New in version 3.3.
Source code: Lib/venv/
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
Running this command creates the target directory (creating any parent
directories that don’t exist already) and places a
pyvenv.cfg file in it
home key pointing to the Python installation the command was run
from. It also creates a
Scripts on Windows) subdirectory
containing a copy of the
python binary (or binaries, in the case of
Windows). It also creates an (initially empty)
subdirectory (on Windows, this is
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
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
Changed in version 3.4: Installs pip by default, added the
Changed in version 3.4: In earlier versions, if the target directory already existed, an error was
raised, unless the
--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.
pyvenv.cfg file also includes the
include-system-site-packages key, set to
run with 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
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|
|PowerShell||PS C:\> <venv>\Scripts\Activate.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.ps1 which are installed when the venv is
New in version 3.4:
csh activation scripts.
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
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
sys.exec_prefix point to the base
directory of the venv, whereas
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.exec_prefix is the same as
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
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
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
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(system_site_packages=False, clear=False, symlinks=False, upgrade=False, with_pip=False)¶
EnvBuilderclass 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
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
Trueon Linux and Unix systems, but
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
with_pip– a Boolean value which, if true, ensures pip is installed in the virtual environment. This uses
Changed in version 3.4: Added the
Creators of third-party virtual environment tools will be free to use the provided
EnvBuilderclass as a base class.
The returned env-builder is an object which has a method,
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
createmethod will either create the environment in the specified directory, or raise an appropriate exception.
createmethod of the
EnvBuilderclass 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)
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
upgradewere specified to allow operating on an existing environment directory.
pyvenv.cfgconfiguration file in the environment.
Creates a copy of the Python executable (and, under Windows, DLLs) in the environment. On a POSIX system, if a specific executable
python3.xwas used, symlinks to
python3will be created pointing to that executable, unless files with those names already exist.
Installs activation scripts appropriate to the platform into the virtual environment.
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.
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.nameare 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
__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:
28.3.3. An example of extending
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.