Hi everyone, Cython 0.29.5 was released with an important bug fix for extension types that have "cpdef" methods. If they get subclassed by Python classes that do not define additional attributes (or anything that would require them to have an instance `__dict__`), then calling the cpdef-methods repeatedly triggers a crash in the optimised code in 0.29.x that checks for subclass overrides. The issue was reported here: https://github.com/cython/cython/issues/2823 It's a new issue in the 0.29.x series and only appears under Python 3.6 and later, so older versions were not affected. Everyone using 0.29.x with cpdef methods is encouraged to upgrade. Stefan