class_docstr: Abstract base class for calculating when futures contracts are the active
  contract.
class_docstr_tok: ["Abstract","base","class","for","calculating","when","futures","contracts","are","the","active","contract","."]
code: def _active_contract(self, oc, front, back, dt):
      raise NotImplementedError
  
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 1 for calling the RollFinder obj's underlying member method code with example usage: obj._active_contract(oc, front, back, dt) without return types