Use of _slots_ for known variables is supposed to help reduce memory usage in classes and to some extent speed up execution.

"By default, instances of both old and new-style classes have a dictionary for attribute storage. This wastes space for objects having very few instance variables. The space consumption can become acute when creating large numbers of instances."

So this may be an issue for users who load large numbers of files.

See https://docs.python.org/2/reference/datamodel.html#slots