[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:
> - Perhaps mention Kivy Properties, as prior art:
Those are all good. Thanks.
> - For mutable default values:
> class C:
> x: list # = field(default_factory=list)
> Could it detect list as a mutable class "type" and set it as a factory
> The PEP/bug #3 mentions using copy, but that's not exactly what I'm
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.
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.
More information about the Python-Dev