'Transform a single export-side field value to dict property.
Args:
value: Value from the entity or model instance.
property_name: Name of the value in the entity or model instance.
transform: Transform property, either an ExportEntry or PropertyEntry
export_dict: output dictionary.
bulkload_state: Passed bulkload state.
Raises:
ErrorOnTransform, encapsulating an error encountered during the transform.'

def __prop_to_dict(self, value, property_name, transform, export_dict, bulkload_state):

    if transform.export_transform:
        try:
            if transform.export_transform.supports_bulkload_state:
                transformed_value = transform.export_transform(value, bulkload_state=bulkload_state)
            else:
                transformed_value = transform.export_transform(value)
        except Exception as err:
            raise bulkloader_errors.ErrorOnTransform(('Error    on    transform.    Property:    %s    External    Name:    %s.    Code:    %s    Details:    %s' % (property_name, transform.external_name, transform.export_transform, err)))
    else:
        transformed_value = default_export_transform(value)
    export_dict[transform.external_name] = transformed_value
