Should you decorate dataclass subclasses if not making additional fields

Question

If you don't add any more fields to your subclass is there a need to add the @dataclass decorator to it and would it do anything?

If there is no difference, which is the usual convention?

from dataclasses import dataclass

@dataclass
class AAA:
    x: str
    y: str
    
    ...

# decorate?
class BBB(AAA):
    ...

Answer 1

From documentation, dataclasses source code and experimenting with class instances, I don't see any difference, even if new fields are added to a subclass¹.

If we are talking conventions, then I would advise against decorating a subclass. Class BBB's definition is supposed to say "this class behaves just like AAA, but with these changes" (likely a couple of new methods in your case).

Re-decorating BBB:

• serves no purpose
• violates DRY principle: although it's just one line, but still there's no particular difference between re-decorating and copy-pasting a short method from superclass
• could (potentially) get in the way of changing AAA: you might theoretically switch to another library for dataclassing or decide to use @dataclass with non-default parameters, and that'd require maintaining subclass as well (for no good reason).

---

Update

¹ As Andrius corrected me, if you add new fields in a subclass, you'll encounter the following problem:

>>> class BBB(AAA):
...     z: str
... 
>>> BBB('a', 'b', 'c')
Traceback (most recent call last):
  File "<input>", line 1, in <module>
    BBB('a', 'b', 'c')
TypeError: AAA.__init__() takes 3 positional arguments but 4 were given

Original question specifically says that new fields won't be added, so this is just a correction of my initial answer.