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1.. _setup-config:
2
3************************************
4Writing the Setup Configuration File
5************************************
6
7Often, it's not possible to write down everything needed to build a distribution
8*a priori*: you may need to get some information from the user, or from the
9user's system, in order to proceed.  As long as that information is fairly
10simple---a list of directories to search for C header files or libraries, for
11example---then providing a configuration file, :file:`setup.cfg`, for users to
12edit is a cheap and easy way to solicit it.  Configuration files also let you
13provide default values for any command option, which the installer can then
14override either on the command-line or by editing the config file.
15
16The setup configuration file is a useful middle-ground between the setup script
17---which, ideally, would be opaque to installers [#]_---and the command-line to
18the setup script, which is outside of your control and entirely up to the
19installer.  In fact, :file:`setup.cfg` (and any other Distutils configuration
20files present on the target system) are processed after the contents of the
21setup script, but before the command-line.  This has  several useful
22consequences:
23
24.. % (If you have more advanced needs, such as determining which extensions
25.. % to build based on what capabilities are present on the target system,
26.. % then you need the Distutils ``auto-configuration'' facility.  This
27.. % started to appear in Distutils 0.9 but, as of this writing, isn't mature
28.. % or stable enough yet for real-world use.)
29
30* installers can override some of what you put in :file:`setup.py` by editing
31  :file:`setup.cfg`
32
33* you can provide non-standard defaults for options that are not easily set in
34  :file:`setup.py`
35
36* installers can override anything in :file:`setup.cfg` using the command-line
37  options to :file:`setup.py`
38
39The basic syntax of the configuration file is simple::
40
41   [command]
42   option=value
43   ...
44
45where *command* is one of the Distutils commands (e.g. :command:`build_py`,
46:command:`install`), and *option* is one of the options that command supports.
47Any number of options can be supplied for each command, and any number of
48command sections can be included in the file.  Blank lines are ignored, as are
49comments, which run from a ``'#'`` character until the end of the line.  Long
50option values can be split across multiple lines simply by indenting the
51continuation lines.
52
53You can find out the list of options supported by a particular command with the
54universal :option:`!--help` option, e.g. ::
55
56   > python setup.py --help build_ext
57   [...]
58   Options for 'build_ext' command:
59     --build-lib (-b)     directory for compiled extension modules
60     --build-temp (-t)    directory for temporary files (build by-products)
61     --inplace (-i)       ignore build-lib and put compiled extensions into the
62                          source directory alongside your pure Python modules
63     --include-dirs (-I)  list of directories to search for header files
64     --define (-D)        C preprocessor macros to define
65     --undef (-U)         C preprocessor macros to undefine
66     --swig-opts          list of SWIG command line options
67   [...]
68
69Note that an option spelled :option:`!--foo-bar` on the command-line  is spelled
70``foo_bar`` in configuration files.
71
72.. _distutils-build-ext-inplace:
73
74For example, say you want your extensions to be built "in-place"---that is, you
75have an extension :mod:`pkg.ext`, and you want the compiled extension file
76(:file:`ext.so` on Unix, say) to be put in the same source directory as your
77pure Python modules :mod:`pkg.mod1` and :mod:`pkg.mod2`.  You can always use the
78:option:`!--inplace` option on the command-line to ensure this::
79
80   python setup.py build_ext --inplace
81
82But this requires that you always specify the :command:`build_ext` command
83explicitly, and remember to provide :option:`!--inplace`. An easier way is to
84"set and forget" this option, by encoding it in :file:`setup.cfg`, the
85configuration file for this distribution::
86
87   [build_ext]
88   inplace=1
89
90This will affect all builds of this module distribution, whether or not you
91explicitly specify :command:`build_ext`.  If you include :file:`setup.cfg` in
92your source distribution, it will also affect end-user builds---which is
93probably a bad idea for this option, since always building extensions in-place
94would break installation of the module distribution.  In certain peculiar cases,
95though, modules are built right in their installation directory, so this is
96conceivably a useful ability.  (Distributing extensions that expect to be built
97in their installation directory is almost always a bad idea, though.)
98
99Another example: certain commands take a lot of options that don't change from
100run to run; for example, :command:`bdist_rpm` needs to know everything required
101to generate a "spec" file for creating an RPM distribution.  Some of this
102information comes from the setup script, and some is automatically generated by
103the Distutils (such as the list of files installed).  But some of it has to be
104supplied as options to :command:`bdist_rpm`, which would be very tedious to do
105on the command-line for every run.  Hence, here is a snippet from the Distutils'
106own :file:`setup.cfg`::
107
108   [bdist_rpm]
109   release = 1
110   packager = Greg Ward <gward@python.net>
111   doc_files = CHANGES.txt
112               README.txt
113               USAGE.txt
114               doc/
115               examples/
116
117Note that the ``doc_files`` option is simply a whitespace-separated string
118split across multiple lines for readability.
119
120
121.. seealso::
122
123   :ref:`inst-config-syntax` in "Installing Python Modules"
124      More information on the configuration files is available in the manual for
125      system administrators.
126
127
128.. rubric:: Footnotes
129
130.. [#] This ideal probably won't be achieved until auto-configuration is fully
131   supported by the Distutils.
132
133