cpython/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>

.. versionadded:: 3.3

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

.. index:: pair: Environments; virtual

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

.. _venv-def:
.. _venv-intro:

The :mod:`!venv` module supports creating lightweight "virtual environments",
each with their own independent set of Python packages installed in
their :mod:`site` directories.
A virtual environment is created on top of an existing
Python installation, known as the virtual environment's "base" Python, and may
optionally be isolated from the packages in the base environment,
so only those explicitly installed in the virtual environment are available.

When used from within a virtual environment, common installation tools such as
:pypi:`pip` will install Python packages into a virtual environment
without needing to be told to do so explicitly.

A virtual environment is (amongst other things):

* Used to contain a specific Python interpreter and software libraries and
  binaries which are needed to support a project (library or application). These
  are by default isolated from software in other virtual environments and Python
  interpreters and libraries installed in the operating system.

* Contained in a directory, conventionally either named ``venv`` or ``.venv`` in
  the project directory, or under a container directory for lots of virtual
  environments, such as ``~/.virtualenvs``.

* Not checked into source control systems such as Git.

* Considered as disposable -- it should be simple to delete and recreate it from
  scratch. You don't place any project code in the environment

* Not considered as movable or copyable -- you just recreate the same
  environment in the target location.

See :pep:`405` for more background on Python virtual environments.

.. seealso::

   `Python Packaging User Guide: Creating and using virtual environments
   <https://packaging.python.org/guides/installing-using-pip-and-virtual-environments/#create-and-use-virtual-environments>`__

.. include:: ../includes/wasm-ios-notavail.rst

Creating virtual environments
-----------------------------

.. include:: /using/venv-create.inc

.. _venv-explanation:

How venvs work
--------------

When a Python interpreter is running from a virtual environment,
:data:`sys.prefix` and :data:`sys.exec_prefix`
point to the directories of the virtual environment,
whereas :data:`sys.base_prefix` and :data:`sys.base_exec_prefix`
point to those of the base Python used to create the environment.
It is sufficient to check
``sys.prefix != sys.base_prefix`` to determine if the current interpreter is
running from a virtual environment.

A virtual environment may be "activated" using a script in its binary directory
(``bin`` on POSIX; ``Scripts`` on Windows).
This will prepend that directory to your :envvar:`PATH`, so that running
:program:`python` will invoke the environment's Python interpreter
and you can run installed scripts without having to use their full path.
The invocation of the activation script is platform-specific
(:samp:`{<venv>}` must be replaced by the path to the directory
containing the virtual environment):

+-------------+------------+--------------------------------------------------+
| Platform    | Shell      | Command to activate virtual environment          |
+=============+============+==================================================+
| POSIX       | bash/zsh   | :samp:`$ source {<venv>}/bin/activate`           |
|             +------------+--------------------------------------------------+
|             | fish       | :samp:`$ source {<venv>}/bin/activate.fish`      |
|             +------------+--------------------------------------------------+
|             | csh/tcsh   | :samp:`$ source {<venv>}/bin/activate.csh`       |
|             +------------+--------------------------------------------------+
|             | PowerShell | :samp:`$ {<venv>}/bin/Activate.ps1`              |
+-------------+------------+--------------------------------------------------+
| Windows     | cmd.exe    | :samp:`C:\\> {<venv>}\\Scripts\\activate.bat`    |
|             +------------+--------------------------------------------------+
|             | PowerShell | :samp:`PS C:\\> {<venv>}\\Scripts\\Activate.ps1` |
+-------------+------------+--------------------------------------------------+

.. versionadded:: 3.4
   :program:`fish` and :program:`csh` activation scripts.

.. versionadded:: 3.8
   PowerShell activation scripts installed under POSIX for PowerShell Core
   support.

You don't specifically *need* to activate a virtual environment,
as you can just specify the full path to that environment's
Python interpreter when invoking Python.
Furthermore, all scripts installed in the environment
should be runnable without activating it.

In order to achieve this, scripts installed into virtual environments have
a "shebang" line which points to the environment's Python interpreter,
i.e. :samp:`#!/{<path-to-venv>}/bin/python`.
This means that the script will run with that interpreter regardless of the
value of :envvar:`PATH`. On Windows, "shebang" line processing is supported if
you have the :ref:`launcher` installed. Thus, double-clicking an installed
script in a Windows Explorer window should run it with the correct interpreter
without the environment needing to be activated or on the :envvar:`PATH`.

When a virtual environment has been activated, the :envvar:`!VIRTUAL_ENV`
environment variable is set to the path of the environment.
Since explicitly activating a virtual environment is not required to use it,
:envvar:`!VIRTUAL_ENV` cannot be relied upon to determine
whether a virtual environment is being used.

