Doc/install/index.rst

.. highlightlang:: none

.. _install-index:

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

:Author: Greg Ward

.. TODO: Fill in XXX comments

.. The audience for this document includes people who don't know anything
   about Python and aren't about to learn the language just in order to
   install and maintain it for their users, i.e. system administrators.
   Thus, I have to be sure to explain the basics at some point:
   sys.path and PYTHONPATH at least.  Should probably give pointers to
   other docs on "import site", PYTHONSTARTUP, PYTHONHOME, etc.

   Finally, it might be useful to include all the material from my "Care
   and Feeding of a Python Installation" talk in here somewhere.  Yow!

This document describes the Python Distribution Utilities ("Distutils") from the
end-user's point-of-view, describing how to extend the capabilities of a
standard Python installation by building and installing third-party Python
modules and extensions.


.. note::

   This guide only covers the basic tools for installing extensions that are
   provided as part of this version of Python. Third party tools offer easier
   to use and more secure alternatives. Refer to the
   `quick recommendations section
   <https://python-packaging-user-guide.readthedocs.org/en/latest/current.html>`__
   in the Python Packaging User Guide for more information.


.. _inst-intro:

Introduction
============

Although Python's extensive standard library covers many programming needs,
there often comes a time when you need to add some new functionality to your
Python installation in the form of third-party modules.  This might be necessary
to support your own programming, or to support an application that you want to
use and that happens to be written in Python.

In the past, there has been little support for adding third-party modules to an
existing Python installation.  With the introduction of the Python Distribution
Utilities (Distutils for short) in Python 2.0, this changed.

This document is aimed primarily at the people who need to install third-party
Python modules: end-users and system administrators who just need to get some
Python application running, and existing Python programmers who want to add some
new goodies to their toolbox.  You don't need to know Python to read this
document; there will be some brief forays into using Python's interactive mode
to explore your installation, but that's it.  If you're looking for information
on how to distribute your own Python modules so that others may use them, see
the :ref:`distutils-index` manual.


.. _inst-trivial-install:

Best case: trivial installation
-------------------------------

In the best case, someone will have prepared a special version of the module
distribution you want to install that is targeted specifically at your platform
and is installed just like any other software on your platform.  For example,
the module developer might make an executable installer available for Windows
users, an RPM package for users of RPM-based Linux systems (Red Hat, SuSE,
Mandrake, and many others), a Debian package for users of Debian-based Linux
systems, and so forth.

In that case, you would download the installer appropriate to your platform and
do the obvious thing with it: run it if it's an executable installer, ``rpm
--install`` it if it's an RPM, etc.  You don't need to run Python or a setup
script, you don't need to compile anything---you might not even need to read any
instructions (although it's always a good idea to do so anyway).

Of course, things will not always be that easy.  You might be interested in a
module distribution that doesn't have an easy-to-use installer for your
platform.  In that case, you'll have to start with the source distribution
released by the module's author/maintainer.  Installing from a source
distribution is not too hard, as long as the modules are packaged in the
standard way.  The bulk of this document is about building and installing
modules from standard source distributions.


.. _inst-new-standard:

The new standard: Distutils
---------------------------

If you download a module source distribution, you can tell pretty quickly if it
was packaged and distributed in the standard way, i.e. using the Distutils.
First, the distribution's name and version number will be featured prominently
in the name of the downloaded archive, e.g. :file:`foo-1.0.tar.gz` or
:file:`widget-0.9.7.zip`.  Next, the archive will unpack into a similarly-named
directory: :file:`foo-1.0` or :file:`widget-0.9.7`.  Additionally, the
distribution will contain a setup script :file:`setup.py`, and a file named
:file:`README.txt` or possibly just :file:`README`, which should explain that
building and installing the module distribution is a simple matter of running
one command from a terminal::

   python setup.py install

For Windows, this command should be run from a command prompt window
(:menuselection:`Start --> Accessories`)::

   setup.py install

If all these things are true, then you already know how to build and install the
modules you've just downloaded:  Run the command above. Unless you need to
install things in a non-standard way or customize the build process, you don't
really need this manual.  Or rather, the above command is everything you need to
get out of this manual.


.. _inst-standard-install:

Standard Build and Install
==========================

As described in section :ref:`inst-new-standard`, building and installing a module
distribution using the Distutils is usually one simple command to run from a
terminal::

   python setup.py install


.. _inst-platform-variations:

Platform variations
-------------------

You should always run the setup command from the distribution root directory,
i.e. the top-level subdirectory that the module source distribution unpacks
into.  For example, if you've just downloaded a module source distribution
:file:`foo-1.0.tar.gz` onto a Unix system, the normal thing to do is::

   gunzip -c foo-1.0.tar.gz | tar xf -    # unpacks into directory foo-1.0
   cd foo-1.0
   python setup.py install

On Windows, you'd probably download :file:`foo-1.0.zip`.  If you downloaded the
archive file to :file:`C:\\Temp`, then it would unpack into
:file:`C:\\Temp\\foo-1.0`; you can use either a archive manipulator with a
graphical user interface (such as WinZip) or a command-line tool (such as
:program:`unzip` or :program:`pkunzip`) to unpack the archive.  Then, open a
command prompt window and run::

   cd c:\Temp\foo-1.0
   python setup.py install


.. _inst-splitting-up:

Splitting the job up
--------------------

Running ``setup.py install`` builds and installs all modules in one run.  If you
prefer to work incrementally---especially useful if you want to customize the
build process, or if things are going wrong---you can use the setup script to do
one thing at a time.  This is particularly helpful when the build and install
will be done by different users---for example, you might want to build a module
distribution and hand it off to a system administrator for installation (or do
it yourself, with super-user privileges).

