Installing Python Modules (Legacy version)
******************************************

Author:
   Greg Ward

Note:

  The entire "distutils" package has been deprecated and will be
  removed in Python 3.12. This documentation is retained as a
  reference only, and will be removed with the package. See the What’s
  New entry for more information.

See also:

  Installing Python Modules
     The up to date module installation documentation. For regular
     Python usage, you almost certainly want that document rather than
     this one.

Note:

  This document is being retained solely until the "setuptools"
  documentation at
  https://setuptools.readthedocs.io/en/latest/setuptools.html
  independently covers all of the relevant information currently
  included here.

Note:

  This guide only covers the basic tools for building and distributing
  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 in the Python Packaging
  User Guide for more information.


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

In Python 2.0, the "distutils" API was first added to the standard
library. This provided Linux distro maintainers with a standard way of
converting Python projects into Linux distro packages, and system
administrators with a standard way of installing them directly onto
target systems.

In the many years since Python 2.0 was released, tightly coupling the
build system and package installer to the language runtime release
cycle has turned out to be problematic, and it is now recommended that
projects use the "pip" package installer and the "setuptools" build
system, rather than using "distutils" directly.

See Installing Python Modules and Distributing Python Modules for more
details.

This legacy documentation is being retained only until we’re confident
that the "setuptools" documentation covers everything needed.


Distutils based source distributions
------------------------------------

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. "foo-1.0.tar.gz" or "widget-0.9.7.zip".  Next, the archive will
unpack into a similarly named directory: "foo-1.0" or "widget-0.9.7".
Additionally, the distribution will contain a setup script "setup.py",
and a file named "README.txt" or possibly just "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
(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.


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

As described in section Distutils based source distributions, building
and installing a module distribution using the Distutils is usually
one simple command to run from a terminal:

   python setup.py install


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

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


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 **install** command
first runs the **build** command, which—in this case—quickly notices
that it has nothing to do, since everything in the "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.


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

As implied above, the **build** command is responsible for putting the
files to install into a *build directory*.  By default, this is
"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 "--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 Distutils
Configuration 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
"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 "temp.*plat*" directory holds temporary files
generated by the compile/link process that don’t actually get
installed.  In either case, the "lib" (or "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.


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

After the **build** command runs (whether you run it explicitly, or
the **install** command does it for you), the work of the **install**
command is relatively simple: all it has to do is copy everything
under "build/lib" (or "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 **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 macOS, which is also Unix-based), it also depends on whether
the module distribution being installed is pure Python or contains
extensions (“non-pure”):

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

Notes:

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

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

"*prefix*" and "*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 macOS.  You can find out what your Python installation uses for
"*prefix*" and "*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 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 "*prefix*" and "*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: "*X.Y*" stands for
the version of Python, for example "3.2"; "*abiflags*" will be
replaced by the value of "sys.abiflags" or the empty string for
platforms which don’t define ABI flags; "*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 Alternate Installation.  If you
want to customize your installation directories more heavily, see
section Custom Installation on custom installations.


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 **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 **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.


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 "site.USER_BASE"
(written as "*userbase*" hereafter).  This scheme installs pure Python
modules and extension modules in the same location (also known as
"site.USER_SITE"). Here are the values for UNIX, including macOS:

+-----------------+-------------------------------------------------------------+
| Type of file    | Installation directory                                      |
|=================|=============================================================|
| modules         | "*userbase*/lib/python*X.Y*/site-packages"                  |
+-----------------+-------------------------------------------------------------+
| scripts         | "*userbase*/bin"                                            |
+-----------------+-------------------------------------------------------------+
| data            | "*userbase*"                                                |
+-----------------+-------------------------------------------------------------+
| C headers       | "*userbase*/include/python*X.Y**abiflags*/*distname*"       |
+-----------------+-------------------------------------------------------------+

And here are the values used on Windows:

+-----------------+-------------------------------------------------------------+
| Type of file    | Installation directory                                      |
|=================|=============================================================|
| modules         | "*userbase*\Python*XY*\site-packages"                       |
+-----------------+-------------------------------------------------------------+
| scripts         | "*userbase*\Python*XY*\Scripts"                             |
+-----------------+-------------------------------------------------------------+
| data            | "*userbase*"                                                |
+-----------------+-------------------------------------------------------------+
| C headers       | "*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 "sys.path" (see "site" for more
information), which means that there is no additional step to perform
after running the "setup.py" script to finalize the installation.

