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# 26.9. `venv` — Creation of virtual environments

New in version 3.3.

Source code: Lib/venv.py

The `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.

## 26.9.1. Creating virtual environments

Creation of virtual environments is done by executing the `pyvenv` script:

    pyvenv /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 the command was run from. It also creates a `bin` (or `Scripts` on Windows) subdirectory containing a copy of the `python` binary (or binaries, in the case of Windows). It also creates an (initially empty) `lib/pythonX.Y/site-packages` subdirectory (on Windows, this is `Lib\site-packages`).

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:\Python33\python c:\Python33\Tools\Scripts\pyvenv.py myenv

or equivalently:

    c:\Temp>c:\Python33\python -m venv myenv

The command, if run with `-h`, will show the available options:

    usage: pyvenv [-h] [--system-site-packages] [--symlinks] [--clear]
                  [--upgrade] 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 access to the global site-packages dir to the
                             virtual environment.
      --symlinks             Try to use symlinks rather than copies, when symlinks
                             are not the default for the platform.
      --clear                Delete the environment directory if it already exists.
                             If not specified and the directory exists, an error is
                             raised.
      --upgrade              Upgrade the environment directory to use this version
                             of Python, assuming Python has been upgraded in-place.

If the target directory already exists an error will be raised, unless the `--clear` or `--upgrade` option was provided.

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.

Multiple paths can be given to `pyvenv`, in which case an identical virtualenv will be created, according to the given options, at each provided path.

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: on a Posix platform, you would typically do:

    $ source <venv>/bin/activate

whereas on Windows, you might do:

    C:\> <venv>/Scripts/activate

if you are using the `cmd.exe` shell, or perhaps:

    PS C:\> <venv>/Scripts/Activate.ps1

if you use PowerShell.

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.bat` and `Deactivate.ps1` which are installed when the venv is created.

Note

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 `Distribute` and `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. Of course, you need to install them into the venv first: this could be done by running `distribute_setup.py` with the venv activated, followed by running `easy_install pip`. Alternatively, you could download the source tarballs and run `python setup.py install` after unpacking, with the venv activated.

When a venv is active (i.e. the venv’s Python interpreter is running), the attributes `sys.prefix` and `sys.exec_prefix` point to the base directory of the venv, whereas `sys.base_prefix` and `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.base_prefix` and `sys.exec_prefix` is the same as `sys.base_exec_prefix` (they 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.

## 26.9.2. 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)

  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 any existing target directory instead of raising an exception (defaults to `False`).
  - `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 `True` on Linux and Unix systems, but `False` on Windows.
  - `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`).

  Creators 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)

    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 `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.create_directories(env_dir)
            self.create_configuration(context)
            self.setup_python(context)
            self.setup_scripts(context)
            self.post_setup(context)

    Each of the methods `create_directories()`, `create_configuration()`, `setup_python()`, `setup_scripts()` and `post_setup()` can be overridden.

  `create_directories`(env_dir)

    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.

  `create_configuration`(context)

    Creates the `pyvenv.cfg` configuration file in the environment.

  `setup_python`(context)

    Creates a copy of the Python executable (and, under Windows, DLLs) in the environment.

  `setup_scripts`(context)

    Installs activation scripts appropriate to the platform into the virtual environment.

  `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.

  In addition, `EnvBuilder` provides this utility method that can be called from `setup_scripts()` or `post_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_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.

There is also a module-level convenience function:

`venv.``create`(env_dir, system_site_packages=False, clear=False, symlinks=False)

  Create an `EnvBuilder` with the given keyword arguments, and call its `create()` method with the env_dir argument.

## 26.9.3. An example of extending `EnvBuilder`

The following script shows how to extend `EnvBuilder` by implementing a subclass which installs Distribute 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 DistributeEnvBuilder(venv.EnvBuilder):
        """
        This builder installs Distribute and pip so that you can pip or
        easy_install other packages into the created environment.

        :param nodist: If True, Distribute is not installed into the created
                       environment.
        :param nopip: If True, pip is not installed into the created
                      environment.
        :param progress: If Distribute 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.
            """
            if not self.nodist:
                self.install_distribute(context)
            if not self.nopip:
                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()

        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 progress is not None:
                progress('Installing %s' %name, 'main')
            else:
                sys.stderr.write('Installing %s ' % name)
                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_distribute(self, context):
            """
            Install Distribute in the environment.

            :param context: The information for the environment creation request
                            being processed.
            """
            url = 'http://python-distribute.org/distribute_setup.py'
            self.install_script(context, 'distribute', url)
            # clear up the distribute archive which gets downloaded
            pred = lambda o: o.startswith('distribute-') 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-distribute', default=False,
                                action='store_true', dest='nodist',
                                help="Don't install Distribute 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 Distribute and pip.')
            options = parser.parse_args(args)
            if options.upgrade and options.clear:
                raise ValueError('you cannot supply --upgrade and --clear together.')
            builder = DistributeEnvBuilder(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.

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