For example, you can build everything in one step, and then install everything
in a second step, by invoking the setup script twice::

   python setup.py build
   python setup.py install

If you do this, you will notice that running the :command:`install` command
first runs the :command:`build` command, which---in this case---quickly notices
that it has nothing to do, since everything in the :file:`build` directory is
up-to-date.

You may not need this ability to break things down often if all you do is
install modules downloaded off the 'net, but it's very handy for more advanced
tasks.  If you get into distributing your own Python modules and extensions,
you'll run lots of individual Distutils commands on their own.


.. _inst-how-build-works:

How building works
------------------

As implied above, the :command:`build` command is responsible for putting the
files to install into a *build directory*.  By default, this is :file:`build`
under the distribution root; if you're excessively concerned with speed, or want
to keep the source tree pristine, you can change the build directory with the
:option:`--build-base` option. For example::

   python setup.py build --build-base=/path/to/pybuild/foo-1.0

(Or you could do this permanently with a directive in your system or personal
Distutils configuration file; see section :ref:`inst-config-files`.)  Normally, this
isn't necessary.

The default layout for the build tree is as follows::

   --- build/ --- lib/
   or
   --- build/ --- lib.<plat>/
                  temp.<plat>/

where ``<plat>`` expands to a brief description of the current OS/hardware
platform and Python version.  The first form, with just a :file:`lib` directory,
is used for "pure module distributions"---that is, module distributions that
include only pure Python modules.  If a module distribution contains any
extensions (modules written in C/C++), then the second form, with two ``<plat>``
directories, is used.  In that case, the :file:`temp.{plat}` directory holds
temporary files generated by the compile/link process that don't actually get
installed.  In either case, the :file:`lib` (or :file:`lib.{plat}`) directory
contains all Python modules (pure Python and extensions) that will be installed.

In the future, more directories will be added to handle Python scripts,
documentation, binary executables, and whatever else is needed to handle the job
of installing Python modules and applications.


.. _inst-how-install-works:

How installation works
----------------------

After the :command:`build` command runs (whether you run it explicitly, or the
:command:`install` command does it for you), the work of the :command:`install`
command is relatively simple: all it has to do is copy everything under
:file:`build/lib` (or :file:`build/lib.{plat}`) to your chosen installation
directory.

If you don't choose an installation directory---i.e., if you just run ``setup.py
install``\ ---then the :command:`install` command installs to the standard
location for third-party Python modules.  This location varies by platform and
by how you built/installed Python itself.  On Unix (and Mac OS X, which is also
Unix-based), it also depends on whether the module distribution being installed
is pure Python or contains extensions ("non-pure"):

.. tabularcolumns:: |l|l|l|l|

+-----------------+-----------------------------------------------------+--------------------------------------------------+-------+
| Platform        | Standard installation location                      | Default value                                    | Notes |
+=================+=====================================================+==================================================+=======+
| Unix (pure)     | :file:`{prefix}/lib/python{X.Y}/site-packages`      | :file:`/usr/local/lib/python{X.Y}/site-packages` | \(1)  |
+-----------------+-----------------------------------------------------+--------------------------------------------------+-------+
| Unix (non-pure) | :file:`{exec-prefix}/lib/python{X.Y}/site-packages` | :file:`/usr/local/lib/python{X.Y}/site-packages` | \(1)  |
+-----------------+-----------------------------------------------------+--------------------------------------------------+-------+
| Windows         | :file:`{prefix}\\Lib\\site-packages`                | :file:`C:\\Python{XY}\\Lib\\site-packages`       | \(2)  |
+-----------------+-----------------------------------------------------+--------------------------------------------------+-------+

Notes:

(1)
   Most Linux distributions include Python as a standard part of the system, so
   :file:`{prefix}` and :file:`{exec-prefix}` are usually both :file:`/usr` on
   Linux.  If you build Python yourself on Linux (or any Unix-like system), the
   default :file:`{prefix}` and :file:`{exec-prefix}` are :file:`/usr/local`.

(2)
   The default installation directory on Windows was :file:`C:\\Program
   Files\\Python` under Python 1.6a1, 1.5.2, and earlier.

