cpython_sandbox / Doc / library / venv.rst

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

Source code: :source:`Lib/`

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


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 with the venv activated, followed by running easy_install pip. Alternatively, you could download the source tarballs and run python 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).

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.


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.

There is also a module-level convenience function:

An example of extending EnvBuilder

The following script shows how to extend :class:`EnvBuilder` by implementing a subclass which installs Distribute 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 DistributeEnvBuilder(venv.EnvBuilder):
    This builder installs Distribute and pip so that you can pip or
    easy_install other packages into the created environment.

    :param nodist: If True, Distribute is not installed into the created
    :param nopip: If True, pip is not installed into the created
    :param progress: If Distribute 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.
        if not self.nodist:
        if not self.nopip:

    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:
            if progress is not None:
                progress(s, context)
                if not self.verbose:

    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 progress is not None:
            progress('Installing %s' %name, 'main')
            sys.stderr.write('Installing %s ' % name)
        # 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'))
        t2 = Thread(target=self.reader, args=(p.stderr, 'stderr'))
        if progress is not None:
            progress('done.', 'main')
        # Clean up - no longer needed

    def install_distribute(self, context):
        Install Distribute in the environment.

        :param context: The information for the environment creation request
                        being processed.
        url = ''
        self.install_script(context, 'distribute', url)
        # clear up the distribute archive which gets downloaded
        pred = lambda o: o.startswith('distribute-') 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)

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

        :param context: The information for the environment creation request
                        being processed.
        url = ''
        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')
        import argparse

        parser = argparse.ArgumentParser(prog=__name__,
                                         description='Creates virtual Python '
                                                     'environments in one or '
                                                     'more target '
        parser.add_argument('dirs', metavar='ENV_DIR', nargs='+',
                            help='A directory to create the environment in.')
        parser.add_argument('--no-distribute', default=False,
                            action='store_true', dest='nodist',
                            help="Don't install Distribute in the virtual "
        parser.add_argument('--no-pip', default=False,
                            action='store_true', dest='nopip',
                            help="Don't install pip in the virtual "
        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 == 'nt':
            use_symlinks = False
            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 Distribute and pip.')
        options = parser.parse_args(args)
        if options.upgrade and options.clear:
            raise ValueError('you cannot supply --upgrade and --clear together.')
        builder = DistributeEnvBuilder(system_site_packages=options.system_site,
        for d in options.dirs:

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

This script is also available for download online.