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+: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
+-----------------------------
+
+.. include:: /using/venv-create.inc
+
+
+.. _venv-def:
+
+.. 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 :attr:`sys.prefix` and :attr:`sys.exec_prefix` point to the base
+   directory of the venv, whereas :attr:`sys.base_prefix` and
+   :attr:`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
+   :attr:`sys.prefix` is the same as :attr:`sys.base_prefix` and
+   :attr:`sys.exec_prefix` is the same as :attr:`sys.base_exec_prefix` (they
+   all point to a non-venv Python installation).
+
+
+API
+---
+
+.. highlight:: python
+
+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.
+
+.. class:: EnvBuilder(system_site_packages=False, clear=False, symlinks=False, upgrade=False)
+
+    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.
+
+    * ``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``:
+
+    .. method:: 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 :meth:`create_directories`,
+        :meth:`create_configuration`, :meth:`setup_python`,
+        :meth:`setup_scripts` and :meth:`post_setup` can be overridden.
+
+    .. method:: 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.
+
+    .. method:: create_configuration(context)
+
+        Creates the ``pyvenv.cfg`` configuration file in the environment.
+
+    .. method:: setup_python(context)
+
+        Creates a copy of the Python executable (and, under Windows, DLLs) in
+        the environment.
+
+    .. method:: setup_scripts(context)
+
+        Installs activation scripts appropriate to the platform into the virtual
+        environment.
+
+    .. method:: 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, :class:`EnvBuilder` provides this utility method that can be
+    called from :meth:`setup_scripts` or :meth:`post_setup` in subclasses to
+    assist in installing custom scripts into the virtual environment.
+
+    .. method:: 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 :data:`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:
+
+.. function:: create(env_dir, system_site_packages=False, clear=False, symlinks=False)
+
+    Create an :class:`EnvBuilder` with the given keyword arguments, and call its
+    :meth:`~EnvBuilder.create` method with the *env_dir* argument.