'Args:
store_builders: A list of modulestore builder objects. These will be instantiated, in order,
as the backing stores for the MixedModuleStore.
mappings: Any course mappings to pass to the MixedModuleStore on instantiation.'

def __init__(self, store_builders, mappings=None):

    self.store_builders = store_builders
    self.mappings = (mappings or {})
    self.mixed_modulestore = None
