[Python-ideas] PEP 557 Dataclasses evolution: supporting implicit field creation with __init__ signature introspection

smarie sylvain.marie at schneider-electric.com
Tue Jan 16 04:43:45 EST 2018

(my first post seems to have been accepted by google groups but rejected by 
the mailing list. The first repost attempt was completely destructured by 
outlook > Reposting a 2d time… VERY sorry for the inconvenience, I’m not 
familiar with google groups)

Hello there
I recently found out that while I was developing autoclass 
<https://smarie.github.io/python-autoclass/> in complete, naïve ignorance 
of the python evolution process, there was a PEP (557, here 
<https://www.python.org/dev/peps/pep-0557/>) growing, being proposed by 
Eric V. Smith; and now accepted since december. While this PEP is a GREAT 
step forward developer-friendly classes in python, I do not feel that it 
covers all of the use cases I wanted to cover with autoclass. I discussed 
with Eric about the particular one below and he suggested to post it here:

In the case users want to benefit from the code generation while keeping 
custom __init__ methods the current proposal leads to double declaration as 
shown in the pep. 
<https://www.python.org/dev/peps/pep-0557/#custom-init-method> The choice I 
made with autoclass is opposite/complementary: rather than using class 
descriptor to define fields, the developer includes all of them in the 
__init__ method signature, and an optional include/exclude parameter allows 
him/her to further customize the list if need be. See here 

I think that both ways to declare fields actually make sense, depending on 
the use case. Compact pure-data classes will be more compact and readable 
with fields defined as class descriptors. Hybrid data+logic classes with 
custom constructors will be more compact if the fields used in the custom 
constructor signature are not defined twice (also in a descriptor).

Supporting both ways to declare the fields, possibly both styles in the 
same class definition (but does it make sense? not sure), would avoid 
duplication when users want custom constructors. The example in the pep 
<https://www.python.org/dev/peps/pep-0557/#custom-init-method> would become: 


class ArgHolder:


    def __init__(self, *args: Any, **kwargs: Any):



Does that make sense ?

Kind regards


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