Making classfactory that derive from dataclass objects informed by types in python

Question

When using dataclasses.dataclass the type information informs how parameters are parsed. I would like to take a defined dataclass and produce a class that changes all the attribute type declarations from X to Optional[List[X]].

from dataclasses import dataclass
from dataclasses_json import DataClassJsonMixin
from datetime import datetime

@dataclass
class SrcClass(DataClassJsonMixin):
    number: int
    name: str
    at: datetime

SrcClassLister = make_lister(SrcClass)

I want the function make_lister to produce a class similar to the class defined below.

@dataclass
class SrcClassLister(DataClassJsonMixin):
    numbers: Optional[List[int]]
    names: Optional[List[str]]
    ats: Optional[List[datetime]]

I am unsure how type information is captured for parsing by the dataclass.

My reason for doing this is I have a high variety of dataclass definitions and I would like to automatically make a spec for filtering. This spec would take a list of values that are acceptable for a pass-filter.

Answer 1

I'd imagine something like

import dataclasses
import typing
from dataclasses import dataclass
from dataclasses_json import DataClassJsonMixin
from datetime import datetime


@dataclass
class SrcClass(DataClassJsonMixin):
    number: int
    name: str
    at: datetime
    purpose: int = 42


def pluralize(name):
    # TODO: improve this if you will
    return name + "s"


def make_lister(src_cls):
    fields = [
        (pluralize(field.name), typing.Optional[typing.List[field.type]], dataclasses.field(default=None))
        for field in dataclasses.fields(src_cls)
    ]
    name = f"{src_cls.__name__}Lister"
    return dataclasses.make_dataclass(name, fields, bases=(DataClassJsonMixin,))


SrcClassLister = make_lister(SrcClass)

scl = SrcClassLister(numbers=[1, 2])
print(scl)
print(scl.to_json())

works for you - this prints out

SrcClassLister(numbers=[1, 2], names=None, ats=None, purposes=None)
{"numbers": [1, 2], "names": null, "ats": null, "purposes": null}