class_docstr: The CalendarRollFinder calculates contract rolls based purely on the
  contract's auto close date.
class_docstr_tok: ["The","CalendarRollFinder","calculates","contract","rolls","based","purely","on","the","contract","'s","auto","close","date","."]
code: def _active_contract(self, oc, front, back, dt):
      contract = oc.sid_to_contract[front].contract
      auto_close_date = contract.auto_close_date
      auto_closed = dt >= auto_close_date
      return back if auto_closed else front
  
file_path: zipline/zipline/assets/roll_finder.py
imports: from abc import ABCMeta, abstractmethod
  from six import with_metaclass
label_desc: Use image node_id 2 for calling the CalendarRollFinder obj's underlying member method code with example usage: obj._active_contract(oc, front, back, dt) and returns: unknown