On 11/27/2017 7:26 AM, Sebastian Rittau wrote:
On 27.11.2017 12:01, Sebastian Rittau wrote:
The major changes from the previous version are:
- Add InitVar to specify initialize-only fields.
This is the only feature that does not sit right with me. It looks
very obscure and "hacky". From what I understand, we are supposed to
use the field syntax to define constructor arguments. I'd argue that
the name "initialize-only fields" is a misnomer, which only hides the
fact that this has nothing to do with fields at all. Couldn't
dataclassses just pass *args and **kwargs to __post_init__()? Type
checkers need to be special-cases for InitVar anyway, couldn't they
instead be special cased to look at __post_init__ argument types?
I am sorry for the double post, but I thought a bit more about why this
does not right with me:
* As written above, InitVars look like fields, but aren't.
Same as with ClassVars, which is where the inspiration came from.
* InitVar goes against the established way to pass through arguments,
*args and **kwargs. While type checking those is an unsolved
problem, from what I understand, I don't think we should introduce a
second way just for dataclasses.
* InitVars look like a way to satisfy the type checker without
providing any benefit to the programmer. Even when I'm not
interested in type checking, I have to declare init vars.
Same as with ClassVars, if you're using them. And that's not just a
dataclasses thing, although dataclasses is the first place I know of
where it would change the code semantics.
* InitVars force me to repeat myself. I have the InitVar declaration
and then I have the repeat myself in the signature of
__post_init__(). This has all the usual problems of repeated code.
There was some discussion about this starting at
https://github.com/ericvsmith/dataclasses/issues/17#issuecomment-345529717,
in particular a few messages down where we discussed what would be
repeated, and what mypy would be able to deduce. You won't need to
repeat the type declaration.
I hope I did not misunderstood the purpose of InitVar.
I think you understand it perfectly well, especially with the "context"
discussion. Thanks for bringing it up.
Eric.
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