avoid-modification-with-frozen-dataclass.md

Avoid Modification With Frozen Dataclass

The @dataclass decorator can be set as frozen to prevent modification of values on instances of that dataclass.

Without making it frozen, I can easily subvert validations by changing the value of attributes after the __post_init__ validations are called.

>>> config = BPEConfig(300, []) # passes validations
>>> config.vocab_size = 22 # this is invalid, wish this was prevented

Here is the updated @dataclass declaration with frozen=True passed as a parameter.

from dataclasses import dataclass
from typing import ClassVar

@dataclass(frozen=True)
class BPEConfig:
    BASE_VOCAB_SIZE: ClassVar[int] = 256

    vocab_size: int
    special_tokens: list[str]

    def __post_init__(self):
        if self.vocab_size < self.BASE_VOCAB_SIZE:
            msg = f"vocab_size ({self.vocab_size}) must be greater than or equal to BASE_VOCAB_SIZE ({self.BASE_VOCAB_SIZE})"
            raise ValueError(msg)

Now I am prevented from modifying a scalar value like vocab_size after the instance has been created.

>>> config = BPEConfig(300, [])
>>> config.vocab_size = 22
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 4, in __setattr__
dataclasses.FrozenInstanceError: cannot assign to field 'vocab_size'

This doesn't prevent you from modifying the contents of attributes that are list or dict types.