Additional Python Guidelines

When byte compiling a .py file, python embeds a magic number in the byte compiled files that correspond to the runtime. Files in %{python?_sitelib} and %{python?_sitearch} MUST correspond to the runtime for which they were built. For instance, a pure Python module compiled for the 3.4 runtime MUST be below %{_usr}/lib/python3.4/site-packages

The brp-python-bytecompile script tries to figure this out for you. The script determines which interpreter to use when byte compiling the module by checking what directory the file is installed in. If it’s /usr/lib{,64}/pythonX.Y, then pythonX.Y is used to byte compile the module. If pythonX.Y is not installed, then an error is returned and the rpm build process will exit on an error so remember to BuildRequire the proper python package.

If you have *.py files outside of the /usr/lib(64)?/pythonX.Y/ directories and you require those files to be byte compiled (e.g. it’s an importable Python module) you MUST compile them explicitly using the %py_byte_compile macro. Note that not all Python files are importable Python modules; when in doubt, grep the sources for the appropriate import statement.

An example for a package that has both Python versions:

The %py_byte_compile macro takes two arguments. The first is the python interpreter to use for byte compiling. The second is a file or directory to byte compile. If the second argument is a directory, the macro will recursively byte compile any *.py file in the directory.

The script interpreter defined in %{__python} is used to compile the modules outside of /usr/lib(64)?/pythonX.Y/ directories. This defaults to /usr/bin/python (that’s Python 2.6 or on EPEL 6 and 2.7 on EPEL 7). If you need to compile the modules for python3, set it to /usr/bin/python3 instead:

Doing this is useful when you have a python3 application that’s installing a private module into its own directory. For instance, if the foobar application installs a module for use only by the command line application in %{_datadir}/foobar. Since these files are not in one of the python3 library paths (i.e., /usr/lib/python3.6) you have to override %{__python} to tell brp-python-bytecompile to use the python3 interpreter for byte compiling.

These settings are enough to properly byte compile any package that builds Python modules in %{python?_sitelib} or %{python?_sitearch} or builds for only a single Python interpreter. However, if the application you’re packaging needs to build with both python2 and python3 and install into a private module directory (perhaps because it provides one utility written in python2 and a second utility written in python3) then you need to do this manually. Here’s a sample spec file snippet that shows what to do:

Note that this does disable the compilation of files in /usr/lib(64)?/pythonX.Y/.

This subsection only applies to Fedora ⇐ 40, ELN, and EPEL. In later Fedora releases, this is implemented automatically.

For two Python files with the exact same content and metadata, byte compilation might produce different results. The resulting .pyc files are functionally identical but are not bit-by-bit identical. In most cases, internal Python reference counter is here to blame because it might have a different internal state during each byte compilation. If you want a deeper explanation, take a look at this Bugzilla comment.

This inconvenience might cause a problem in Koji where noarch packages built as a part of an arch build might be rejected because they have different content.

To work around this issue, BuildRequire marshalparser BuildRequires: /usr/bin/marshalparser (a tool that makes .pyc files more reproducible) and instruct it to process the .pyc files in certain paths by setting the %py_reproducible_pyc_path macro:

With that setting, marshalparser recursively finds all byte-compiled Python files in %{buildroot}%{_datadir}/llamafarm/plugins and attempts to fix them. This happens at the very end of the build process when all previous byte compilation steps are finished. If marshalparser cannot parse some of the cache files, the build fails.