: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:: 3.3Source code: :source:`Lib/venv/`
.. index:: pair: Environments; virtualThe :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 pip will install Python packages into a virtual environment without needing to be told to do so explicitly.
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/#creating-a-virtual-environment>`__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).
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, \
upgrade_deps=False)
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`` parameterThe 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.