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.
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
).
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>Scriptsactivate.bat |
PowerShell | PS C:> <venv>ScriptsActivate.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 toFalse
).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 toTrue
on Linux and Unix systems, butFalse
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 toFalse
).with_pip
– a Boolean value which, if true, ensures pip is installed in the virtual environment. This usesensurepip
with the--default-pip
option.
Changed in version 3.4: Added the
with_pip
parameterCreators 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 theEnvBuilder
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()
andpost_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
orupgrade
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 topython
andpython3
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 fromsetup_scripts()
orpost_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 (eitherbin
orScripts
).__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 itscreate()
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.