venv.rst

.. _tut-venv:

Virtual Environments and Packages

Introduction

Python applications will often use packages and modules that don't

come as part of the standard library. Applications will sometimes

need a specific version of a library, because the application may

require that a particular bug has been fixed or the application may be

written using an obsolete version of the library's interface.

This means it may not be possible for one Python installation to meet

the requirements of every application. If application A needs version

1.0 of a particular module but application B needs version 2.0, then

the requirements are in conflict and installing either version 1.0 or 2.0

will leave one application unable to run.

The solution for this problem is to create a :term:`virtual

environment` (often shortened to "virtualenv"), a self-contained

directory tree that contains a Python installation for a particular

version of Python, plus a number of additional packages.

Different applications can then use different virtual environments.

To resolve the earlier example of conflicting requirements,

application A can have its own virtual environment with version 1.0

installed while application B has another virtualenv with version 2.0.

If application B requires a library be upgraded to version 3.0, this will

not affect application A's environment.

Creating Virtual Environments

The script used to create and manage virtual environments is called

:program:`pyvenv`. :program:`pyvenv` will usually install the most

recent version of Python that you have available; the script is also

installed with a version number, so if you have multiple versions of

Python on your system you can select a specific Python version by

running ``pyvenv-3.4`` or whichever version you want.

To create a virtualenv, decide upon a directory

where you want to place it and run :program:`pyvenv` with the

directory path::

This will create the ``tutorial-env`` directory if it doesn't exist,

and also create directories inside it containing a copy of the Python

interpreter, the standard library, and various supporting files.

Once you've created a virtual environment, you need to

activate it.

On Windows, run::

On Unix or MacOS, run::

(This script is written for the bash shell. If you use the

:program:`csh` or :program:`fish` shells, there are alternate

``activate.csh`` and ``activate.fish`` scripts you should use

instead.)

Activating the virtualenv will change your shell's prompt to show what

virtualenv you're using, and modify the environment so that running

``python`` will get you that particular version and installation of

Python. For example::

Managing Packages with pip

Once you've activated a virtual environment, you can install, upgrade,

and remove packages using a program called :program:`pip`. By default

``pip`` will install packages from the Python Package Index,

<https://pypi.python.org/pypi>. You can browse the Python Package Index

by going to it in your web browser, or you can use ``pip``'s

limited search feature::

``pip`` has a number of subcommands: "search", "install", "uninstall",

"freeze", etc. (Consult the :ref:`installing-index` guide for

complete documentation for ``pip``.)

You can install the latest version of a package by specifying a package's name::

You can also install a specific version of a package by giving the

package name followed by ``==`` and the version number::

If you re-run this command, ``pip`` will notice that the requested

version is already installed and do nothing. You can supply a

different version number to get that version, or you can run ``pip

install --upgrade`` to upgrade the package to the latest version::

-> pip install --upgrade requests

Collecting requests

Installing collected packages: requests

Found existing installation: requests 2.6.0

Uninstalling requests-2.6.0:

Successfully uninstalled requests-2.6.0

Successfully installed requests-2.7.0

``pip uninstall`` followed by one or more package names will remove the

packages from the virtual environment.

``pip show`` will display information about a particular package::

``pip list`` will display all of the packages installed in the virtual

environment::

``pip freeze`` will produce a similar list of the installed packages,

but the output uses the format that ``pip install`` expects.

A common convention is to put this list in a ``requirements.txt`` file::

The ``requirements.txt`` can then be committed to version control and

shipped as part of an application. Users can then install all the

necessary packages with ``install -r``::

``pip`` has many more options. Consult the :ref:`installing-index`

guide for complete documentation for ``pip``. When you've written

a package and want to make it available on the Python Package Index,

consult the :ref:`distributing-index` guide.

venv.rst