3.2.1.3. Union

Internally constructing unions in the library poses challenges. The current implementation uses the predefined behavior of the Sequence class for union types. It selects the field with the greatest length as its representational size. Unions, much like BitFields, must store a static size.

In essence, they behave similarly to C unions. A traditional function hook will be installed on the model to capture field assignments. What that means will be illustrated by the following example:

>>> @union
... class Format:
...     foo: uint16
...     bar: uint32
...     baz: boolean
...
>>> obj = Format()      # union does not need any values

Right now, all attributes store the default value (None). If we assign a new value to one field, it will be applied to all others. Hence,

>>> obj.bar = 0xFF00FF00

will result in

>>> obj
Format(foo=65280, bar=4278255360, baz=False)

Implementation Detail

The constructor is the only place where there is no synchronization between fields. Additionally, the current implementation may produce some overhead, because every refresh will first pack the new value and then executes unpack on all other fields.