'Handler for call events.
Adds information about who called who to the self._callers dict.'

def globaltrace_trackcallers(self, frame, why, arg):

    if (why == 'call'):
        this_func = self.file_module_function_of(frame)
        parent_func = self.file_module_function_of(frame.f_back)
        self._callers[(parent_func, this_func)] = 1
