def __init__(self, p_range, ns_range, query_spec):
    self._property_range = p_range
    self._ns_range = ns_range
    self._query_spec = query_spec
    self._cursor = None
    self._query = None
L{DeferredContext.addCallback} returns the L{DeferredContext}.
dataset/ETHPy150Open GoogleCloudPlatform/appengine-mapreduce/python/src/mapreduce/datastore_range_iterators.py/_PropertyRangeModelIterator.__init__--crossover with--dataset/ETHPy150Open ClusterHQ/eliot/eliot/tests/test_twisted.py/DeferredContextTests.test_addCallbackReturnsSelf