.. warning:: Because scripts installed in environments should not expect the
   environment to be activated, their shebang lines contain the absolute paths
   to their environment's interpreters. Because of this, environments are
   inherently non-portable, in the general case. You should always have a
   simple means of recreating an environment (for example, if you have a
   requirements file ``requirements.txt``, you can invoke ``pip install -r
   requirements.txt`` using the environment's ``pip`` to install all of the
   packages needed by the environment). If for any reason you need to move the
   environment to a new location, you should recreate it at the desired
   location and delete the one at the old location. If you move an environment
   because you moved a parent directory of it, you should recreate the
   environment in its new location. Otherwise, software installed into the
   environment may not work as expected.

You can deactivate a virtual environment by typing ``deactivate`` in your shell.
The exact mechanism is platform-specific and is an internal implementation
detail (typically, a script or shell function will be used).


.. _venv-api:

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, with_pip=False, \
                      prompt=None, upgrade_deps=False, \
                      *, scm_ignore_files=frozenset())

    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 the contents of
      any existing target directory, before creating the environment.

    * ``symlinks`` -- a Boolean value indicating whether to attempt to symlink the
      Python binary rather than copying.

    * ``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``).

    * ``with_pip`` -- a Boolean value which, if true, ensures pip is
      installed in the virtual environment. This uses :mod:`ensurepip` with
      the ``--default-pip`` option.

    * ``prompt`` -- a String to be used after virtual environment is activated
      (defaults to ``None`` which means directory name of the environment would
      be used). If the special string ``"."`` is provided, the basename of the
      current directory is used as the prompt.

    * ``upgrade_deps`` -- Update the base venv modules to the latest on PyPI

    * ``scm_ignore_files`` -- Create ignore files based for the specified source
      control managers (SCM) in the iterable. Support is defined by having a
      method named ``create_{scm}_ignore_file``. The only value supported by
      default is ``"git"`` via :meth:`create_git_ignore_file`.


    .. versionchanged:: 3.4
       Added the ``with_pip`` parameter

    .. versionchanged:: 3.6
       Added the ``prompt`` parameter

    .. versionchanged:: 3.9
       Added the ``upgrade_deps`` parameter

    .. versionchanged:: 3.13
       Added the ``scm_ignore_files`` parameter

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

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

    .. method:: create(env_dir)

        Create a virtual environment by specifying the target directory
        (absolute or relative to the current 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 :class:`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.ensure_directories(env_dir)
                self.create_configuration(context)
                self.setup_python(context)
                self.setup_scripts(context)
                self.post_setup(context)

        Each of the methods :meth:`ensure_directories`,
        :meth:`create_configuration`, :meth:`setup_python`,
        :meth:`setup_scripts` and :meth:`post_setup` can be overridden.

    .. method:: ensure_directories(env_dir)

        Creates the environment directory and all necessary subdirectories that
        don't already exist, and returns a context object.  This context object
        is just a holder for attributes (such as paths) for use by the other
        methods.  If the :class:`EnvBuilder` is created with the arg
        ``clear=True``, contents of the environment directory will be cleared
        and then all necessary subdirectories will be recreated.

        The returned context object is a :class:`types.SimpleNamespace` with the
        following attributes:

        * ``env_dir`` - The location of the virtual environment. Used for
          ``__VENV_DIR__`` in activation scripts (see :meth:`install_scripts`).

        * ``env_name`` - The name of the virtual environment. Used for
          ``__VENV_NAME__`` in activation scripts (see :meth:`install_scripts`).

        * ``prompt`` - The prompt to be used by the activation scripts. Used for
          ``__VENV_PROMPT__`` in activation scripts (see :meth:`install_scripts`).

        * ``executable`` - The underlying Python executable used by the virtual
          environment. This takes into account the case where a virtual environment
          is created from another virtual environment.

        * ``inc_path`` - The include path for the virtual environment.

        * ``lib_path`` - The purelib path for the virtual environment.

        * ``bin_path`` - The script path for the virtual environment.

        * ``bin_name`` - The name of the script path relative to the virtual
          environment location. Used for ``__VENV_BIN_NAME__`` in activation
          scripts (see :meth:`install_scripts`).

        * ``env_exe`` - The name of the Python interpreter in the virtual
          environment. Used for ``__VENV_PYTHON__`` in activation scripts
          (see :meth:`install_scripts`).

        * ``env_exec_cmd`` - The name of the Python interpreter, taking into
          account filesystem redirections. This can be used to run Python in
          the virtual environment.


        .. versionchanged:: 3.11
           The *venv*
           :ref:`sysconfig installation scheme <installation_paths>`
           is used to construct the paths of the created directories.

