Installing Python Modules
*************************

Email:
   distutils-sig@python.org

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
distribution 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. Starting with Python 3.4,
  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, and
  has been part of Python since Python 3.3. Starting with Python 3.4,
  it defaults to installing "pip" into all created virtual
  environments.

* "virtualenv" is a third party alternative (and predecessor) to
  "venv". It allows virtual environments to be used on versions of
  Python prior to 3.4, which either don't provide "venv" at all, or
  aren't able to automatically install "pip" into created
  environments.

* The Python Package Index 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 both GitHub and Bitbucket.

* "distutils" is the original build and distribution system first
  added to the Python standard library in 1998. While direct use of
  "distutils" is being phased out, it still laid the foundation for
  the current packaging and distribution infrastructure, and it not
  only remains part of the standard library, but its name lives on in
  other ways (such as the name of the mailing list used to coordinate
  Python packaging standards development).

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 the Python Package Index:

   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 "pip" in versions of Python prior to Python 3.4?
------------------------------------------------------------

Python only started bundling "pip" with Python 3.4. For earlier
versions, "pip" needs to be "bootstrapped" as described in the Python
Packaging User Guide.

Vedi anche:

  Python Packaging User Guide: Requirements for Installing Packages


... 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. At this point in time, 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":

   python2   -m pip install SomePackage  # default Python 2
   python2.7 -m pip install SomePackage  # specifically Python 2.7
   python3   -m pip install SomePackage  # default Python 3
   python3.4 -m pip install SomePackage  # specifically Python 3.4

Appropriately versioned "pip" commands may also be available.

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

   py -2   -m pip install SomePackage  # default Python 2
   py -2.7 -m pip install SomePackage  # specifically Python 2.7
   py -3   -m pip install SomePackage  # default Python 3
   py -3.4 -m pip install SomePackage  # specifically Python 3.4


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 has typically 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 support for the binary "wheel" format, and
the ability to publish wheels for at least Windows and macOS through
the Python Package Index, this problem is expected to diminish over
time, 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
