.. _packaging-setup-config:

************************************
Writing the Setup Configuration File
************************************

Often, it's not possible to write down everything needed to build a distribution
*a priori*: you may need to get some information from the user, or from the
user's system, in order to proceed.  As long as that information is fairly
simple---a list of directories to search for C header files or libraries, for
example---then providing a configuration file, :file:`setup.cfg`, for users to
edit is a cheap and easy way to solicit it.  Configuration files also let you
provide default values for any command option, which the installer can then
override either on the command line or by editing the config file.

The setup configuration file is a useful middle-ground between the setup script
---which, ideally, would be opaque to installers [#]_---and the command line to
the setup script, which is outside of your control and entirely up to the
installer.  In fact, :file:`setup.cfg` (and any other Distutils configuration
files present on the target system) are processed after the contents of the
setup script, but before the command line.  This has  several useful
consequences:

.. If you have more advanced needs, such as determining which extensions to
   build based on what capabilities are present on the target system, then you
   need the Distutils auto-configuration facility.  This started to appear in
   Distutils 0.9 but, as of this writing, isn't mature or stable enough yet
   for real-world use.

* installers can override some of what you put in :file:`setup.py` by editing
  :file:`setup.cfg`

* you can provide non-standard defaults for options that are not easily set in
  :file:`setup.py`

* installers can override anything in :file:`setup.cfg` using the command-line
  options to :file:`setup.py`

The basic syntax of the configuration file is simple::

   [command]
   option = value
   ...

where *command* is one of the Distutils commands (e.g. :command:`build_py`,
:command:`install_dist`), and *option* is one of the options that command supports.
Any number of options can be supplied for each command, and any number of
command sections can be included in the file.  Blank lines are ignored, as are
comments, which run from a ``'#'`` character until the end of the line.  Long
option values can be split across multiple lines simply by indenting the
continuation lines.

You can find out the list of options supported by a particular command with the
universal :option:`--help` option, e.g. ::

   > python setup.py --help build_ext
   [...]
   Options for 'build_ext' command:
     --build-lib (-b)     directory for compiled extension modules
     --build-temp (-t)    directory for temporary files (build by-products)
     --inplace (-i)       ignore build-lib and put compiled extensions into the
                          source directory alongside your pure Python modules
     --include-dirs (-I)  list of directories to search for header files
     --define (-D)        C preprocessor macros to define
     --undef (-U)         C preprocessor macros to undefine
     --swig-opts          list of SWIG command-line options
   [...]

.. XXX do we want to support ``setup.py --help metadata``?

Note that an option spelled :option:`--foo-bar` on the command line  is spelled
:option:`foo_bar` in configuration files.

For example, say you want your extensions to be built "in-place"---that is, you
have an extension :mod:`pkg.ext`, and you want the compiled extension file
(:file:`ext.so` on Unix, say) to be put in the same source directory as your
pure Python modules :mod:`pkg.mod1` and :mod:`pkg.mod2`.  You can always use the
:option:`--inplace` option on the command line to ensure this::

   python setup.py build_ext --inplace

But this requires that you always specify the :command:`build_ext` command
explicitly, and remember to provide :option:`--inplace`. An easier way is to
"set and forget" this option, by encoding it in :file:`setup.cfg`, the
configuration file for this distribution::

   [build_ext]
   inplace = 1

This will affect all builds of this module distribution, whether or not you
explicitly specify :command:`build_ext`.  If you include :file:`setup.cfg` in
your source distribution, it will also affect end-user builds---which is
probably a bad idea for this option, since always building extensions in-place
would break installation of the module distribution.  In certain peculiar cases,
though, modules are built right in their installation directory, so this is
conceivably a useful ability.  (Distributing extensions that expect to be built
in their installation directory is almost always a bad idea, though.)

Another example: certain commands take options that vary from project to
project but not depending on the installation system, for example,
:command:`test` needs to know where your test suite is located and what test
runner to use; likewise, :command:`upload_docs` can find HTML documentation in
a :file:`doc` or :file:`docs` directory, but needs an option to find files in
:file:`docs/build/html`.  Instead of having to type out these options each
time you want to run the command, you can put them in the project's
:file:`setup.cfg`::

   [test]
   suite = packaging.tests

   [upload_docs]
   upload-dir = docs/build/html


.. seealso::

   :ref:`packaging-config-syntax` in "Installing Python Projects"
      More information on the configuration files is available in the manual for
      system administrators.


.. rubric:: Footnotes

.. [#] This ideal probably won't be achieved until auto-configuration is fully
   supported by the Distutils.