We are about to see our first Python code in the executable spec. For specification purposes, these Container data structures are just Python data classes that are derived from the base SSZ
SSZ is the serialisation and Merkleization format used everywhere in Ethereum 2.0. It is not self-describing, so you need to know ahead of time what you are unpacking when deserialising. SSZ deals with basic types and composite types. Classes like the below are handled as SSZ containers, a composite type defined as an "ordered heterogeneous collection of values".
Client implementations in different languages will obviously use their own paradigms to represent these data structures.
Two notes directly from the spec:
- The definitions are ordered topologically to facilitate execution of the spec.
- Fields missing in container instantiations default to their zero value.