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


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
"/usr/" or "/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 "--home" option.
On Unix, lazy typists can just type a tilde ("~"); the **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 modify Python’s search path or edit "sitecustomize" (see
"site") to call "site.addsitedir()" or edit "sys.path".

The "--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         | "*home*/lib/python"                                         |
+-----------------+-------------------------------------------------------------+
| scripts         | "*home*/bin"                                                |
+-----------------+-------------------------------------------------------------+
| data            | "*home*"                                                    |
+-----------------+-------------------------------------------------------------+
| C headers       | "*home*/include/python/*distname*"                          |
+-----------------+-------------------------------------------------------------+

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


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 "/usr",
rather than the more traditional "/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
"/usr/local/lib/python2.*X*" rather than "/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
"/usr/local/bin/python" might search for modules in
"/usr/local/lib/python2.*X*", but those modules would have to be
installed to, say, "/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 "--prefix" option defines the installation base,
and the "--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 "--exec-
prefix" is not supplied, it defaults to "--prefix".  Files are
installed as follows:

+-------------------+------------------------------------------------------------+
| Type of file      | Installation directory                                     |
|===================|============================================================|
| Python modules    | "*prefix*/lib/python*X.Y*/site-packages"                   |
+-------------------+------------------------------------------------------------+
| extension modules | "*exec-prefix*/lib/python*X.Y*/site-packages"              |
+-------------------+------------------------------------------------------------+
| scripts           | "*prefix*/bin"                                             |
+-------------------+------------------------------------------------------------+
| data              | "*prefix*"                                                 |
+-------------------+------------------------------------------------------------+
| C headers         | "*prefix*/include/python*X.Y**abiflags*/*distname*"        |
+-------------------+------------------------------------------------------------+

There is no requirement that "--prefix" or "--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 "--
prefix" and "--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 ("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 "--prefix" and "--exec-prefix" don’t even point to an
alternate Python installation, this is immaterial.)


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 "--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 "\Temp\Python" directory on the current
drive.

The installation base is defined by the "--prefix" option; the "--
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         | "*prefix*\Lib\site-packages"                               |
+-----------------+------------------------------------------------------------+
| scripts         | "*prefix*\Scripts"                                         |
+-----------------+------------------------------------------------------------+
| data            | "*prefix*"                                                 |
+-----------------+------------------------------------------------------------+
| C headers       | "*prefix*\Include{distname}"                               |
+-----------------+------------------------------------------------------------+


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

Sometimes, the alternate installation schemes described in section
Alternate Installation 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 "~/scripts" rather
than "~/bin". As you might expect, you can override this directory
with the "--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 "/usr/local/python", so under a standard
installation scripts will wind up in "/usr/local/python/bin".  If you
want them in "/usr/local/bin" instead, you would supply this absolute
directory for the "--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—
"/usr/local/python" in this case.)

If you maintain Python on Windows, you might want third-party modules
to live in a subdirectory of "*prefix*", rather than right in
"*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 "*prefix*".  Of
course, you also have to ensure that this directory is in Python’s
module search path, such as by putting a ".pth" file in a site
directory (see "site").  See section Modifying Python’s 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 "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
Distutils Configuration 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 "/tmp/python/lib" in the first case, and
to "/tmp/lib" in the second case.  (For the second case, you probably
want to supply an installation base of "/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 Distutils
Configuration Files for details.

