When making a dataclass, how can I use a custom type for the field objects themselves?

Question

I'm writing a library that works a lot like dataclass but with some additional functionality. Obviously I want to reuse dataclass as much as possible, but for my added functionality, I'd like to extend dataclass.Field. That is, I'd like to define:

class MyField(dataclasses.Field):
  ... # additional functionality

And then do something so that when the user does:

@mydataclass
class UserDefinedClass:
  foo: int
  y: Bar

(Where mydataclass is a decorator that eventually calls dataclass...)

The actual field objects inside UserDefinedClass will be of type MyField rather than dataclasses.Field.

I can't find anything in the dataclass docs that allows me to do this, but I think I might be missing something.

(I am aware of the metadata keyword argument to field, and I will probably use that if the answer to this question is "that's impossible". But it'll be more work.)

Answer 1

It certainly doesn't exactly look as though dataclasses.Field is supposed to be subclassed, and convincing the dataclass decorator to use a custom field is, well, a bit grotesque, but this works – even if I'd advise against this.

It would certainly be nicer if you could tell dataclass() to use a custom field class, but alas...

import dataclasses
from unittest import mock


def mydataclass(**kwargs):
    def deco(cls):
        with mock.patch("dataclasses.Field", MyField):  # NB: not thread-safe
            return dataclasses.dataclass(**kwargs)(cls)

    return deco


class MyField(dataclasses.Field):
    def honk(self):
        print(
            f"You hear {self.name!r} the {self.type.__name__} emitting a loud HONK!"
        )


@mydataclass()
class MyData:
    a: int
    b: str


d = MyData(1, "hello")
for f in dataclasses.fields(d):
    f.honk()

This prints out

You hear 'a' the int emitting a loud HONK!
You hear 'b' the str emitting a loud HONK!

as you might imagine.