[Python-Dev] PEP 557: Data Classes (original) (raw)

Mike Miller python-dev at mgmiller.net
Thu Oct 12 04:20:30 EDT 2017

On 2017-10-12 00:36, Stéfane Fermigier wrote:

"An object that is not defined by its attributes, but rather by a thread of continuity and its identity." (from https://en.wikipedia.org/wiki/Domain-drivendesign#Buildingblocks)

Not sure I follow all this, but Python objects do have identities once instantiated. e.g. >>> id('')

See also the more general Wikipedia definition "An entity is something that exists as itself, as a subject or as an object, actually or potentially, concretely or abstractly, physically or not." (https://en.wikipedia.org/wiki/Entity).

In the context of DDD, entities are usually opposed to value objects: "An object that contains attributes but has no conceptual identity. They should be treated as immutable.". (https://en.wikipedia.org/wiki/Domain-drivendesign#Buildingblocks) Attrs, and by extension the dataclass proposal (I guess), provide some support for both: - Providing support for quickly constructing immutable objects from a bag of attributes, and providing equality based on those attributes, it helps implement Value Objects (not sure much more is needed actually) - By supporting equality based on some "primary key", it will also help with maintaining the concept of "equality" in entities.

I don't believe either module particularly supports or restricts immutability?

-Mike

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