Note:

  When a virtual environment 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.


Modifying Python’s Search Path
------------------------------

When the Python interpreter executes an "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 "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 "*…*/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 "/www". Add-on Python modules might then belong in
"/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 ".../site-
packages/" directory.  Path configuration files have an extension of
".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 ".pth" file.  See the documentation of
the "site" module for more information.

A slightly less convenient way is to edit the "site.py" file in
Python’s standard library, and modify "sys.path".  "site.py" is
automatically imported when the Python interpreter is executed, unless
the "-S" switch is supplied to suppress this behaviour.  So you could
simply edit "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) "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".
"PYTHONHOME" sets an alternate value for the prefix of the Python
installation.  For example, if "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 "PYTHONPATH" variable can be set to a list of paths that will be
added to the beginning of "sys.path".  For example, if "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 "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.


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.


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

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

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

And on Windows, the configuration files are:

+----------------+---------------------------------------------------+---------+
| Type of file   | Location and filename                             | Notes   |
|================|===================================================|=========|
| system         | "*prefix*\Lib\distutils\distutils.cfg"            | (4)     |
+----------------+---------------------------------------------------+---------+
| personal       | "%HOME%\pydistutils.cfg"                          | (5)     |
+----------------+---------------------------------------------------+---------+
| local          | "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 "*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 "HOME" environment variable is not defined, the
   user’s home directory will be determined with the "getpwuid()"
   function from the standard "pwd" module. This is done by the
   "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 "C:\Python", so the system configuration
   file is normally "C:\Python\Lib\distutils\distutils.cfg". Under
   Python 1.5.2, the default prefix was "C:\Program Files\Python", and
   the Distutils were not part of the standard library—so the system
   configuration file would be "C:\Program
   Files\Python\distutils\distutils.cfg" in a standard Python 1.5.2
   installation under Windows.

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


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 "setup.cfg" for a
particular module distribution, it affects only that distribution.

You could override the default “build base” directory and make the
**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 **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 "--help" option, e.g.:

   python setup.py build --help

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

   python setup.py --help

See also the “Reference” section of the “Distributing Python Modules”
manual.


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
"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.


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
"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 "Setup" file, if present, is parsed in order to get a list of
extensions to build.  Each line in a "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 ".c" are assumed
  to be written in C, filenames ending in ".C", ".cc", and ".c++" are
  assumed to be C++, and filenames ending in ".m" or ".mm" are assumed
  to be in Objective C.

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

* *library* is anything ending in ".a" or beginning with "-l" or "-L".

If a particular platform requires a special library on your platform,
you can add it by editing the "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 "libm.a" on your platform, simply
add "-lm" to the line:

   foo foomodule.c -lm

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

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

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

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


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


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 "python25.lib" into the Borland format.  You can do
this as follows:

   coff2omf python25.lib python25_bcpp.lib

The "coff2omf" program comes with the Borland compiler.  The file
"python25.lib" is in the "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. "foo") Distutils checks
first if it finds a library with suffix "_bcpp" (eg. "foo_bcpp.lib")
and then uses this library.  In the case it doesn’t find such a
special library it uses the default name ("foo.lib".) [1]

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 Distutils Configuration Files.)

See also:

  C++Builder Compiler
     Information about the free C++ compiler from Borland, including
     links to the download pages.

  Creating Python Extensions Using Borland’s Free Compiler
     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. [2] 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 [3] 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 Distutils Configuration
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 htt
ps://sourceforge.net/projects/mingw/files/MinGW/Extension/pexports/).

   pexports python25.dll >python25.def

The location of an installed "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
"python25.lib". (Should be the "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.

See also:

  Building Python modules on MS Windows platform with MinGW
     Information about building the required libraries for the MinGW
     environment.

-[ Footnotes ]-

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

[2] Check https://www.sourceware.org/cygwin/ for more information

[3] Then you have no POSIX emulation available, but you also don’t
    need "cygwin1.dll".
