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

Eric V. Smith eric at trueblade.com
Fri Sep 8 18:40:27 EDT 2017

On 9/8/17 3:20 PM, Mike Miller wrote:

On 2017-09-08 07:57, Eric V. Smith wrote: I've written a PEP for… Apologies for the following list dumb questions and bikesheds:

- 'Classes can be thought of as "mutable namedtuples with defaults".' - A C/C++ (struct)ure sounds like a simpler description that many more would understand.

Yes, other people have pointed out that this might not be the best "elevator pitch" example. I'm thinking about it.

- dataclass name:

- class, redundant - data, good but very common - struct, used? - Record? (best I could come up with)

There was a bunch of discussions on this. We're delaying the name bikeshedding for later (and maybe never).

- Source needs blanks between functions, hard to read.

It's supposed to be hard to read! You're just supposed to think "am I glad I don't have to read or write that". But I'll look at it.

- Are types required?

Annotations are required, the typing module is not.

Maybe an example or two with Any?

I'd rather leave it like it is: typing is referenced only once, for ClassVar.

- Intro discounts inheritance and metaclasses as "potentially interfering", but unclear why that would be the case. Inheritance is easy to override, metaclasses not sure?

I don't really want to get in to the history of why people don't like inheritance, single and multi. Or how metaclass magic can make life difficult. I just want to point out that Data Classes don't interfere at all.

- Perhaps mention ORMs/metaclass approach, as prior art:

https://docs.djangoproject.com/en/dev/topics/db/models/ - Perhaps mention Kivy Properties, as prior art: https://kivy.org/docs/api-kivy.properties.html

Those are all good. Thanks.

- For mutable default values:

@dataclass class C: x: list # = field(defaultfactory=list) Could it detect list as a mutable class "type" and set it as a factory automatically? The PEP/bug #3 mentions using copy, but that's not exactly what I'm asking above.

The problem is: how do you know what's a mutable type? There's no general way to know. The behavior in the PEP is just mean to stop the worst of it.

I guess we could have an option that says: call the type to create a new, empty instance.

@dataclass class C: x: list = field(default_type_is_factory=True)

Thanks for the critical reading and your comments. I'm going to push a new version early next week, when I get back from traveling.

Eric.

More information about the Python-Dev mailing list