[Python-Dev] Concerns about method overriding and subclassing with dataclasses (original) (raw)
Antoine Pitrou solipsis at pitrou.net
Fri Dec 29 05:45:21 EST 2017
- Previous message (by thread): [Python-Dev] Concerns about method overriding and subclassing with dataclasses
- Next message (by thread): [Python-Dev] Concerns about method overriding and subclassing with dataclasses
- Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]
On Fri, 29 Dec 2017 02:23:56 -0800 Ethan Smith <ethan at ethanhs.me> wrote:
In a few cases I want to override the repr of the AST nodes. I wrote a repr and ran the code but lo and behold I got a type error. I couldn't override it. I quickly learned that one needs to pass a keyword to the dataclass decorator to tell it not to auto generate methods you override. I have two usability concerns with the current implementation. I emailed Eric about the first, and he said I should ask for thoughts here. The second I found after a couple of days sitting on this message. The first is that needing both a keyword and method is duplicative and unnecessary. Eric agreed it was a hassle, but felt it was justified considering someone may accidentally override a dataclass method. I disagree with this point of view as dataclasses are billed as providing automatic methods. Overriding via method definition is very natural and idiomatic.
Agreed. We shouldn't take magic too far just for the sake of protecting users against their own (alleged) mistakes. And I'm not sure how you "accidentally" override a dataclass method (if I'm implementing a repr I'm doing so deliberately :-)).
The second concern, which I came across more recently, is if I have a base class, and dataclasses inherit from this base class, inherited repr & co are silently overridden by dataclass. This is both unexpected, and also means I need to pass a repr=False to each subclass' decorator to get correct behavior, which somewhat defeats the utility of subclassing. Im not as sure a whole lot can be done about this though.
Agreed as well. If I make the effort of having a dataclass inherit from a base class, I probably don't want the base class' methods to be silently overriden by machine-generated methods. Of course, that can be worked around by using multiple inheritance, you just need to be careful and add a small amount of class definition boilerplate.
I would expect dataclass parameters such as repr to be tri-state:
- repr=None (the default): only provide a machine-generated implementation if none is already defined (either on a base class or in the dataclass namespace... ignoring runtime-provided defaults such as object.repr)
- repr=False: never provide a machine-generated implementation
- repr=True: always provide a machine-generated implementation, even overriding a previous user-defined implementation
Regards
Antoine.
- Previous message (by thread): [Python-Dev] Concerns about method overriding and subclassing with dataclasses
- Next message (by thread): [Python-Dev] Concerns about method overriding and subclassing with dataclasses
- Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]