5. 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_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 --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 options that vary from project to project but not depending on the installation system, for example, test needs to know where your test suite is located and what test runner to use; likewise, upload_docs can find HTML documentation in a doc or docs directory, but needs an option to find files in 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 setup.cfg:

[test]
suite = packaging.tests

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

See also

Syntax of configuration files in “Installing Python Projects”

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.