# `venv` — Creation of virtual environments

Νέο στην έκδοση 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.

See PEP 405 for more background on Python virtual environments.

Δείτε επίσης

Python Packaging User Guide: Creating and using virtual environments

## Creating virtual environments

Creation of virtual environments is done by executing the command `venv`:

    python -m venv /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 from which the command was run (a common name for the target directory is `.venv`). It also creates a `bin` (or `Scripts` on Windows) subdirectory containing a copy/symlink of the Python binary/binaries (as appropriate for the platform or arguments used at environment creation time). It also creates an (initially empty) `lib/pythonX.Y/site-packages` subdirectory (on Windows, this is `Lib\site-packages`). If an existing directory is specified, it will be re-used.

Αποσύρθηκε στην έκδοση 3.6: `pyvenv` was the recommended tool for creating virtual environments for Python 3.3 and 3.4, and is deprecated in Python 3.6.

Άλλαξε στην έκδοση 3.5: The use of `venv` is now recommended for creating virtual environments.

On Windows, invoke the `venv` command as follows:

    c:\>c:\Python35\python -m venv c:\path\to\myenv

Alternatively, if you configured the `PATH` and `PATHEXT` variables for your Python installation:

    c:\>python -m venv c:\path\to\myenv

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]
                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)
      --prompt PROMPT       Provides an alternative prompt prefix for this
                            environment.
      --upgrade-deps        Upgrade core dependencies: pip setuptools to the
                            latest version in PyPI

    Once an environment has been created, you may wish to activate it, e.g. by
    sourcing an activate script in its bin directory.

Άλλαξε στην έκδοση 3.9: Add `--upgrade-deps` option to upgrade pip + setuptools to the latest on PyPI

Άλλαξε στην έκδοση 3.4: Installs pip by default, added the `--without-pip` and `--copies` options

Άλλαξε στην έκδοση 3.4: In earlier versions, if the target directory already existed, an error was raised, unless the `--clear` or `--upgrade` option was provided.

Σημείωση

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.

Σημείωση

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`
PowerShell | `$ <venv>/bin/Activate.ps1`
Windows | cmd.exe | `C:\> <venv>\Scripts\activate.bat`
PowerShell | `PS C:\> <venv>\Scripts\Activate.ps1`

Νέο στην έκδοση 3.4: fish and csh activation scripts.

Νέο στην έκδοση 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, i.e. `#!/<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.

Προειδοποίηση

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)

  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

  Άλλαξε στην έκδοση 3.4: Added the `with_pip` parameter

  Νέο στην έκδοση 3.6: Added the `prompt` parameter

  Νέο στην έκδοση 3.9: Added the `upgrade_deps` parameter

  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)

    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.

    Άλλαξε στην έκδοση 3.12: The attribute `lib_path` was added to the context, and the context object was documented.

    Άλλαξε στην έκδοση 3.11: The venv sysconfig installation scheme is used to construct the paths of the created directories.

  `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` and `setuptools`) in the environment. This is done by shelling out to the `pip` executable in the environment.

    Νέο στην έκδοση 3.9.

  `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.

  Άλλαξε στην έκδοση 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.

  Άλλαξε στην έκδοση 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.

  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_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).

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)

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

  Νέο στην έκδοση 3.3.

  Άλλαξε στην έκδοση 3.4: Added the `with_pip` parameter

  Άλλαξε στην έκδοση 3.6: Added the `prompt` parameter

  Άλλαξε στην έκδοση 3.9: Added the `upgrade_deps` 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://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 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):
        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 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.