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, 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.
(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.)
The setup configuration file is a useful middle-ground between the setup script--which, ideally, would be opaque to installers2--and the command-line to the setup script, which is outside of your control and entirely up to the installer. In fact, 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:
The basic syntax of the configuration file is simple:
[command] option=value ...
where command is one of the Distutils commands (e.g.
install), 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 (from a
"#" character to end-of-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 --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 [...]
Or consult section 7 of this document (the command reference).
Note that an option spelled --foo-bar on the command-line is spelled foo_bar in configuration files.
For example, say you want your extensions to be built ``in-place''--that is, you have an extension pkg.ext, and you want the compiled extension file (ext.so on Unix, say) to be put in the same source directory as your pure Python modules pkg.mod1 and pkg.mod2. You can always use the --inplace option on the command-line to ensure this:
python setup.py build_ext --inplace
But this requires that you always specify the
command explicitly, and remember to provide --inplace.
An easier way is to ``set and forget'' this option, by encoding it in
setup.cfg, the configuration file for this distribution:
This will affect all builds of this module distribution, whether or not
you explcitly specify
build_ext. If you include
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 a lot of options that don't
change from run-to-run; for example,
bdist_rpm needs to know
everything required to generate a ``spec'' file for creating an RPM
distribution. Some of this information comes from the setup script, and
some is automatically generated by the Distutils (such as the list of
files installed). But some of it has to be supplied as options to
bdist_rpm, which would be very tedious to do on the
command-line for every run. Hence, here is a snippet from the
Distutils' own setup.cfg:
[bdist_rpm] release = 1 packager = Greg Ward <firstname.lastname@example.org> doc_files = CHANGES.txt README.txt USAGE.txt doc/ examples/
Note that the doc_files option is simply a whitespace-separated string split across multiple lines for readability.