I think it would be useful to write 1-2 sentences about the problem with inheritance -- in that case you pretty much have to use a metaclass, and the use of a metaclass makes life harder for people who want to use their own metaclass (since metaclasses don't combine without some manual intervention).

On Fri, Sep 8, 2017 at 3:40 PM, Eric V. Smith <eric@trueblade.com> wrote:
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(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.

@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.


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