Doc/library/venv.rst

:mod:`venv` --- Creation of virtual environments
================================================

.. module:: venv
   :synopsis: Creation of virtual environments.
.. moduleauthor:: Vinay Sajip <vinay_sajip@yahoo.co.uk>
.. sectionauthor:: Vinay Sajip <vinay_sajip@yahoo.co.uk>


.. index:: pair: Environments; virtual

.. versionadded:: 3.3

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

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

The :mod:`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 simplest 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) and the ``pysetup3`` script (to
facilitate easy installation of packages from PyPI into the new virtualenv).
It also creates an (initially empty) ``lib/pythonX.Y/site-packages``
subdirectory (on Windows, this is ``Lib\site-packages``).

.. highlight:: none

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] [--symlink] [--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.
      --symlink              Attempt to symlink rather than copy.
      --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.


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 :class:`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 and
     Mac OS X.

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

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

.. highlight:: python

The ``venv`` module will also provide a module-level function as a
convenience::

    def create(env_dir,
               system_site_packages=False, clear=False, symlinks=False):
        builder = EnvBuilder(
            system_site_packages=system_site_packages,
            clear=clear,
            symlinks=symlinks)
        builder.create(env_dir)

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.

        :param env_dir: 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.  The functions of these methods are:

   * ``create_directories`` - 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`` - creates the ``pyvenv.cfg``
     configuration file in the environment.

   * ``setup_python`` - creates a copy of the Python executable (and,
     under Windows, DLLs) in the environment.

   * ``setup_scripts`` - Installs activation scripts appropriate to the
     platform into the virtual environment.

   * ``post_setup`` - 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 an ``install_scripts`` utility
method that can be called from ``setup_scripts`` or ``post_setup`` in
subclasses to assist in installing custom scripts into the virtual
environment. The method accepts as arguments the context object (see
above) and a path to a directory. The directory 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.