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omit_default=True evaluates default_factory value at declaration time which can cause a circular reference about mashumaro HOT 3 CLOSED

m3talstorm commented on May 23, 2024

omit_default=True evaluates default_factory value at declaration time which can cause a circular reference

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Comments (3)

Fatal1ty commented on May 23, 2024

Hi @m3talstorm

I guess there would be a performance hit on the first serialize, maybe add a lazy_omit_default config value?

Yes, there would be a performance hit. I do think that the new config option is not worth it in terms of cost and profit. Nevertheless, I'm sure there are workarounds or alternative approaches. The question is, do you really need to be able to pass an explicit sequence number to the constructor? Depending on the answer, there are some ways to avoid non-idempotent default_factory usage:

change sequence type to Optional[int] with default None and set it in post-init:

@dataclass(slots=True, frozen=False)
class Thing(DataClassJSONMixin, DataClassDictMixin):
    class Config(BaseConfig):
        omit_none = True
        omit_default = True

    sequence: Optional[int] = field(default=None)

    SEQUENCE: ClassVar[int] = 0

    def __post_init__(self):
        if self.sequence is None:
            self.sequence = sequence_incr()

the same as above but instead of Optional[int] keep it as int with default -1 (I suppose, sequence number must be positive):

@dataclass(slots=True, frozen=False)
class Thing(DataClassJSONMixin, DataClassDictMixin):
    class Config(BaseConfig):
        omit_none = True
        omit_default = True

    sequence: int = field(default=-1)

    SEQUENCE: ClassVar[int] = 0

    def __post_init__(self):
        if self.sequence < 0:
            self.sequence = sequence_incr()

remove sequence from the constructor and set it in post-init instead:

@dataclass(slots=True, frozen=False)
class Thing(DataClassJSONMixin, DataClassDictMixin):
    class Config(BaseConfig):
        omit_none = True
        omit_default = True

    sequence: int = field(init=False)

    SEQUENCE: ClassVar[int] = 0

    def __post_init__(self):
        self.sequence = sequence_incr()