Sometimes is it impossible to build both versions from the same source directory. Most often this happens when sources are "translated" to python3 in the source directory and made incompatible with python2 in the process. This used to be fairly common, but is fortunately much rarer now. Some things to look for are:

Our method in building from the same code to make the two separate modules is to keep each build as independent as possible. To do that, we copy the source tree to python3 so that the python 2 sources are entirely independent from the python 3 sources.

Some things to watch out for:

rpmbuild resets the directory at the end of each phase, so you don’t need to restore the directory at the end of %prep.

You’ll notice that the %build, %install, and %check sections again follow a pattern similar to the previous example. They switch to the python2 directory and do the normal steps for building the python2 module, and then switch to the python3 directory and run the same steps for python3. The usage of pushd/popd commands will ensure that the directories are logged.

Sometimes, upstream hasn’t integrated running 2to3 on the code into their build scripts but they support making a python3 module from it if you manually run 2to3 on the source. This is the case when it’s documented on the upstream’s website, in a file in the tarball, or even when email with the module’s author has instructions for building a python3 module from the python2 source and the authors are willing to support the result. In these cases it’s usually just a matter of the upstream not having written the build script that can turn the python2 source into python3. When this happens you can run 2to3 from the spec file. Once you have it working, you can also help upstream integrate it into their build scripts which will benefit everyone in the long term.

You should usually follow upstream’s directions on how to run 2to3 and build the python3 module in these cases but there’s a few things you should check to make sure upstream is doing it correctly.

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 needs to 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 following these steps:

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:

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.