:file:`{prefix}` and :file:`{exec-prefix}` stand for the directories that Python
is installed to, and where it finds its libraries at run-time.  They are always
the same under Windows, and very often the same under Unix and Mac OS X.  You
can find out what your Python installation uses for :file:`{prefix}` and
:file:`{exec-prefix}` by running Python in interactive mode and typing a few
simple commands. Under Unix, just type ``python`` at the shell prompt.  Under
Windows, choose :menuselection:`Start --> Programs --> Python X.Y -->
Python (command line)`.   Once the interpreter is started, you type Python code
at the prompt.  For example, on my Linux system, I type the three Python
statements shown below, and get the output as shown, to find out my
:file:`{prefix}` and :file:`{exec-prefix}`::

   Python 2.4 (#26, Aug  7 2004, 17:19:02)
   Type "help", "copyright", "credits" or "license" for more information.
   >>> import sys
   >>> sys.prefix
   '/usr'
   >>> sys.exec_prefix
   '/usr'

A few other placeholders are used in this document: :file:`{X.Y}` stands for the
version of Python, for example ``3.2``; :file:`{abiflags}` will be replaced by
the value of :data:`sys.abiflags` or the empty string for platforms which don't
define ABI flags; :file:`{distname}` will be replaced by the name of the module
distribution being installed.  Dots and capitalization are important in the
paths; for example, a value that uses ``python3.2`` on UNIX will typically use
``Python32`` on Windows.

If you don't want to install modules to the standard location, or if you don't
have permission to write there, then you need to read about alternate
installations in section :ref:`inst-alt-install`.  If you want to customize your
installation directories more heavily, see section :ref:`inst-custom-install` on
custom installations.


.. _inst-alt-install:

Alternate Installation
======================

Often, it is necessary or desirable to install modules to a location other than
the standard location for third-party Python modules.  For example, on a Unix
system you might not have permission to write to the standard third-party module
directory.  Or you might wish to try out a module before making it a standard
part of your local Python installation.  This is especially true when upgrading
a distribution already present: you want to make sure your existing base of
scripts still works with the new version before actually upgrading.

The Distutils :command:`install` command is designed to make installing module
distributions to an alternate location simple and painless.  The basic idea is
that you supply a base directory for the installation, and the
:command:`install` command picks a set of directories (called an *installation
scheme*) under this base directory in which to install files.  The details
differ across platforms, so read whichever of the following sections applies to
you.

Note that the various alternate installation schemes are mutually exclusive: you
can pass ``--user``, or ``--home``, or ``--prefix`` and ``--exec-prefix``, or
``--install-base`` and ``--install-platbase``, but you can't mix from these
groups.


.. _inst-alt-install-user:

Alternate installation: the user scheme
---------------------------------------

This scheme is designed to be the most convenient solution for users that don't
have write permission to the global site-packages directory or don't want to
install into it.  It is enabled with a simple option::

   python setup.py install --user

Files will be installed into subdirectories of :data:`site.USER_BASE` (written
as :file:`{userbase}` hereafter).  This scheme installs pure Python modules and
extension modules in the same location (also known as :data:`site.USER_SITE`).
Here are the values for UNIX, including Mac OS X:

=============== ===========================================================
Type of file    Installation directory
=============== ===========================================================
modules         :file:`{userbase}/lib/python{X.Y}/site-packages`
scripts         :file:`{userbase}/bin`
data            :file:`{userbase}`
C headers       :file:`{userbase}/include/python{X.Y}{abiflags}/{distname}`
=============== ===========================================================

And here are the values used on Windows:

=============== ===========================================================
Type of file    Installation directory
=============== ===========================================================
modules         :file:`{userbase}\\Python{XY}\\site-packages`
scripts         :file:`{userbase}\\Scripts`
data            :file:`{userbase}`
C headers       :file:`{userbase}\\Python{XY}\\Include\\{distname}`
=============== ===========================================================

The advantage of using this scheme compared to the other ones described below is
that the user site-packages directory is under normal conditions always included
in :data:`sys.path` (see :mod:`site` for more information), which means that
there is no additional step to perform after running the :file:`setup.py` script
to finalize the installation.

The :command:`build_ext` command also has a ``--user`` option to add
:file:`{userbase}/include` to the compiler search path for header files and
:file:`{userbase}/lib` to the compiler search path for libraries as well as to
the runtime search path for shared C libraries (rpath).


.. _inst-alt-install-home:

Alternate installation: the home scheme
---------------------------------------

The idea behind the "home scheme" is that you build and maintain a personal
stash of Python modules.  This scheme's name is derived from the idea of a
"home" directory on Unix, since it's not unusual for a Unix user to make their
home directory have a layout similar to :file:`/usr/` or :file:`/usr/local/`.
This scheme can be used by anyone, regardless of the operating system they
are installing for.

Installing a new module distribution is as simple as ::

   python setup.py install --home=<dir>

where you can supply any directory you like for the :option:`--home` option.  On
Unix, lazy typists can just type a tilde (``~``); the :command:`install` command
will expand this to your home directory::

   python setup.py install --home=~

To make Python find the distributions installed with this scheme, you may have
to :ref:`modify Python's search path <inst-search-path>` or edit
:mod:`sitecustomize` (see :mod:`site`) to call :func:`site.addsitedir` or edit
:data:`sys.path`.

The :option:`--home` option defines the installation base directory.  Files are
installed to the following directories under the installation base as follows:

=============== ===========================================================
Type of file    Installation directory
=============== ===========================================================
modules         :file:`{home}/lib/python`
scripts         :file:`{home}/bin`
data            :file:`{home}`
C headers       :file:`{home}/include/python/{distname}`
=============== ===========================================================

(Mentally replace slashes with backslashes if you're on Windows.)


