OK, it was a stupid question.

In the experiment of automatic updating binaries, I mistakenly downloaded a cn.pyd to src.

Then I wasted almost a whole day trying to figure out why breakpoints in cn.py do not work trying this and that.

Finally my colleagues suggseted me to check cn._file_ and find out what I reallly imported.

Maybe this experience could save someone's time in the future when they read this.