'Returns the collected value of the xBlock field for the
requested block for the requested field_name; returns default if
not found.
Arguments:
usage_key (UsageKey) - Usage key of the block whose xBlock
field is requested.
field_name (string) - The name of the field that is
requested.
default (any type) - The value to return if a field value is
not found.'

def get_xblock_field(self, usage_key, field_name, default=None):

    block_data = self._block_data_map.get(usage_key)
    return (getattr(block_data, field_name, default) if block_data else default)