.. _inst-alt-install-prefix-unix:

Alternate installation: Unix (the prefix scheme)
------------------------------------------------

The "prefix scheme" is useful when you wish to use one Python installation to
perform the build/install (i.e., to run the setup script), but install modules
into the third-party module directory of a different Python installation (or
something that looks like a different Python installation).  If this sounds a
trifle unusual, it is---that's why the user and home schemes come before.  However,
there are at least two known cases where the prefix scheme will be useful.

First, consider that many Linux distributions put Python in :file:`/usr`, rather
than the more traditional :file:`/usr/local`.  This is entirely appropriate,
since in those cases Python is part of "the system" rather than a local add-on.
However, if you are installing Python modules from source, you probably want
them to go in :file:`/usr/local/lib/python2.{X}` rather than
:file:`/usr/lib/python2.{X}`.  This can be done with ::

   /usr/bin/python setup.py install --prefix=/usr/local

Another possibility is a network filesystem where the name used to write to a
remote directory is different from the name used to read it: for example, the
Python interpreter accessed as :file:`/usr/local/bin/python` might search for
modules in :file:`/usr/local/lib/python2.{X}`, but those modules would have to
be installed to, say, :file:`/mnt/{@server}/export/lib/python2.{X}`.  This could
be done with ::

   /usr/local/bin/python setup.py install --prefix=/mnt/@server/export

In either case, the :option:`--prefix` option defines the installation base, and
the :option:`--exec-prefix` option defines the platform-specific installation
base, which is used for platform-specific files.  (Currently, this just means
non-pure module distributions, but could be expanded to C libraries, binary
executables, etc.)  If :option:`--exec-prefix` is not supplied, it defaults to
:option:`--prefix`.  Files are installed as follows:

================= ==========================================================
Type of file      Installation directory
================= ==========================================================
Python modules    :file:`{prefix}/lib/python{X.Y}/site-packages`
extension modules :file:`{exec-prefix}/lib/python{X.Y}/site-packages`
scripts           :file:`{prefix}/bin`
data              :file:`{prefix}`
C headers         :file:`{prefix}/include/python{X.Y}{abiflags}/{distname}`
================= ==========================================================

There is no requirement that :option:`--prefix` or :option:`--exec-prefix`
actually point to an alternate Python installation; if the directories listed
above do not already exist, they are created at installation time.

Incidentally, the real reason the prefix scheme is important is simply that a
standard Unix installation uses the prefix scheme, but with :option:`--prefix`
and :option:`--exec-prefix` supplied by Python itself as ``sys.prefix`` and
``sys.exec_prefix``.  Thus, you might think you'll never use the prefix scheme,
but every time you run ``python setup.py install`` without any other options,
you're using it.

Note that installing extensions to an alternate Python installation has no
effect on how those extensions are built: in particular, the Python header files
(:file:`Python.h` and friends) installed with the Python interpreter used to run
the setup script will be used in compiling extensions.  It is your
responsibility to ensure that the interpreter used to run extensions installed
in this way is compatible with the interpreter used to build them.  The best way
to do this is to ensure that the two interpreters are the same version of Python
(possibly different builds, or possibly copies of the same build).  (Of course,
if your :option:`--prefix` and :option:`--exec-prefix` don't even point to an
alternate Python installation, this is immaterial.)


.. _inst-alt-install-prefix-windows:

Alternate installation: Windows (the prefix scheme)
---------------------------------------------------

Windows has no concept of a user's home directory, and since the standard Python
installation under Windows is simpler than under Unix, the :option:`--prefix`
option has traditionally been used to install additional packages in separate
locations on Windows. ::

   python setup.py install --prefix="\Temp\Python"

to install modules to the :file:`\\Temp\\Python` directory on the current drive.

The installation base is defined by the :option:`--prefix` option; the
:option:`--exec-prefix` option is not supported under Windows, which means that
pure Python modules and extension modules are installed into the same location.
Files are installed as follows:

=============== ==========================================================
Type of file    Installation directory
=============== ==========================================================
modules         :file:`{prefix}\\Lib\\site-packages`
scripts         :file:`{prefix}\\Scripts`
data            :file:`{prefix}`
C headers       :file:`{prefix}\\Include\\{distname}`
=============== ==========================================================


.. _inst-custom-install:

Custom Installation
===================

Sometimes, the alternate installation schemes described in section
:ref:`inst-alt-install` just don't do what you want.  You might want to tweak just
one or two directories while keeping everything under the same base directory,
or you might want to completely redefine the installation scheme.  In either
case, you're creating a *custom installation scheme*.

To create a custom installation scheme, you start with one of the alternate
schemes and override some of the installation directories used for the various
types of files, using these options:

====================== =======================
Type of file           Override option
====================== =======================
Python modules         ``--install-purelib``
extension modules      ``--install-platlib``
all modules            ``--install-lib``
scripts                ``--install-scripts``
data                   ``--install-data``
C headers              ``--install-headers``
====================== =======================

These override options can be relative, absolute,
or explicitly defined in terms of one of the installation base directories.
(There are two installation base directories, and they are normally the same---
they only differ when you use the Unix "prefix scheme" and supply different
``--prefix`` and ``--exec-prefix`` options; using ``--install-lib`` will
override values computed or given for ``--install-purelib`` and
``--install-platlib``, and is recommended for schemes that don't make a
difference between Python and extension modules.)

