'Constructor. See class docstring for more info.
Args:
transformer_spec: A single transformer from a parsed bulkloader.yaml.
This assumes that the transformer_spec is valid. It does not
double check things like use_model_on_export requiring model.'

def __init__(self, transformer_spec):

    self._transformer_spec = transformer_spec
    self._create_key = None
    for prop in self._transformer_spec.property_map:
        if (prop.property == '__key__'):
            self._create_key = prop
