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************************************
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Writing the Setup Configuration File
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************************************
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Often, it's not possible to write down everything needed to build a distribution
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*a priori*: you may need to get some information from the user, or from the
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user's system, in order to proceed. As long as that information is fairly
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simple---a list of directories to search for C header files or libraries, for
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example---then providing a configuration file, :file:`setup.cfg`, for users to
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edit is a cheap and easy way to solicit it. Configuration files also let you
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provide default values for any command option, which the installer can then
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override either on the command-line or by editing the config file.
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The setup configuration file is a useful middle-ground between the setup script
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---which, ideally, would be opaque to installers [#]_---and the command-line to
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the setup script, which is outside of your control and entirely up to the
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installer. In fact, :file:`setup.cfg` (and any other Distutils configuration
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files present on the target system) are processed after the contents of the
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setup script, but before the command-line. This has several useful
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.. % (If you have more advanced needs, such as determining which extensions
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.. % to build based on what capabilities are present on the target system,
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.. % then you need the Distutils ``auto-configuration'' facility. This
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.. % started to appear in Distutils 0.9 but, as of this writing, isn't mature
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.. % or stable enough yet for real-world use.)
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* installers can override some of what you put in :file:`setup.py` by editing
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* you can provide non-standard defaults for options that are not easily set in
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* installers can override anything in :file:`setup.cfg` using the command-line
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options to :file:`setup.py`
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The basic syntax of the configuration file is simple::
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where *command* is one of the Distutils commands (e.g. :command:`build_py`,
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:command:`install`), and *option* is one of the options that command supports.
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Any number of options can be supplied for each command, and any number of
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command sections can be included in the file. Blank lines are ignored, as are
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comments, which run from a ``'#'`` character until the end of the line. Long
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option values can be split across multiple lines simply by indenting the
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You can find out the list of options supported by a particular command with the
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universal :option:`--help` option, e.g. ::
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> python setup.py --help build_ext
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Options for 'build_ext' command:
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--build-lib (-b) directory for compiled extension modules
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--build-temp (-t) directory for temporary files (build by-products)
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--inplace (-i) ignore build-lib and put compiled extensions into the
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source directory alongside your pure Python modules
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--include-dirs (-I) list of directories to search for header files
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--define (-D) C preprocessor macros to define
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--undef (-U) C preprocessor macros to undefine
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--swig-opts list of SWIG command line options
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Note that an option spelled :option:`--foo-bar` on the command-line is spelled
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:option:`foo_bar` in configuration files.
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For example, say you want your extensions to be built "in-place"---that is, you
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have an extension :mod:`pkg.ext`, and you want the compiled extension file
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(:file:`ext.so` on Unix, say) to be put in the same source directory as your
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pure Python modules :mod:`pkg.mod1` and :mod:`pkg.mod2`. You can always use the
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:option:`--inplace` option on the command-line to ensure this::
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python setup.py build_ext --inplace
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But this requires that you always specify the :command:`build_ext` command
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explicitly, and remember to provide :option:`--inplace`. An easier way is to
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"set and forget" this option, by encoding it in :file:`setup.cfg`, the
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configuration file for this distribution::
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This will affect all builds of this module distribution, whether or not you
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explicitly specify :command:`build_ext`. If you include :file:`setup.cfg` in
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your source distribution, it will also affect end-user builds---which is
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probably a bad idea for this option, since always building extensions in-place
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would break installation of the module distribution. In certain peculiar cases,
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though, modules are built right in their installation directory, so this is
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conceivably a useful ability. (Distributing extensions that expect to be built
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in their installation directory is almost always a bad idea, though.)
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Another example: certain commands take a lot of options that don't change from
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run to run; for example, :command:`bdist_rpm` needs to know everything required
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to generate a "spec" file for creating an RPM distribution. Some of this
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information comes from the setup script, and some is automatically generated by
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the Distutils (such as the list of files installed). But some of it has to be
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supplied as options to :command:`bdist_rpm`, which would be very tedious to do
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on the command-line for every run. Hence, here is a snippet from the Distutils'
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own :file:`setup.cfg`::
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packager = Greg Ward <gward@python.net>
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doc_files = CHANGES.txt
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Note that the :option:`doc_files` option is simply a whitespace-separated string
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split across multiple lines for readability.
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:ref:`inst-config-syntax` in "Installing Python Modules"
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More information on the configuration files is available in the manual for
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system administrators.
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.. rubric:: Footnotes
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.. [#] This ideal probably won't be achieved until auto-configuration is fully
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supported by the Distutils.