        .. versionchanged:: 3.12
           The attribute ``lib_path`` was added to the context, and the context
           object was documented.

    .. method:: create_configuration(context)

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

    .. method:: setup_python(context)

        Creates a copy or symlink to the Python executable in the environment.
        On POSIX systems, if a specific executable ``python3.x`` was used,
        symlinks to ``python`` and ``python3`` will be created pointing to that
        executable, unless files with those names already exist.

    .. method:: setup_scripts(context)

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

    .. method:: upgrade_dependencies(context)

       Upgrades the core venv dependency packages (currently ``pip``)
       in the environment. This is done by shelling out to the
       ``pip`` executable in the environment.

       .. versionadded:: 3.9
       .. versionchanged:: 3.12

          ``setuptools`` is no longer a core venv dependency.

    .. 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.

    .. versionchanged:: 3.7.2
       Windows now uses redirector scripts for ``python[w].exe`` instead of
       copying the actual binaries. In 3.7.2 only :meth:`setup_python` does
       nothing unless running from a build in the source tree.

    .. versionchanged:: 3.7.3
       Windows copies the redirector scripts as part of :meth:`setup_python`
       instead of :meth:`setup_scripts`. This was not the case in 3.7.2.
       When using symlinks, the original executables will be linked.

    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_PROMPT__`` is replaced with the prompt (the environment
          name surrounded by parentheses and with a following space)

        * ``__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.

        The directories are allowed to exist (for when an existing environment
        is being upgraded).

    .. method:: create_git_ignore_file(context)

       Creates a ``.gitignore`` file within the virtual environment that causes
       the entire directory to be ignored by the ``git`` source control manager.

       .. versionadded:: 3.13

There is also a module-level convenience function:

.. function:: create(env_dir, system_site_packages=False, clear=False, \
                     symlinks=False, with_pip=False, prompt=None, \
                     upgrade_deps=False, *, scm_ignore_files=frozenset())

    Create an :class:`EnvBuilder` with the given keyword arguments, and call its
    :meth:`~EnvBuilder.create` method with the *env_dir* argument.

    .. versionadded:: 3.3

    .. versionchanged:: 3.4
       Added the ``with_pip`` parameter

    .. versionchanged:: 3.6
       Added the ``prompt`` parameter

    .. versionchanged:: 3.9
       Added the ``upgrade_deps`` parameter

    .. versionchanged:: 3.13
       Added the ``scm_ignore_files`` parameter

An example of extending ``EnvBuilder``
--------------------------------------

The following script shows how to extend :class:`EnvBuilder` by implementing a
subclass which installs setuptools and pip into a created virtual environment::

    import os
    import os.path
    from subprocess import Popen, PIPE
    import sys
    from threading import Thread
    from urllib.parse import urlparse
    from urllib.request import urlretrieve
    import venv

    class ExtendedEnvBuilder(venv.EnvBuilder):
        """
        This builder installs setuptools and pip so that you can pip or
        easy_install other packages into the created virtual environment.

        :param nodist: If true, setuptools and pip are not installed into the
                       created virtual environment.
        :param nopip: If true, pip is not installed into the created
                      virtual environment.
        :param progress: If setuptools or pip are installed, the progress of the
                         installation can be monitored by passing a progress
                         callable. If specified, it is called with two
                         arguments: a string indicating some progress, and a
                         context indicating where the string is coming from.
                         The context argument can have one of three values:
                         'main', indicating that it is called from virtualize()
                         itself, and 'stdout' and 'stderr', which are obtained
                         by reading lines from the output streams of a subprocess
                         which is used to install the app.

                         If a callable is not specified, default progress
                         information is output to sys.stderr.
        """

        def __init__(self, *args, **kwargs):
            self.nodist = kwargs.pop('nodist', False)
            self.nopip = kwargs.pop('nopip', False)
            self.progress = kwargs.pop('progress', None)
            self.verbose = kwargs.pop('verbose', False)
            super().__init__(*args, **kwargs)

        def post_setup(self, context):
            """
            Set up any packages which need to be pre-installed into the
            virtual environment being created.

