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Python 3.6.5 Documentation > "venv" — Creation of virtual environments

"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.

Creating virtual environments
=============================

Creation of virtual environments is done by executing the command
"venv":

python3 -m venv /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 from which
the command was run. 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"). If an existing directory is specified, it will
be re-used.

Deprecated since version 3.6: "pyvenv" was the recommended tool for
creating virtual environments for Python 3.3 and 3.4, and is
deprecated in Python 3.6.

Changed in version 3.5: The use of "venv" is now recommended for
creating virtual environments.

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

On Windows, invoke the "venv" command as follows:

c:\>c:\Python35\python -m venv c:\path\to\myenv

Alternatively, if you configured the "PATH" and "PATHEXT" variables
for your Python installation:

c:\>python -m venv c:\path\to\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)

Once an environment has been created, you may wish to activate it, e.g. by
sourcing an activate script in its bin directory.

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.

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 "venv", in which case an identical
virtual environment will be created, according to the given options,
at each provided path.

Once a virtual environment has been created, it can be “activated”
using a script in the virtual environment’s binary directory. The
invocation of the script is platform-specific:

You don’t specifically *need* to activate an environment; activation
just prepends the virtual environment’s binary directory to your path,
so that “python” invokes the virtual environment’s Python interpreter
and you can run installed scripts without having to use their full
path. However, all scripts installed in a virtual environment should
be runnable without activating it, and run with the virtual
environment’s Python automatically.

You can deactivate a virtual environment 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 virtual environment is
created.

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

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 "Setuptools" and "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
"sys.prefix" and "sys.exec_prefix" point to the base directory of
the virtual environment, whereas "sys.base_prefix" and
"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 "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-virtual environment
Python installation).When a virtual environment 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 virtual environment
active by running an "activate" script in the virtual environment’s
executables directory (the precise filename 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 **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 virtual
environment in "PATH".

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, prompt=None)

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.

* "prompt" – a String to be used after virtual environment is
activated (defaults to "None" which means directory name of the
environment would be used).

Changed in version 3.4: Added the "with_pip" parameter

New in version 3.6: Added the "prompt" 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

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 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 online.

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Siebel Articles Python Perl 5 Documentation Manuel PHP7 français
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