Python dataclass: Forcing a dictionary field to be a deep copy

Question

I am working with a dataclass that contains a dict.

I want the dict to be a deepcopy, without having to rely on a post_init call, which would basically void to interest of a dataclass

What would be a nice solution ?

from dataclasses import dataclass, field
from typing import Dict


@dataclass
class ClassWithDict:
    the_dict: Dict = field(default_factory=dict, kw_only=True)


toto = {"toto": "tata"}
the_class = ClassWithDict(the_dict=toto)

assert toto == the_class.the_dict
assert toto is not the_class.the_dict  # FALSE

Asked By: Milan

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Source

Answer 1

Solution, if not wanting to use a __post__init__ using the object.setattr method to force a copy once the object is initialized, is to use a metaclass:

import copy
from dataclasses import dataclass, field
from typing import Dict


class DataClassWithDeepCopyMeta(type):

    def __call__(cls, *args, **kwargs):
        args = copy.deepcopy(args)
        kwargs = copy.deepcopy(kwargs)
        return super().__call__(*args, **kwargs)


@dataclass
class ClassWithDict(metaclass=DataClassWithDeepCopyMeta):
    the_dict: Dict = field(default_factory=dict)


toto = {"toto": "tata"}
the_class = ClassWithDict(toto)

assert toto == the_class.the_dict
assert toto is not the_class.the_dict

Answered By: Milan

Python dataclass: Forcing a dictionary field to be a deep copy

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