setup(context)

      A placeholder method which can be overridden in third party
      implementations to pre-install packages in the virtual
      environment or perform other post-creation steps.

   In addition, ``EnvBuilder`` provides this utility method that can
   be called from ``setup_scripts()`` or ``post_setup()`` in
   subclasses to assist in installing custom scripts into the virtual
   environment.

   install_scripts(context, path)

      *path* is the path to a directory that should contain
      subdirectories "common", "posix", "nt", each containing scripts
      destined for the bin directory in the environment.  The contents
      of "common" and the directory corresponding to ``os.name`` are
      copied after some text replacement of placeholders:

      * ``__VENV_DIR__`` is replaced with the absolute path of the
        environment directory.

      * ``__VENV_NAME__`` is replaced with the environment name (final
        path segment of environment directory).

      * ``__VENV_BIN_NAME__`` is replaced with the name of the bin
        directory (either ``bin`` or ``Scripts``).

      * ``__VENV_PYTHON__`` is replaced with the absolute path of the
        environment's executable.

There is also a module-level convenience function:

venv.create(env_dir, system_site_packages=False, clear=False, symlinks=False)

   Create an ``EnvBuilder`` with the given keyword arguments, and call
   its ``create()`` method with the *env_dir* argument.
