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+.. _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.