[Python-Dev] PEP 557: Data Classes

Steve Holden steve at holdenweb.com
Thu Oct 12 06:20:14 EDT 2017

On Thu, Oct 12, 2017 at 9:20 AM, Mike Miller <python-dev at mgmiller.net>

> 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-driven_design#Building_blocks)
> Not sure I follow all this, but Python objects do have identities once
> instantiated.  e.g. >>> id('')
> ​It seems to me that the quoted document is attempting to make a
distinction ​similar to the one between classes (entities) and instances
(value objects). The reason I liked "row" as a name is because it
resembles  "vector" and hence is loosely assocaited with the concept of a
tuple as well as being familiar to database users. In fact the answer to a
relational query was, I believe, originally formally defined as a set of

​Sometimes one can simply be too hifalutin' [
http://www.dictionary.com/browse/hifalutin]​, and language that attempts to
be precise obscures meaning to the less specialised reader.

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-driven_design#Building_blocks)
>> 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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