code: def __init__(self, lookBack, lookForward):
      assert lookBack > 0
      assert lookForward > 0
      self.__lookBack = lookBack
      self.__lookForward = lookForward
      self.__values = np.empty((lookBack + lookForward + 1))
      self.__values[:] = np.NAN
  
imports: import numpy
  import matplotlib.pyplot
  from six.moves import xrange
  from pyalgotrade.technical import roc
  from pyalgotrade import dispatcher
label_desc: Use image node_id 1 to create a new Event object from inherited base classes: object with example: obj = Event(lookBack, lookForward)