Python dataclasses, inheritance and alternate class constructors

Question

In Python, I am using dataclass with the following class hierarchy:

from dataclasses import dataclass

@dataclass
class Foo:
  id: str

  @classmethod
  def fromRaw(cls, raw: dict[str, str]) -> 'Foo':
    return Foo(raw['id'])

@dataclass
class Bar(Foo):
  name: str

  @classmethod
  def fromRaw(cls, raw: dict[str, str]) -> 'Bar':
    return Bar(raw['id'], raw['name'])

However, it feels a bit double to instantiate all Foo members manually again in Bar::fromRaw().

Is there a better way to re-use the ancestor's alternate constructor for example?

deceze · Accepted Answer · 2025-04-11 13:03:05Z

4

This'll do:

@classmethod
def from_raw(cls, raw: dict[str, str]):
    return cls(**raw)

Use cls instead of hardcoding Foo, so the method instantiates child classes when called on child classes.
Use dict unpacking to turn a dict directly into named arguments, so you don't have to spell them out one by one.

Of course, now that you have this, you may see that you don't really need this method at all, and you can just do Bar(**data) anywhere, instead of Bar.from_raw(data).

If data may contain keys the dataclass doesn't, you'd need to do a little introspection on the fields of the dataclass, and filter out nonexistent fields:

from dataclasses import fields
from typing import Self

...

@classmethod
def from_raw(cls, raw: dict[str, str]) -> Self:
    keys = {f.name for f in fields(cls)}
    return cls(**{k: v for k, v in raw.items() if k in keys})

edited Apr 11 at 13:03

answered Apr 11 at 12:45

deceze♦

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2 Comments

chepner Apr 11 at 12:59

Use typing.Self for the return annotation as well.

deceze Apr 11 at 13:03

Yeah, was debating whether to include that detail or not; did so now.

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