3. 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, "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 [1]—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:

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

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

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

The basic syntax of the configuration file is simple:

   [command]
   option=value
   ...

where *command* is one of the Distutils commands (e.g. **build_py**,
**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, 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 "--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
   [...]

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 **build_ext** 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:

   [build_ext]
   inplace=1

This will affect all builds of this module distribution, whether or
not you explicitly 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 <gward@python.net>
   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.

See also:

  Syntax of config files in “Installing Python Modules”
     More information on the configuration files is available in the
     manual for system administrators.

-[ Footnotes ]-

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