venv
— Creation of virtual environments¶
Added 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 named
.venv
orvenv
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 Android, not iOS, not WASI.
This module is not supported on mobile platforms or WebAssembly platforms.
Creating virtual environments¶
Virtual environments are created by executing the venv
module:
python -m venv /path/to/new/virtual/environment
This creates the target directory (including parent directories as needed)
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 or symlink of the Python executable
(as appropriate for the platform or arguments used at environment creation time).
It also creates a lib/pythonX.Y/site-packages
subdirectory
(on Windows, this is Libsite-packages
).
If an existing directory is specified, it will be re-used.
Changed in version 3.5: The use of venv
is now recommended for creating virtual environments.
Deprecated since version 3.6, removed in version 3.8: pyvenv was the recommended tool for creating virtual environments
for Python 3.3 and 3.4, and replaced in 3.5 by executing venv
directly.
On Windows, invoke the venv
command as follows:
PS> python -m venv C:\path\to\new\virtual\environment
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]
[--without-scm-ignore-files]
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.
options:
-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) to the latest
version in PyPI
--without-scm-ignore-files
Skips adding SCM ignore files to the environment
directory (Git is supported 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.
Changed in version 3.9: Add --upgrade-deps
option to upgrade pip + setuptools to the latest on PyPI.
Changed in version 3.12: setuptools
is no longer a core venv dependency.
Changed in version 3.13: Added the --without-scm-ignore-files
option.
Changed in version 3.13: venv
now creates a .gitignore
file for Git by default.
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 |
|
|
pwsh |
|
|
Windows |
cmd.exe |
|
PowerShell |
|
Added in version 3.4: fish and csh activation scripts.
Added 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,
#!/<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, *, scm_ignore_files=frozenset())¶
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 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 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). 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
scm_ignore_files – Create ignore files based for the specified source control managers (SCM) in the iterable. Support is defined by having a method named
create_{scm}_ignore_file
. The only value supported by default is"git"
viacreate_git_ignore_file()
.
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
parameterChanged in version 3.13: Added the
scm_ignore_files
parameterEnvBuilder
may be used as a base class.- 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 3.11: The venv sysconfig installation scheme is used to construct the paths of the created directories.
Changed in version 3.12: The attribute
lib_path
was added to the context, and the context object was documented.
- 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) in the environment. This is done by shelling out to the
pip
executable in the environment.Added in version 3.9.
Changed in version 3.12: setuptools is no longer a core venv dependency.
- 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.
- install_scripts(context, path)¶
This method can be called from
setup_scripts()
orpost_setup()
in subclasses to assist in installing custom scripts into the virtual environment.path is the path to a directory that should contain subdirectories
common
,posix
,nt
; each containing scripts destined for thebin
directory in the environment. The contents ofcommon
and the directory corresponding toos.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).
- create_git_ignore_file(context)¶
Creates a
.gitignore
file within the virtual environment that causes the entire directory to be ignored by the Git source control manager.Added in version 3.13.
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.
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, *, scm_ignore_files=frozenset())¶
Create an
EnvBuilder
with the given keyword arguments, and call itscreate()
method with the env_dir argument.Added in version 3.3.
Changed in version 3.4: Added the with_pip parameter
Changed in version 3.6: Added the prompt parameter
Changed in version 3.9: Added the upgrade_deps parameter
Changed in version 3.13: Added the scm_ignore_files 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://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.