Cython debugging is not well documented
The documentation for debugging cython code is not very helpful. Using gdb this way one cannot specify breakpoints in the cython code, but has to instead work with the compiled c-functions which are not human readable.
Instead one should use the cython debugger cygdb. The documentation for it is poor. I have written a small how to about it in the logbook. At the minimum this should be added to the documentation, ideally with some tips on how to use it added.