[Python-Dev] PEP 557: Data Classes

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 

> - 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 

> - 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(default_factory=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.

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.


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