virtualenv/docs/user_guide.rst

User Guide
==========

Introduction
------------

Virtualenv has one basic command:

.. code-block:: console

    virtualenv

this will create a python virtual environment of the same version as virtualenv is installed into under path
``venv``. The path where to generate the virtual environment can be changed via a positional argument being passed in,
see the :option:`dest` flag. The command line tool has quite a few of flags that modify the components behaviour, for a
full list make sure to check out :ref:`cli_flags`.

The tool works in two phases:

- **Phase 1** discovers a python interpreter to create a virtual environment from (by default this is the same python
  as the one ``virtualenv`` is running from, however we can change this via the :option:`p` option).
- **Phase 2** creates a virtual environment at the specified destination (:option:`dest`), this can be broken down into
  three further sub-steps:

  - create a python that matches the target python interpreter from phase 1,
  - install (bootstrap) seed packages (one or more of :pypi:`pip`, :pypi:`setuptools`, :pypi:`wheel`) in the created
    virtual environment,
  - install activation scripts into the binary directory of the virtual environment (these will allow end user to
    *activate* the virtual environment from various shells).

The python in your new virtualenv is effectively isolated from the python that was used to create it.

Python discovery
----------------

The first thing we need to be able to create a virtual environment is a python interpreter. This will describe to the
tool what type of virtual environment you would like to create, think of it as: version, architecture, implementation.

``virtualenv`` being a python application has always at least one such available, the one ``virtualenv`` itself is
using it, and as such this is the default discovered element. This means that if you install ``virtualenv`` under
python ``3.8``, virtualenv will by default create virtual environments that are also of version ``3.8``.

Created python virtual environments are usually not self-contained. A complete python packaging is usually made up of
thousand of files, so it's not efficient to install the entire python again into a new folder. Instead virtual
environments are mere shells, that contain very little within itself, and borrow most from the system python (this is
what you installed, when you installed python itself). This does mean that if you upgrade your system python your
virtual environments *might* break, so watch out. The upside of this referring to the system python is that creating
virtual environments can be very fast.

Here we'll describe the builtin mechanism (note this can be extended though by plugins). The CLI flag :option:`p` or
:option:`python` allows you to specify a python specifier for what type of virtual environment you would like, the
format is either:

- a relative/absolute path to a Python interpreter,

- a specifier identifying the Python implementation, version, architecture in the following format:

    .. code-block::

       {python implementation name}{version}{architecture}

    We have the following restrictions:

    - the python implementation is all alphabetic characters (``python`` means any implementation, and if is missing it
      defaults to ``python``),
    - the version is a dot separated version number,
    - the architecture is either ``-64`` or ``-32`` (missing means ``any``).

    For example:

    - ``python3.8.1`` means any python implementation having the version ``3.8.1``,
    - ``3`` means any python implementation having the major version ``3``,
    - ``cpython3`` means a ``CPython`` implementation havin the version ``3``,
    - ``pypy2`` means a python interpreter with the ``PyPy`` implementation and major version ``2``.

  Given the specifier ``virtualenv`` will apply the following strategy to discover/find the system executable:

   - If we're on Windows look into the Windows registry, and check if we see any registered Python implementations that
     match the specification. This is in line with expectation laid out inside
     `PEP-514 <https://www.python.org/dev/peps/pep-0514/>`_
   - Try to discover a matching python executable within the folders enumerated on the ``PATH`` environment variable.
     In this case we'll try to find an executable that has a name roughly similar to the specification (for exact logic,
     please see the implementation code).

.. warning::

   As detailed above virtual environments usually just borrow things from the system Python, they don't actually contain
   all the data from the system Python. The version of the python executable is hardcoded within the python exe itself.
   Therefore if you upgrade your system Python, your virtual environment will still report the version before the
   upgrade, even though now other than the executable all additional content (standard library, binary libs, etc) are
   of the new version.

   Baring any major incompatibilities (rarely the case) the virtual environment will continue working, but other than
   the content embedded within the python executable it will behave like the upgraded version. If a such virtual
   environment python is specified as the target python interpreter, we will create virtual environments that match the
   new system Python version, not the version reported by the virtual environment.

