venv — Creation of virtual environments (original) (raw)

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 aspip will install Python packages into a virtual environment without needing to be told to do so explicitly.

A virtual environment is (amongst other things):

See PEP 405 for more background on Python virtual environments.

Creating virtual environments

Virtual environments are created by executing the venvmodule:

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 --copiesoptions.

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.ps1script 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 Policiesfor more information.

The created pyvenv.cfg file also includes theinclude-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](ensurepip.html#module-ensurepip "ensurepip: Bootstrapping the "pip" installer into an existing Python installation or virtual environment.") 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_prefixpoint to the directories of the virtual environment, whereas sys.base_prefix and sys.base_exec_prefixpoint to those of the base Python used to create the environment. It is sufficient to checksys.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 runningpython 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 $ source /bin/activate
fish $ source /bin/activate.fish
csh/tcsh $ source /bin/activate.csh
pwsh $ /bin/Activate.ps1
Windows cmd.exe C:\> __\Scripts\activate.bat
PowerShell PS C:\> __\Scripts\Activate.ps1

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_ENVenvironment 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:

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

EnvBuilder 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 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 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:

Changed in version 3.11: The venv sysconfig installation schemeis 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 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.

upgrade_dependencies(context)

Upgrades the core venv dependency packages (currently pip) in the environment. This is done by shelling out to thepip 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() or post_setup() in subclasses to assist in installing custom scripts into the virtual environment.

path is the path to a directory that should contain subdirectoriescommon, posix, nt; each containing scripts destined for thebin directory in the environment. The contents of common and the directory corresponding to os.name are copied after some text replacement of placeholders:

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 only setup_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 of setup_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.