            :param context: The information for the virtual environment
                            creation request being processed.
            """
            os.environ['VIRTUAL_ENV'] = context.env_dir
            if not self.nodist:
                self.install_setuptools(context)
            # Can't install pip without setuptools
            if not self.nopip and not self.nodist:
                self.install_pip(context)

        def reader(self, stream, context):
            """
            Read lines from a subprocess' output stream and either pass to a progress
            callable (if specified) or write progress information to sys.stderr.
            """
            progress = self.progress
            while True:
                s = stream.readline()
                if not s:
                    break
                if progress is not None:
                    progress(s, context)
                else:
                    if not self.verbose:
                        sys.stderr.write('.')
                    else:
                        sys.stderr.write(s.decode('utf-8'))
                    sys.stderr.flush()
            stream.close()

        def install_script(self, context, name, url):
            _, _, path, _, _, _ = urlparse(url)
            fn = os.path.split(path)[-1]
            binpath = context.bin_path
            distpath = os.path.join(binpath, fn)
            # Download script into the virtual environment's binaries folder
            urlretrieve(url, distpath)
            progress = self.progress
            if self.verbose:
                term = '\n'
            else:
                term = ''
            if progress is not None:
                progress('Installing %s ...%s' % (name, term), 'main')
            else:
                sys.stderr.write('Installing %s ...%s' % (name, term))
                sys.stderr.flush()
            # Install in the virtual environment
            args = [context.env_exe, fn]
            p = Popen(args, stdout=PIPE, stderr=PIPE, cwd=binpath)
            t1 = Thread(target=self.reader, args=(p.stdout, 'stdout'))
            t1.start()
            t2 = Thread(target=self.reader, args=(p.stderr, 'stderr'))
            t2.start()
            p.wait()
            t1.join()
            t2.join()
            if progress is not None:
                progress('done.', 'main')
            else:
                sys.stderr.write('done.\n')
            # Clean up - no longer needed
            os.unlink(distpath)

        def install_setuptools(self, context):
            """
            Install setuptools in the virtual environment.

            :param context: The information for the virtual environment
                            creation request being processed.
            """
            url = "https://bootstrap.pypa.io/ez_setup.py"
            self.install_script(context, 'setuptools', url)
            # clear up the setuptools archive which gets downloaded
            pred = lambda o: o.startswith('setuptools-') and o.endswith('.tar.gz')
            files = filter(pred, os.listdir(context.bin_path))
            for f in files:
                f = os.path.join(context.bin_path, f)
                os.unlink(f)

        def install_pip(self, context):
            """
            Install pip in the virtual environment.

            :param context: The information for the virtual environment
                            creation request being processed.
            """
            url = 'https://bootstrap.pypa.io/get-pip.py'
            self.install_script(context, 'pip', url)


    def main(args=None):
        import argparse

        parser = argparse.ArgumentParser(prog=__name__,
                                         description='Creates virtual Python '
                                                     'environments in one or '
                                                     'more target '
                                                     'directories.')
        parser.add_argument('dirs', metavar='ENV_DIR', nargs='+',
                            help='A directory in which to create the '
                                 'virtual environment.')
        parser.add_argument('--no-setuptools', default=False,
                            action='store_true', dest='nodist',
                            help="Don't install setuptools or pip in the "
                                 "virtual environment.")
        parser.add_argument('--no-pip', default=False,
                            action='store_true', dest='nopip',
                            help="Don't install pip in the virtual "
                                 "environment.")
        parser.add_argument('--system-site-packages', default=False,
                            action='store_true', dest='system_site',
                            help='Give the virtual environment access to the '
                                 'system site-packages dir.')
        if os.name == 'nt':
            use_symlinks = False
        else:
            use_symlinks = True
        parser.add_argument('--symlinks', default=use_symlinks,
                            action='store_true', dest='symlinks',
                            help='Try to use symlinks rather than copies, '
                                 'when symlinks are not the default for '
                                 'the platform.')
        parser.add_argument('--clear', default=False, action='store_true',
                            dest='clear', help='Delete the contents of the '
                                               'virtual environment '
                                               'directory if it already '
                                               'exists, before virtual '
                                               'environment creation.')
        parser.add_argument('--upgrade', default=False, action='store_true',
                            dest='upgrade', help='Upgrade the virtual '
                                                 'environment directory to '
                                                 'use this version of '
                                                 'Python, assuming Python '
                                                 'has been upgraded '
                                                 'in-place.')
        parser.add_argument('--verbose', default=False, action='store_true',
                            dest='verbose', help='Display the output '
                                                 'from the scripts which '
                                                 'install setuptools and pip.')
        options = parser.parse_args(args)
        if options.upgrade and options.clear:
            raise ValueError('you cannot supply --upgrade and --clear together.')
        builder = ExtendedEnvBuilder(system_site_packages=options.system_site,
                                       clear=options.clear,
                                       symlinks=options.symlinks,
                                       upgrade=options.upgrade,
                                       nodist=options.nodist,
                                       nopip=options.nopip,
                                       verbose=options.verbose)
        for d in options.dirs:
            builder.create(d)

    if __name__ == '__main__':
        rc = 1
        try:
            main()
            rc = 0
        except Exception as e:
            print('Error: %s' % e, file=sys.stderr)
        sys.exit(rc)


This script is also available for download `online
<https://gist.github.com/vsajip/4673395>`_.