For example, say you're installing a module distribution to your home directory
under Unix---but you want scripts to go in :file:`~/scripts` rather than
:file:`~/bin`. As you might expect, you can override this directory with the
:option:`--install-scripts` option; in this case, it makes most sense to supply
a relative path, which will be interpreted relative to the installation base
directory (your home directory, in this case)::

   python setup.py install --home=~ --install-scripts=scripts

Another Unix example: suppose your Python installation was built and installed
with a prefix of :file:`/usr/local/python`, so under a standard  installation
scripts will wind up in :file:`/usr/local/python/bin`.  If you want them in
:file:`/usr/local/bin` instead, you would supply this absolute directory for the
:option:`--install-scripts` option::

   python setup.py install --install-scripts=/usr/local/bin

(This performs an installation using the "prefix scheme," where the prefix is
whatever your Python interpreter was installed with--- :file:`/usr/local/python`
in this case.)

If you maintain Python on Windows, you might want third-party modules to live in
a subdirectory of :file:`{prefix}`, rather than right in :file:`{prefix}`
itself.  This is almost as easy as customizing the script installation directory
---you just have to remember that there are two types of modules to worry about,
Python and extension modules, which can conveniently be both controlled by one
option::

   python setup.py install --install-lib=Site

The specified installation directory is relative to :file:`{prefix}`.  Of
course, you also have to ensure that this directory is in Python's module
search path, such as by putting a :file:`.pth` file in a site directory (see
:mod:`site`).  See section :ref:`inst-search-path` to find out how to modify
Python's search path.

If you want to define an entire installation scheme, you just have to supply all
of the installation directory options.  The recommended way to do this is to
supply relative paths; for example, if you want to maintain all Python
module-related files under :file:`python` in your home directory, and you want a
separate directory for each platform that you use your home directory from, you
might define the following installation scheme::

   python setup.py install --home=~ \
                           --install-purelib=python/lib \
                           --install-platlib=python/lib.$PLAT \
                           --install-scripts=python/scripts
                           --install-data=python/data

or, equivalently, ::

   python setup.py install --home=~/python \
                           --install-purelib=lib \
                           --install-platlib='lib.$PLAT' \
                           --install-scripts=scripts
                           --install-data=data

``$PLAT`` is not (necessarily) an environment variable---it will be expanded by
the Distutils as it parses your command line options, just as it does when
parsing your configuration file(s).

Obviously, specifying the entire installation scheme every time you install a
new module distribution would be very tedious.  Thus, you can put these options
into your Distutils config file (see section :ref:`inst-config-files`)::

   [install]
   install-base=$HOME
   install-purelib=python/lib
   install-platlib=python/lib.$PLAT
   install-scripts=python/scripts
   install-data=python/data

or, equivalently, ::

   [install]
   install-base=$HOME/python
   install-purelib=lib
   install-platlib=lib.$PLAT
   install-scripts=scripts
   install-data=data

Note that these two are *not* equivalent if you supply a different installation
base directory when you run the setup script.  For example, ::

   python setup.py install --install-base=/tmp

would install pure modules to :file:`/tmp/python/lib` in the first case, and
to :file:`/tmp/lib` in the second case.  (For the second case, you probably
want to supply an installation base of :file:`/tmp/python`.)

You probably noticed the use of ``$HOME`` and ``$PLAT`` in the sample
configuration file input.  These are Distutils configuration variables, which
bear a strong resemblance to environment variables. In fact, you can use
environment variables in config files on platforms that have such a notion but
the Distutils additionally define a few extra variables that may not be in your
environment, such as ``$PLAT``.  (And of course, on systems that don't have
environment variables, such as Mac OS 9, the configuration variables supplied by
the Distutils are the only ones you can use.) See section :ref:`inst-config-files`
for details.

.. note:: When a :ref:`virtual environment <venv-def>` is activated, any options
   that change the installation path will be ignored from all distutils configuration
   files to prevent inadvertently installing projects outside of the virtual
   environment.

.. XXX need some Windows examples---when would custom installation schemes be
   needed on those platforms?


.. XXX Move this to Doc/using

.. _inst-search-path:

Modifying Python's Search Path
------------------------------

When the Python interpreter executes an :keyword:`import` statement, it searches
for both Python code and extension modules along a search path.  A default value
for the path is configured into the Python binary when the interpreter is built.
You can determine the path by importing the :mod:`sys` module and printing the
value of ``sys.path``.   ::

   $ python
   Python 2.2 (#11, Oct  3 2002, 13:31:27)
   [GCC 2.96 20000731 (Red Hat Linux 7.3 2.96-112)] on linux2
   Type "help", "copyright", "credits" or "license" for more information.
   >>> import sys
   >>> sys.path
   ['', '/usr/local/lib/python2.3', '/usr/local/lib/python2.3/plat-linux2',
    '/usr/local/lib/python2.3/lib-tk', '/usr/local/lib/python2.3/lib-dynload',
    '/usr/local/lib/python2.3/site-packages']
   >>>

The null string in ``sys.path`` represents the current working directory.

