'Returns the value associated with the given key for the given
transformer for the block identified by the given usage_key;
returns default if not found.
Arguments:
usage_key (UsageKey) - Usage key of the block whose
transformer data is requested.
transformer (BlockStructureTransformer) - The transformer
whose dictionary data is requested.
key (string) - A dictionary key to the transformer\'s data
that is requested.
default (any type) - The value to return if a dictionary
entry is not found.'

def get_transformer_block_field(self, usage_key, transformer, key, default=None):

    try:
        transformer_data = self.get_transformer_block_data(usage_key, transformer)
    except KeyError:
        return default
    return getattr(transformer_data, key, default)
