"venv" --- Creation of virtual environments
*******************************************

Nuovo nella versione 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.

Vedi anche:

  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.

Deprecato dalla versione 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.

Cambiato nella versione 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.

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

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.

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
  CurrentUserSee 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"            |
+---------------+--------------+----------------------------------------------------+

Nuovo nella versione 3.4: **fish** and **csh** activation scripts.

Nuovo nella versione 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.

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)

   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

   Cambiato nella versione 3.4: Added the "with_pip" parameter

   Nuovo nella versione 3.6: Added the "prompt" parameter

   Nuovo nella versione 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.

      Cambiato nella versione 3.12: The attribute "lib_path" was added
      to the context, and the context object was documented.

      Cambiato nella versione 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.

      Nuovo nella versione 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.

   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.

   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.

   Nuovo nella versione 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


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.
