code: def _pre_run(self, pre_run=None):
          " Called during `run`. "
          if pre_run:
              pre_run()
  
          # Process registered "pre_run_callables" and clear list.
          for c in self.pre_run_callables:
              c()
          del self.pre_run_callables[:f _pre_run(self, pre_run=None):
          " Called during `run`. "
          if pre_run:
              pre_run()
  
          # Process registered "pre_run_callables" and clear list.
          for c in self.pre_run_callables:
              c()
          del self.pre_run_callables[:]
docs: Called during `run`.