[Python-Dev] Second post: PEP 557, Data Classes

Sebastian Rittau srittau at rittau.biz
Mon Nov 27 07:26:48 EST 2017

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.
  * 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.
  * 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.

I hope I did not misunderstood the purpose of InitVar.

  - Sebastian

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