cpython/Doc/packaging/introduction.rst

.. _packaging-intro:

*****************************
An Introduction to Packaging
*****************************

This document covers using Packaging to distribute your Python modules,
concentrating on the role of developer/distributor.  If you're looking for
information on installing Python modules you should refer to the
:ref:`packaging-install-index` chapter.

Throughout this documentation, the terms "Distutils", "the Distutils" and
"Packaging" will be used interchangeably.

.. _packaging-concepts:

Concepts & Terminology
======================

Using Distutils is quite simple both for module developers and for
users/administrators installing third-party modules.  As a developer, your
responsibilities (apart from writing solid, well-documented and well-tested
code, of course!) are:

* writing a setup script (:file:`setup.py` by convention)

* (optional) writing a setup configuration file

* creating a source distribution

* (optional) creating one or more "built" (binary) distributions of your
  project

All of these tasks are covered in this document.

Not all module developers have access to multiple platforms, so one cannot 
expect them to create buildt distributions for every platform.  To remedy
this, it is hoped that intermediaries called *packagers* will arise to address
this need.  Packagers take source distributions released by module developers,
build them on one or more platforms and release the resulting built 
distributions.  Thus, users on a greater range of platforms will be able to 
install the most popular Python modules in the most natural way for their 
platform without having to run a setup script or compile a single line of code.


.. _packaging-simple-example:

A Simple Example
================

A setup script is usually quite simple, although since it's written in Python
there are no arbitrary limits to what you can do with it, though you should be
careful about putting expensive operations in your setup script.
Unlike, say, Autoconf-style configure scripts the setup script may be run
multiple times in the course of building and installing a module
distribution.

If all you want to do is distribute a module called :mod:`foo`, contained in a
file :file:`foo.py`, then your setup script can be as simple as::

   from packaging.core import setup
   setup(name='foo',
         version='1.0',
         py_modules=['foo'])

Some observations:

* most information that you supply to the Distutils is supplied as keyword
  arguments to the :func:`setup` function

* those keyword arguments fall into two categories: package metadata (name,
  version number, etc.) and information about what's in the package (a list 
  of pure Python modules in this case)

* modules are specified by module name, not filename (the same will hold true
  for packages and extensions)

* it's recommended that you supply a little more metadata than we have in the 
  example.  In particular your name, email address and a URL for the 
  project if appropriate (see section :ref:`packaging-setup-script` for an example)

To create a source distribution for this module you would create a setup
script, :file:`setup.py`, containing the above code and run::

   python setup.py sdist

which will create an archive file (e.g., tarball on Unix, ZIP file on Windows)
containing your setup script :file:`setup.py`, and your module :file:`foo.py`.
The archive file will be named :file:`foo-1.0.tar.gz` (or :file:`.zip`), and
will unpack into a directory :file:`foo-1.0`.

If an end-user wishes to install your :mod:`foo` module all he has to do is
download :file:`foo-1.0.tar.gz` (or :file:`.zip`), unpack it, and from the
:file:`foo-1.0` directory run ::

   python setup.py install

which will copy :file:`foo.py` to the appropriate directory for
third-party modules in their Python installation.

This simple example demonstrates some fundamental concepts of Distutils.
First, both developers and installers have the same basic user interface, i.e.
the setup script.  The difference is which Distutils *commands* they use: the
:command:`sdist` command is almost exclusively for module developers, while
:command:`install` is more often used by installers (although some developers 
will want to install their own code occasionally).

If you want to make things really easy for your users, you can create more 
than one built distributions for them.  For instance, if you are running on a
Windows machine and want to make things easy for other Windows users, you can
create an executable installer (the most appropriate type of built distribution
for this platform) with the :command:`bdist_wininst` command.  For example::

   python setup.py bdist_wininst

will create an executable installer, :file:`foo-1.0.win32.exe`, in the current
directory.  You can find out what distribution formats are available at any time
by running ::

   python setup.py bdist --help-formats


.. _packaging-python-terms:

General Python terminology
==========================

If you're reading this document, you probably have a good idea of what Python 
modules, extensions and so forth are.  Nevertheless, just to be sure that 
everyone is on the same page, here's a quick overview of Python terms:

module
   The basic unit of code reusability in Python: a block of code imported by 
   some other code.  Three types of modules are important to us here: pure 
   Python modules, extension modules and packages.

pure Python module
   A module written in Python and contained in a single :file:`.py` file (and
   possibly associated :file:`.pyc` and/or :file:`.pyo` files).  Sometimes 
   referred to as a "pure module."

extension module
   A module written in the low-level language of the Python implementation: C/C++
   for Python, Java for Jython.  Typically contained in a single dynamically
   loaded pre-compiled file, e.g. a shared object (:file:`.so`) file for Python
   extensions on Unix, a DLL (given the :file:`.pyd` extension) for Python
   extensions on Windows, or a Java class file for Jython extensions.  Note that
   currently Distutils only handles C/C++ extensions for Python.

package
   A module that contains other modules, typically contained in a directory of 
   the filesystem and distinguished from other directories by the presence of a 
   file :file:`__init__.py`.

root package
   The root of the hierarchy of packages.  (This isn't really a package, 
   since it doesn't have an :file:`__init__.py` file.  But... we have to 
   call it something, right?)  The vast majority of the standard library is 
   in the root package, as are many small standalone third-party modules that 
   don't belong to a larger module collection.  Unlike regular packages, 
   modules in the root package can be found in many directories: in fact, 
   every directory listed in ``sys.path`` contributes modules to the root 
   package.


.. _packaging-term:

Distutils-specific terminology
==============================

The following terms apply more specifically to the domain of distributing Python
modules using Distutils:

module distribution
   A collection of Python modules distributed together as a single downloadable
   resource and meant to be installed all as one.  Examples of some well-known
   module distributions are NumPy, SciPy, PIL (the Python Imaging
   Library) or mxBase.  (Module distributions would be called a *package*, 
   except that term is already taken in the Python context: a single module 
   distribution may contain zero, one, or many Python packages.)

pure module distribution
   A module distribution that contains only pure Python modules and packages.
   Sometimes referred to as a "pure distribution."

non-pure module distribution
   A module distribution that contains at least one extension module.  Sometimes
   referred to as a "non-pure distribution."

distribution root
   The top-level directory of your source tree (or  source distribution).  The
   directory where :file:`setup.py` exists.  Generally  :file:`setup.py` will 
   be run from this directory.