28.3. "venv" — Creation of virtual environments
***********************************************

New in version 3.3.

**Source code:** Lib/venv/

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

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.

See **PEP 405** for more information about Python virtual
environments.

Note: The "pyvenv" script has been deprecated as of Python 3.6 in
  favor of using "python3 -m venv" to help prevent any potential
  confusion as to which Python interpreter a virtual environment will
  be based on.


28.3.1. 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").

See also: Python Packaging User Guide: Creating and using virtual
  environments

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:\Python35\python c:\Python35\Tools\Scripts\pyvenv.py myenv

or equivalently:

   c:\Temp>c:\Python35\python -m venv myenv

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

   usage: venv [-h] [--system-site-packages] [--symlinks | --copies] [--clear]
               [--upgrade] [--without-pip]
               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 the virtual environment access to the system
                            site-packages dir.
     --symlinks             Try to use symlinks rather than copies, when symlinks
                            are not the default for the platform.
     --copies               Try to use copies rather than symlinks, even when
                            symlinks are the default for the platform.
     --clear                Delete the contents of the environment directory if it
                            already exists, before environment creation.
     --upgrade              Upgrade the environment directory to use this version
                            of Python, assuming Python has been upgraded in-place.
     --without-pip          Skips installing or upgrading pip in the virtual
                            environment (pip is bootstrapped by default)

Depending on how the "venv" functionality has been invoked, the usage
message may vary slightly, e.g. referencing "pyvenv" rather than
"venv".

Changed in version 3.4: Installs pip by default, added the "--without-
pip"  and "--copies" options

Changed in version 3.4: In earlier versions, if the target directory
already existed, an error was raised, unless the "--clear" or "--
upgrade" option was provided. Now, if an existing directory is
specified, its contents are removed and the directory is processed as
if it had been newly created.

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.

Unless the "--without-pip" option is given, "ensurepip" will be
invoked to bootstrap "pip" into the virtual environment.

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:

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

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.

New in version 3.4: "fish" and "csh" activation scripts.

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 "Setuptools" 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.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).When a venv is active, any options that change
  the installation path will be ignored from all distutils
  configuration files to prevent projects being inadvertently
  installed outside of the virtual environment.When working in a
  command shell, users can make a venv active by running an "activate"
  script in the venv’s executables directory (the precise filename is
  shell-dependent), which prepends the venv’s directory for
  executables to the "PATH" environment variable for the running
  shell. There should be no need in other circumstances to activate a
  venv – scripts installed into venvs have a shebang line which points
  to the venv’s Python interpreter. This means that the script will
  run with that interpreter regardless of the value of "PATH". On
  Windows, shebang line processing is supported if you have the Python
  Launcher for Windows installed (this was added to Python in 3.3 -
  see **PEP 397** for more details). Thus, double-clicking an
  installed script in a Windows Explorer window should run the script
  with the correct interpreter without there needing to be any
  reference to its venv in "PATH".


28.3.2. 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 venv.EnvBuilder(system_site_packages=False, clear=False, symlinks=False, upgrade=False, with_pip=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 the
     contents of any existing target directory, before creating the
     environment.

   * "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").

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

   Changed in version 3.4: Added the "with_pip" parameter

   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.ensure_directories(env_dir)
             self.create_configuration(context)
             self.setup_python(context)
             self.setup_scripts(context)
             self.post_setup(context)

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

   ensure_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. The
      directories are allowed to exist already, as long as either
      "clear" or "upgrade" were specified to allow operating on an
      existing environment directory.

   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. On a POSIX system, 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.

   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_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).

There is also a module-level convenience function:

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

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

   Changed in version 3.4: Added the "with_pip" parameter


28.3.3. An example of extending "EnvBuilder"
============================================

The following script shows how to extend "EnvBuilder" by implementing
a subclass which installs setuptools and pip into a created venv:

   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 environment.

       :param nodist: If True, setuptools and pip are not installed into the
                      created environment.
       :param nopip: If True, pip is not installed into the created
                     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
           environment being created.

           :param context: The information for the 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 env'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 env
           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 environment.

           :param context: The information for the environment creation request
                           being processed.
           """
           url = 'https://bitbucket.org/pypa/setuptools/downloads/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 environment.

           :param context: The information for the environment creation request
                           being processed.
           """
           url = 'https://raw.github.com/pypa/pip/master/contrib/get-pip.py'
           self.install_script(context, 'pip', url)

   def main(args=None):
       compatible = True
       if sys.version_info < (3, 3):
           compatible = False
       elif not hasattr(sys, 'base_prefix'):
           compatible = False
       if not compatible:
           raise ValueError('This script is only for use with '
                            'Python 3.3 or later')
       else:
           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 to create the environment in.')
           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 '
                                                  'environment directory if it '
                                                  'already exists, before '
                                                  'environment creation.')
           parser.add_argument('--upgrade', default=False, action='store_true',
                               dest='upgrade', help='Upgrade the 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.
