`ContainerSet`'s `_representation` attribute is not set upon initialisation (possibly related to unused `data_specs` constructor parameter)

[thehrh]

Trying to access the representation attribute of a newly instantiated ContainerSet results in:

AttributeError: 'ContainerSet' object has no attribute '_representation'. Did you mean: 'representation'?

because self._representation is not assigned a default value . This does not seem like intended behaviour.

Possible solution: replace the constructor parameter data_specs (which is unused right now, bug!) with representation, and call the representation setter with that argument?