venv
— Creation of virtual environments¶
New in version 3.3.
Source code: Lib/venv/
The venv
module supports creating lightweight “virtual environments”,
each with their own independent set of Python packages installed in
their 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.
A virtual environment is (amongst other things):
Used to contain a specific Python interpreter and software libraries and binaries which are needed to support a project (library or application). These are by default isolated from software in other virtual environments and Python interpreters and libraries installed in the operating system.
Contained in a directory, conventionally either named
venv
or.venv
in the project directory, or under a container directory for lots of virtual environments, such as~/.virtualenvs
.Not checked into source control systems such as Git.
Considered as disposable – it should be simple to delete and recreate it from scratch. You don’t place any project code in the environment
Not considered as movable or copyable – you just recreate the same environment in the target location.
See PEP 405 for more background on Python virtual environments.
Availability: not Emscripten, not WASI.
This module does not work or is not available on WebAssembly platforms
wasm32-emscripten
and wasm32-wasi
. See
WebAssembly platforms for more information.
Creating virtual environments¶
Creation of virtual environments is done by executing the
command venv
:
python -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.
How venvs work¶
When a Python interpreter is running from a virtual environment,
sys.prefix
and sys.exec_prefix
point to the directories of the virtual environment,
whereas sys.base_prefix
and 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 PATH
, so that running
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
(<venv>
must be replaced by the path to the directory
containing the virtual environment):
Platform |
Shell |
Command to activate virtual environment |
---|---|---|
POSIX |
bash/zsh |
|
fish |
|
|
csh/tcsh |
|
|
PowerShell |
|
|
Windows |
cmd.exe |
|
PowerShell |
|
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.
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. #!/<path-to-venv>/bin/python
.
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. 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 PATH
.
When a virtual environment has been activated, the VIRTUAL_ENV
environment variable is set to the path of the environment.
Since explicitly activating a virtual environment is not required to use it,
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).
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 subdirectories that don’t already exist, and returns a context object. This context object is just a holder for attributes (such as paths) for use by the other methods. If the
EnvBuilder
is created with the argclear=True
, contents of the environment directory will be cleared and then all necessary subdirectories will be recreated.The returned context object is a
types.SimpleNamespace
with the following attributes:env_dir
- The location of the virtual environment. Used for__VENV_DIR__
in activation scripts (seeinstall_scripts()
).env_name
- The name of the virtual environment. Used for__VENV_NAME__
in activation scripts (seeinstall_scripts()
).prompt
- The prompt to be used by the activation scripts. Used for__VENV_PROMPT__
in activation scripts (seeinstall_scripts()
).executable
- The underlying Python executable used by the virtual environment. This takes into account the case where a virtual environment is created from another virtual environment.inc_path
- The include path for the virtual environment.lib_path
- The purelib path for the virtual environment.bin_path
- The script path for the virtual environment.bin_name
- The name of the script path relative to the virtual environment location. Used for__VENV_BIN_NAME__
in activation scripts (seeinstall_scripts()
).env_exe
- The name of the Python interpreter in the virtual environment. Used for__VENV_PYTHON__
in activation scripts (seeinstall_scripts()
).env_exec_cmd
- The name of the Python interpreter, taking into account filesystem redirections. This can be used to run Python in the virtual environment.
Changed in version 3.12: The attribute
lib_path
was added to the context, and the context object was documented.Changed in version 3.11: The venv sysconfig installation scheme is used to construct the paths of the created directories.
- 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, upgrade_deps=False)¶
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
parameterChanged in version 3.9: Added the
upgrade_deps
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://bootstrap.pypa.io/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.