Creators
--------

These are what actually setup the virtual environment, usually as a reference against the system python. virtualenv
at the moment has two types of virtual environments:

- ``venv`` - this delegates the creation process towards the ``venv`` module, as described in
  `PEP 404 <https://www.python.org/dev/peps/pep-0405>`_. This is only available on Python interpreters having version
  ``3.4`` or later, and also has the downside that virtualenv **must** create a process to invoke that module (unless
  virtualenv is installed in the system python), which can be an expensive operation (especially true on Windows).

- ``builtin`` - this means ``virtualenv`` is able to do the creation operation itself (by knowing exactly what files to
  create and what system files needs to be referenced). The creator with name ``builtin`` is an alias on the first
  creator that's of this type (we provide creators for various target environments, that all differ in actual create
  operations, such as CPython 2 on Windows, PyPy2 on Windows, CPython3 on Posix, PyPy3 on Posix, and so on; for a full
  list see :option:`creator`).

Seeders
-------
These will install for you some seed packages (one or more of the: :pypi:`pip`, :pypi:`setuptools`, :pypi:`wheel`) that
enables you to install additional python packages into the created virtual environment (by invoking pip). There are two
main seed mechanism available:

- ``pip`` - this method uses the bundled pip with virtualenv to install the seed packages (note, a new child process
  needs to be created to do this).
- ``app-data`` - this method uses the user application data directory to create install images. These images are needed
  to be created only once, and subsequent virtual environments can just link/copy those images into their pure python
  library path (the ``site-packages`` folder). This allows all but the first virtual environment creation to be blazing
  fast (a ``pip`` mechanism takes usually 98% of the virtualenv creation time, so by creating this install image that
  we can just link into the virtual environments install directory we can achieve speedups of shaving the initial
  1 minutes 10 seconds down to just 8 seconds in case of copy, or ``0.8`` seconds in case symlinks are available -
  this is on Windows, Linux/macOS with symlinks this can be as low as ``100ms`` from 3+ seconds).

Activators
----------
These are activation scripts that will mangle with your shells settings to ensure that commands from within the python
virtual environment take priority over your system paths. For example if invoking ``pip`` from your shell returned the
system pythons pip before activation, once you do the activation this should refer to the virtual environments ``pip``.
Note, though that all we do is change priority; so if your virtual environments ``bin``/``Scripts`` folder does not
contain some executable, this will still resolve to the same executable it would have resolved before the activation.

For a list of shells we provide activators see :option:`activators`. The location of these is right alongside the python
executables ( usually ``Scripts`` folder on Windows, ``bin`` on POSIX), and are named as ``activate`` (and some
extension that's specific per activator; no extension is bash). You can invoke them, usually by source-ing (the source
command might vary by shell - e.g. bash is ``.``):

.. code-block:: console

   source bin/activate

This is all it does; it's purely a convenience of prepending the virtual environments binary folder onto the ``PATH``
environment variable. Note you don't have to activate a virtual environment to use it. In this case though you would
need to type out the path to the executables, rather than relying on your shell to resolve them to your virtual
environment.

The ``activate`` script will also modify your shell prompt to indicate which environment is currently active. The script
also provisions a ``decativate`` command that will allow you to undo the operation:

.. code-block:: console

   deactivate


.. note::

    If using Powershell, the ``activate`` script is subject to the
    `execution policies <http://technet.microsoft.com/en-us/library/dd347641.aspx>`_ on the system. By default Windows
    7 and later, the system's execution policy is set to ``Restricted``, meaning no scripts like the ``activate`` script
    are allowed to be executed.

    However, that can't stop us from changing that slightly to allow it to be executed. You may relax the system
    execution policy to allow running of local scripts without verifying the code signature using the following:

    .. code-block:: powershell

       Set-ExecutionPolicy RemoteSigned

    Since the ``activate.ps1`` script is generated locally for each virtualenv, it is not considered a remote script and
    can then be executed.

A  longer explanation of this can be found within Allison Kaptur's 2013 blog post: `There's no magic: virtualenv
edition <https://www.recurse.com/blog/14-there-is-no-magic-virtualenv-edition>`_ explains how virtualenv uses bash and
Python and ``PATH`` and ``PYTHONHOME`` to isolate virtual environments' paths.