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 (which
matches the version of the binary that was used to create this environment) 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.
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 (a common name for the target directory is .venv
). It also creates
a bin
(or Scripts
on Windows) subdirectory containing a copy/symlink
of the Python binary/binaries (as appropriate for the platform or arguments
used at environment creation time). 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.
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] [--prompt PROMPT] [--upgrade-deps]
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)
--prompt PROMPT Provides an alternative prompt prefix for this
environment.
--upgrade-deps Upgrade core dependencies: pip setuptools to the
latest version in PyPI
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 3.9: Add --upgrade-deps
option to upgrade pip + setuptools to the latest on PyPI
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.
Note
While symlinks are supported on Windows, they are not recommended. Of
particular note is that double-clicking python.exe
in File Explorer
will resolve the symlink eagerly and ignore the virtual environment.
Note
On Microsoft Windows, it may be required to enable the Activate.ps1
script by setting the execution policy for the user. You can do this by
issuing the following PowerShell command:
PS C:> Set-ExecutionPolicy -ExecutionPolicy RemoteSigned -Scope CurrentUser
See About Execution Policies for more information.
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 (<venv> must be replaced by the path of the directory containing the virtual environment):
Platform |
Shell |
Command to activate virtual environment |
---|---|---|
POSIX |
bash/zsh |
$ source <venv>/bin/activate |
fish |
$ source <venv>/bin/activate.fish |
|
csh/tcsh |
$ source <venv>/bin/activate.csh |
|
PowerShell Core |
$ <venv>/bin/Activate.ps1 |
|
Windows |
cmd.exe |
C:\> <venv>\Scripts\activate.bat |
PowerShell |
PS C:\> <venv>\Scripts\Activate.ps1 |
When a virtual environment is active, the VIRTUAL_ENV
environment
variable is set to the path of the virtual environment. This can be used to
check if one is running inside a virtual environment.
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 and is an internal implementation detail (typically a script or shell function will be used).
New in version 3.4: fish
and csh
activation scripts.
New in version 3.8: PowerShell activation scripts installed under POSIX for PowerShell Core support.
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 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
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, upgrade_deps=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 rather than copying.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.prompt
– a String to be used after virtual environment is activated (defaults toNone
which means directory name of the environment would be used). If the special string"."
is provided, the basename of the current directory is used as the prompt.upgrade_deps
– Update the base venv modules to the latest on PyPI
Changed in version 3.4: Added the
with_pip
parameterNew in version 3.6: Added the
prompt
parameterNew in version 3.9: Added the
upgrade_deps
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)¶ Create a virtual environment by specifying the target directory (absolute or relative to the current 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 or symlink to the Python executable in the environment. On POSIX systems, 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.
-
upgrade_dependencies
(context)¶ Upgrades the core venv dependency packages (currently
pip
andsetuptools
) in the environment. This is done by shelling out to thepip
executable in the environment.New in version 3.9.
-
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.
Changed in version 3.7.2: Windows now uses redirector scripts for
python[w].exe
instead of copying the actual binaries. In 3.7.2 onlysetup_python()
does nothing unless running from a build in the source tree.Changed in version 3.7.3: Windows copies the redirector scripts as part of
setup_python()
instead ofsetup_scripts()
. This was not the case in 3.7.2. When using symlinks, the original executables will be linked.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, prompt=None)¶ Create an
EnvBuilder
with the given keyword arguments, and call itscreate()
method with the env_dir argument.New in version 3.3.
Changed in version 3.4: Added the
with_pip
parameterChanged in version 3.6: Added the
prompt
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.