The expected convention for locally installed packages is to put them in the
:file:`{...}/site-packages/` directory, but you may want to install Python
modules into some arbitrary directory.  For example, your site may have a
convention of keeping all software related to the web server under :file:`/www`.
Add-on Python modules might then belong in :file:`/www/python`, and in order to
import them, this directory must be added to ``sys.path``.  There are several
different ways to add the directory.

The most convenient way is to add a path configuration file to a directory
that's already on Python's path, usually to the :file:`.../site-packages/`
directory.  Path configuration files have an extension of :file:`.pth`, and each
line must contain a single path that will be appended to ``sys.path``.  (Because
the new paths are appended to ``sys.path``, modules in the added directories
will not override standard modules.  This means you can't use this mechanism for
installing fixed versions of standard modules.)

Paths can be absolute or relative, in which case they're relative to the
directory containing the :file:`.pth` file.  See the documentation of
the :mod:`site` module for more information.

A slightly less convenient way is to edit the :file:`site.py` file in Python's
standard library, and modify ``sys.path``.  :file:`site.py` is automatically
imported when the Python interpreter is executed, unless the :option:`-S` switch
is supplied to suppress this behaviour.  So you could simply edit
:file:`site.py` and add two lines to it::

   import sys
   sys.path.append('/www/python/')

However, if you reinstall the same major version of Python (perhaps when
upgrading from 2.2 to 2.2.2, for example) :file:`site.py` will be overwritten by
the stock version.  You'd have to remember that it was modified and save a copy
before doing the installation.

There are two environment variables that can modify ``sys.path``.
:envvar:`PYTHONHOME` sets an alternate value for the prefix of the Python
installation.  For example, if :envvar:`PYTHONHOME` is set to ``/www/python``,
the search path will be set to ``['', '/www/python/lib/pythonX.Y/',
'/www/python/lib/pythonX.Y/plat-linux2', ...]``.

The :envvar:`PYTHONPATH` variable can be set to a list of paths that will be
added to the beginning of ``sys.path``.  For example, if :envvar:`PYTHONPATH` is
set to ``/www/python:/opt/py``, the search path will begin with
``['/www/python', '/opt/py']``.  (Note that directories must exist in order to
be added to ``sys.path``; the :mod:`site` module removes paths that don't
exist.)

Finally, ``sys.path`` is just a regular Python list, so any Python application
can modify it by adding or removing entries.


.. _inst-config-files:

Distutils Configuration Files
=============================

As mentioned above, you can use Distutils configuration files to record personal
or site preferences for any Distutils options.  That is, any option to any
command can be stored in one of two or three (depending on your platform)
configuration files, which will be consulted before the command-line is parsed.
This means that configuration files will override default values, and the
command-line will in turn override configuration files.  Furthermore, if
multiple configuration files apply, values from "earlier" files are overridden
by "later" files.


.. _inst-config-filenames:

Location and names of config files
----------------------------------

The names and locations of the configuration files vary slightly across
platforms.  On Unix and Mac OS X, the three configuration files (in the order
they are processed) are:

+--------------+----------------------------------------------------------+-------+
| Type of file | Location and filename                                    | Notes |
+==============+==========================================================+=======+
| system       | :file:`{prefix}/lib/python{ver}/distutils/distutils.cfg` | \(1)  |
+--------------+----------------------------------------------------------+-------+
| personal     | :file:`$HOME/.pydistutils.cfg`                           | \(2)  |
+--------------+----------------------------------------------------------+-------+
| local        | :file:`setup.cfg`                                        | \(3)  |
+--------------+----------------------------------------------------------+-------+

And on Windows, the configuration files are:

+--------------+-------------------------------------------------+-------+
| Type of file | Location and filename                           | Notes |
+==============+=================================================+=======+
| system       | :file:`{prefix}\\Lib\\distutils\\distutils.cfg` | \(4)  |
+--------------+-------------------------------------------------+-------+
| personal     | :file:`%HOME%\\pydistutils.cfg`                 | \(5)  |
+--------------+-------------------------------------------------+-------+
| local        | :file:`setup.cfg`                               | \(3)  |
+--------------+-------------------------------------------------+-------+

On all platforms, the "personal" file can be temporarily disabled by
passing the `--no-user-cfg` option.

Notes:

(1)
   Strictly speaking, the system-wide configuration file lives in the directory
   where the Distutils are installed; under Python 1.6 and later on Unix, this is
   as shown. For Python 1.5.2, the Distutils will normally be installed to
   :file:`{prefix}/lib/python1.5/site-packages/distutils`, so the system
   configuration file should be put there under Python 1.5.2.

(2)
   On Unix, if the :envvar:`HOME` environment variable is not defined, the user's
   home directory will be determined with the :func:`getpwuid` function from the
   standard :mod:`pwd` module. This is done by the :func:`os.path.expanduser`
   function used by Distutils.

