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

[Sebastian Rittau]

On 25.11.2017 22:06, Eric V. Smith wrote:
> The updated version should show up at 
> https://www.python.org/dev/peps/pep-0557/ shortly.

This PEP looks very promising and will make my life quite a bit easier, 
since we are using a pattern involving data classes. Currently, we write 
the constructor by hand.

> 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?

  - Sebastian