`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 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.

Vedi anche

Python Packaging User Guide: Creating and using 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.

Cambiato nella versione 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.

Cambiato nella versione 3.4: Installs pip by default, added the --without-pip and --copies options.

Cambiato nella versione 3.4: In earlier versions, if the target directory already existed, an error was raised, unless the --clear or --upgrade option was provided.

Cambiato nella versione 3.9: Add --upgrade-deps option to upgrade pip + setuptools to the latest on PyPI.

Cambiato nella versione 3.12: setuptools is no longer a core venv dependency.

Cambiato nella versione 3.13: Added the --without-scm-ignore-files option.

Cambiato nella versione 3.13: venv now creates a .gitignore file for Git by default.

Nota

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.

Nota

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	`$ source <venv>/bin/activate`
	fish	`$ source <venv>/bin/activate.fish`
	csh/tcsh	`$ source <venv>/bin/activate.csh`
	pwsh	`$ <venv>/bin/Activate.ps1`
Windows	cmd.exe	`C:\> <venv>\Scripts\activate.bat`
Windows	PowerShell	`PS C:\> <venv>\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_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.

Avvertimento

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" via create_git_ignore_file().

Cambiato nella versione 3.4: Added the with_pip parameter

Cambiato nella versione 3.6: Added the prompt parameter

Cambiato nella versione 3.9: Added the upgrade_deps parameter

Cambiato nella versione 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 arg clear=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 (see install_scripts()).
env_name - The name of the virtual environment. Used for __VENV_NAME__ in activation scripts (see install_scripts()).
prompt - The prompt to be used by the activation scripts. Used for __VENV_PROMPT__ in activation scripts (see install_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 (see install_scripts()).
env_exe - The name of the Python interpreter in the virtual environment. Used for __VENV_PYTHON__ in activation scripts (see install_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.

Cambiato nella versione 3.11: The venv sysconfig installation scheme is used to construct the paths of the created directories.

Cambiato nella versione 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 the pip executable in the environment.

Added in version 3.9.

Cambiato nella versione 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 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 (either bin or Scripts).
__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.

Cambiato nella versione 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.

Cambiato nella versione 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 its create() method with the env_dir argument.

Added in version 3.3.

Cambiato nella versione 3.4: Added the with_pip parameter

Cambiato nella versione 3.6: Added the prompt parameter

Cambiato nella versione 3.9: Added the upgrade_deps parameter

Cambiato nella versione 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.

`venv` — Creation of virtual environments¶

Creating virtual environments¶

How venvs work¶

API¶

An example of extending `EnvBuilder`¶

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venv — Creation of virtual environments¶

Creating virtual environments¶

How venvs work¶

API¶

An example of extending EnvBuilder¶

`venv` — Creation of virtual environments¶

An example of extending `EnvBuilder`¶