(3)
   I.e., in the current directory (usually the location of the setup script).

(4)
   (See also note (1).)  Under Python 1.6 and later, Python's default "installation
   prefix" is :file:`C:\\Python`, so the system configuration file is normally
   :file:`C:\\Python\\Lib\\distutils\\distutils.cfg`. Under Python 1.5.2, the
   default prefix was :file:`C:\\Program Files\\Python`, and the Distutils were not
   part of the standard library---so the system configuration file would be
   :file:`C:\\Program Files\\Python\\distutils\\distutils.cfg` in a standard Python
   1.5.2 installation under Windows.

(5)
   On Windows, if the :envvar:`HOME` environment variable is not defined,
   :envvar:`USERPROFILE` then :envvar:`HOMEDRIVE` and :envvar:`HOMEPATH` will
   be tried. This is done by the :func:`os.path.expanduser` function used
   by Distutils.


.. _inst-config-syntax:

Syntax of config files
----------------------

The Distutils configuration files all have the same syntax.  The config files
are grouped into sections.  There is one section for each Distutils command,
plus a ``global`` section for global options that affect every command.  Each
section consists of one option per line, specified as ``option=value``.

For example, the following is a complete config file that just forces all
commands to run quietly by default::

   [global]
   verbose=0

If this is installed as the system config file, it will affect all processing of
any Python module distribution by any user on the current system.  If it is
installed as your personal config file (on systems that support them), it will
affect only module distributions processed by you.  And if it is used as the
:file:`setup.cfg` for a particular module distribution, it affects only that
distribution.

You could override the default "build base" directory and make the
:command:`build\*` commands always forcibly rebuild all files with the
following::

   [build]
   build-base=blib
   force=1

which corresponds to the command-line arguments ::

   python setup.py build --build-base=blib --force

except that including the :command:`build` command on the command-line means
that command will be run.  Including a particular command in config files has no
such implication; it only means that if the command is run, the options in the
config file will apply.  (Or if other commands that derive values from it are
run, they will use the values in the config file.)

You can find out the complete list of options for any command using the
:option:`--help` option, e.g.::

   python setup.py build --help

and you can find out the complete list of global options by using
:option:`--help` without a command::

   python setup.py --help

See also the "Reference" section of the "Distributing Python Modules" manual.


.. _inst-building-ext:

Building Extensions: Tips and Tricks
====================================

Whenever possible, the Distutils try to use the configuration information made
available by the Python interpreter used to run the :file:`setup.py` script.
For example, the same compiler and linker flags used to compile Python will also
be used for compiling extensions.  Usually this will work well, but in
complicated situations this might be inappropriate.  This section discusses how
to override the usual Distutils behaviour.


.. _inst-tweak-flags:

Tweaking compiler/linker flags
------------------------------

Compiling a Python extension written in C or C++ will sometimes require
specifying custom flags for the compiler and linker in order to use a particular
library or produce a special kind of object code. This is especially true if the
extension hasn't been tested on your platform, or if you're trying to
cross-compile Python.

In the most general case, the extension author might have foreseen that
compiling the extensions would be complicated, and provided a :file:`Setup` file
for you to edit.  This will likely only be done if the module distribution
contains many separate extension modules, or if they often require elaborate
sets of compiler flags in order to work.

A :file:`Setup` file, if present, is parsed in order to get a list of extensions
to build.  Each line in a :file:`Setup` describes a single module.  Lines have
the following structure::

   module ... [sourcefile ...] [cpparg ...] [library ...]


Let's examine each of the fields in turn.

* *module* is the name of the extension module to be built, and should be a
  valid Python identifier.  You can't just change this in order to rename a module
  (edits to the source code would also be needed), so this should be left alone.

* *sourcefile* is anything that's likely to be a source code file, at least
  judging by the filename.  Filenames ending in :file:`.c` are assumed to be
  written in C, filenames ending in :file:`.C`, :file:`.cc`, and :file:`.c++` are
  assumed to be C++, and filenames ending in :file:`.m` or :file:`.mm` are assumed
  to be in Objective C.

* *cpparg* is an argument for the C preprocessor,  and is anything starting with
  :option:`-I`, :option:`-D`, :option:`-U` or :option:`-C`.

* *library* is anything ending in :file:`.a` or beginning with :option:`-l` or
  :option:`-L`.

If a particular platform requires a special library on your platform, you can
add it by editing the :file:`Setup` file and running ``python setup.py build``.
For example, if the module defined by the line ::

   foo foomodule.c

must be linked with the math library :file:`libm.a` on your platform, simply add
:option:`-lm` to the line::

   foo foomodule.c -lm

Arbitrary switches intended for the compiler or the linker can be supplied with
the :option:`-Xcompiler` *arg* and :option:`-Xlinker` *arg* options::

   foo foomodule.c -Xcompiler -o32 -Xlinker -shared -lm

The next option after :option:`-Xcompiler` and :option:`-Xlinker` will be
appended to the proper command line, so in the above example the compiler will
be passed the :option:`-o32` option, and the linker will be passed
:option:`-shared`.  If a compiler option requires an argument, you'll have to
supply multiple :option:`-Xcompiler` options; for example, to pass ``-x c++``
the :file:`Setup` file would have to contain ``-Xcompiler -x -Xcompiler c++``.

