Installing Python modules
*************************

As a popular open source development project, Python has an active
supporting community of contributors and users that also make their
software available for other Python developers to use under open-
source license terms.

This allows Python users to share and collaborate effectively,
benefiting from the solutions others have already created to common
(and sometimes even rare!) problems, as well as potentially
contributing their own solutions to the common pool.

This guide covers the installation part of the process. For a guide to
creating and sharing your own Python projects, refer to the Python
packaging user guide.

Nota:

  For corporate and other institutional users, be aware that many
  organisations have their own policies around using and contributing
  to open source software. Please take such policies into account when
  making use of the distribution and installation tools provided with
  Python.


Key terms
=========

* **pip** is the preferred installer program. It is included by
  default with the Python binary installers.

* A *virtual environment* is a semi-isolated Python environment that
  allows packages to be installed for use by a particular application,
  rather than being installed system wide.

* "venv" is the standard tool for creating virtual environments. It
  defaults to installing **pip** into all created virtual
  environments.

* "virtualenv" is a third-party alternative (and predecessor) to
  "venv".

* The Python Package Index (PyPI) is a public repository of open
  source licensed packages made available for use by other Python
  users.

* The Python Packaging Authority is the group of developers and
  documentation authors responsible for the maintenance and evolution
  of the standard packaging tools and the associated metadata and file
  format standards. They maintain a variety of tools, documentation,
  and issue trackers on GitHub.

Cambiato nella versione 3.5: The use of "venv" is now recommended for
creating virtual environments.

Vedi anche:

  Python Packaging User Guide: Creating and using virtual environments


Basic usage
===========

The standard packaging tools are all designed to be used from the
command line.

The following command will install the latest version of a module and
its dependencies from PyPI:

   python -m pip install SomePackage

Nota:

  For POSIX users (including macOS and Linux users), the examples in
  this guide assume the use of a *virtual environment*.For Windows
  users, the examples in this guide assume that the option to adjust
  the system PATH environment variable was selected when installing
  Python.

It's also possible to specify an exact or minimum version directly on
the command line. When using comparator operators such as ">", "<" or
some other special character which get interpreted by shell, the
package name and the version should be enclosed within double quotes:

   python -m pip install SomePackage==1.0.4    # specific version
   python -m pip install "SomePackage>=1.0.4"  # minimum version

Normally, if a suitable module is already installed, attempting to
install it again will have no effect. Upgrading existing modules must
be requested explicitly:

   python -m pip install --upgrade SomePackage

More information and resources regarding **pip** and its capabilities
can be found in the Python Packaging User Guide.

Creation of virtual environments is done through the "venv" module.
Installing packages into an active virtual environment uses the
commands shown above.

Vedi anche:

  Python Packaging User Guide: Installing Python Distribution Packages


How do I ...?
=============

These are quick answers or links for some common tasks.


... install packages just for the current user?
-----------------------------------------------

Passing the "--user" option to "python -m pip install" will install a
package just for the current user, rather than for all users of the
system.


... install scientific Python packages?
---------------------------------------

A number of scientific Python packages have complex binary
dependencies, and aren't currently easy to install using **pip**
directly. It will often be easier for users to install these packages
by other means rather than attempting to install them with **pip**.

Vedi anche:

  Python Packaging User Guide: Installing Scientific Packages


... work with multiple versions of Python installed in parallel?
----------------------------------------------------------------

On Linux, macOS, and other POSIX systems, use the versioned Python
commands in combination with the "-m" switch to run the appropriate
copy of **pip**:

   python3    -m pip install SomePackage  # default Python 3
   python3.14 -m pip install SomePackage  # specifically Python 3.14

Appropriately versioned **pip** commands may also be available.

On Windows, use the **py** Python launcher in combination with the
"-m" switch:

   py -3    -m pip install SomePackage  # default Python 3
   py -3.14 -m pip install SomePackage  # specifically Python 3.14


Common installation issues
==========================


Installing into the system Python on Linux
------------------------------------------

On Linux systems, a Python installation will typically be included as
part of the distribution. Installing into this Python installation
requires root access to the system, and may interfere with the
operation of the system package manager and other components of the
system if a component is unexpectedly upgraded using **pip**.

On such systems, it is often better to use a virtual environment or a
per-user installation when installing packages with **pip**.


Pip not installed
-----------------

It is possible that **pip** does not get installed by default. One
potential fix is:

   python -m ensurepip --default-pip

There are also additional resources for installing pip.


Installing binary extensions
----------------------------

Python once relied heavily on source-based distribution, with end
users being expected to compile extension modules from source as part
of the installation process.

With the introduction of the binary wheel format, and the ability to
publish wheels through PyPI, this problem is diminishing, as users are
more regularly able to install pre-built extensions rather than
needing to build them themselves.

Some of the solutions for installing scientific software that are not
yet available as pre-built wheel files may also help with obtaining
other binary extensions without needing to build them locally.

Vedi anche: Python Packaging User Guide: Binary Extensions
