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

Note

A virtual environment 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 virtual environment is a directory tree which contains Python executable files and other files which indicate that it is a virtual environment.

Common installation tools such as pip work as expected with virtual environments. In other words, when a virtual environment is active, they install Python packages into the virtual environment without needing to be told to do so explicitly.

When a virtual environment is active (i.e., the virtual environment's Python interpreter is running), the attributes :attr:`sys.prefix` and :attr:`sys.exec_prefix` point to the base directory of the virtual environment, whereas :attr:`sys.base_prefix` and :attr:`sys.base_exec_prefix` point to the non-virtual environment Python installation which was used to create the virtual environment. If a virtual environment 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-virtual environment Python installation).

When a virtual environment is active, any options that change the installation path will be ignored from all :mod:`distutils` configuration files to prevent projects being inadvertently installed outside of the virtual environment.

When working in a command shell, users can make a virtual environment active by running an activate script in the virtual environment's executables directory (the precise filename and command to use the file is shell-dependent), which prepends the virtual environment's directory for executables to the PATH environment variable for the running shell. There should be no need in other circumstances to activate a virtual environment; scripts installed into virtual environments have a "shebang" line which points to the virtual environment'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 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.

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)

 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

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://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 virtual environment.

 :param context: The information for the virtual 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 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 /python_sourcecode/python3.8.1/blob/master/Doc/library/venv.rst