Compiler flags can also be supplied through setting the :envvar:`CFLAGS`
environment variable.  If set, the contents of :envvar:`CFLAGS` will be added to
the compiler flags specified in the  :file:`Setup` file.


.. _inst-non-ms-compilers:

Using non-Microsoft compilers on Windows
----------------------------------------

.. sectionauthor:: Rene Liebscher <R.Liebscher@gmx.de>



Borland/CodeGear C++
^^^^^^^^^^^^^^^^^^^^

This subsection describes the necessary steps to use Distutils with the Borland
C++ compiler version 5.5.  First you have to know that Borland's object file
format (OMF) is different from the format used by the Python version you can
download from the Python or ActiveState Web site.  (Python is built with
Microsoft Visual C++, which uses COFF as the object file format.) For this
reason you have to convert Python's library :file:`python25.lib` into the
Borland format.  You can do this as follows:

.. Should we mention that users have to create cfg-files for the compiler?
.. see also http://community.borland.com/article/0,1410,21205,00.html

::

   coff2omf python25.lib python25_bcpp.lib

The :file:`coff2omf` program comes with the Borland compiler.  The file
:file:`python25.lib` is in the :file:`Libs` directory of your Python
installation.  If your extension uses other libraries (zlib, ...) you have to
convert them too.

The converted files have to reside in the same directories as the normal
libraries.

How does Distutils manage to use these libraries with their changed names?  If
the extension needs a library (eg. :file:`foo`) Distutils checks first if it
finds a library with suffix :file:`_bcpp` (eg. :file:`foo_bcpp.lib`) and then
uses this library.  In the case it doesn't find such a special library it uses
the default name (:file:`foo.lib`.) [#]_

To let Distutils compile your extension with Borland C++ you now have to type::

   python setup.py build --compiler=bcpp

If you want to use the Borland C++ compiler as the default, you could specify
this in your personal or system-wide configuration file for Distutils (see
section :ref:`inst-config-files`.)


.. seealso::

   `C++Builder Compiler <http://www.codegear.com/downloads/free/cppbuilder>`_
      Information about the free C++ compiler from Borland, including links to the
      download pages.

   `Creating Python Extensions Using Borland's Free Compiler <http://www.cyberus.ca/~g_will/pyExtenDL.shtml>`_
      Document describing how to use Borland's free command-line C++ compiler to build
      Python.


GNU C / Cygwin / MinGW
^^^^^^^^^^^^^^^^^^^^^^

This section describes the necessary steps to use Distutils with the GNU C/C++
compilers in their Cygwin and MinGW distributions. [#]_ For a Python interpreter
that was built with Cygwin, everything should work without any of these
following steps.

Not all extensions can be built with MinGW or Cygwin, but many can.  Extensions
most likely to not work are those that use C++ or depend on Microsoft Visual C
extensions.

To let Distutils compile your extension with Cygwin you have to type::

   python setup.py build --compiler=cygwin

and for Cygwin in no-cygwin mode [#]_ or for MinGW type::

   python setup.py build --compiler=mingw32

If you want to use any of these options/compilers as default, you should
consider writing it in your personal or system-wide configuration file for
Distutils (see section :ref:`inst-config-files`.)

Older Versions of Python and MinGW
""""""""""""""""""""""""""""""""""
The following instructions only apply if you're using a version of Python
inferior to 2.4.1 with a MinGW inferior to 3.0.0 (with
binutils-2.13.90-20030111-1).

These compilers require some special libraries.  This task is more complex than
for Borland's C++, because there is no program to convert the library.  First
you have to create a list of symbols which the Python DLL exports. (You can find
a good program for this task at
http://sourceforge.net/projects/mingw/files/MinGW/Extension/pexports/).

.. I don't understand what the next line means. --amk
.. (inclusive the references on data structures.)

::

   pexports python25.dll >python25.def

The location of an installed :file:`python25.dll` will depend on the
installation options and the version and language of Windows.  In a "just for
me" installation, it will appear in the root of the installation directory.  In
a shared installation, it will be located in the system directory.

Then you can create from these information an import library for gcc. ::

   /cygwin/bin/dlltool --dllname python25.dll --def python25.def --output-lib libpython25.a

The resulting library has to be placed in the same directory as
:file:`python25.lib`. (Should be the :file:`libs` directory under your Python
installation directory.)

If your extension uses other libraries (zlib,...) you might  have to convert
them too. The converted files have to reside in the same directories as the
normal libraries do.


.. seealso::

   `Building Python modules on MS Windows platform with MinGW <http://www.zope.org/Members/als/tips/win32_mingw_modules>`_
      Information about building the required libraries for the MinGW environment.


.. rubric:: Footnotes

.. [#] This also means you could replace all existing COFF-libraries with OMF-libraries
   of the same name.

.. [#] Check http://sources.redhat.com/cygwin/ and http://www.mingw.org/ for more
   information

.. [#] Then you have no POSIX emulation available, but you also don't need
   :file:`cygwin1.dll`.