default behavior doesn't work for non JSON serializable types (because default is used for both marshmallow's missing and default fields)

[lovasoa]

default behavior doesn't work for non JSON serializable types (because default is used for both marshmallow's missing and default fields)

[evanfwelch]

Hey @lovasoa , I'm encountering this issue again when trying to implement a schema that has a field of type uuid.UUID with a default_factory=uuid.uuid4...

Basically, the concept of default has changed between marshmallow 2.x and 3.x... For a dataclass you obviously want your default or default_factory to be a "deserialized" type like UUID or datetime--something you want to use in your code. However for marshmallow 2.x your a field's default attribute actually correspond to the serialized output type. Here's a snippet from the codebase of marshmallow 2.x that shows serialize() just output the raw field.default value.

    def serialize(self, attr, obj, accessor=None):
        """Pulls the value for the given key from the object, applies the
        field's formatting and returns the result.

        :param str attr: The attibute or key to get from the object.
        :param str obj: The object to pull the key from.
        :param callable accessor: Function used to pull values from ``obj``.
        :raise ValidationError: In case of formatting problem
        """
        if self._CHECK_ATTRIBUTE:
            value = self.get_value(attr, obj, accessor=accessor)
            if value is missing_:
                if hasattr(self, 'default'):
                    if callable(self.default):
                        return self.default()
                    else:
                        return self.default
        else:
            value = None
        return self._serialize(value, attr, obj)

And notice the difference in 3.x

def serialize(self, attr, obj, accessor=None, **kwargs):
        """Pulls the value for the given key from the object, applies the
        field's formatting and returns the result.

        :param str attr: The attribute or key to get from the object.
        :param str obj: The object to pull the key from.
        :param callable accessor: Function used to pull values from ``obj``.
        :param dict kwargs': Field-specific keyword arguments.
        :raise ValidationError: In case of formatting problem
        """
        if self._CHECK_ATTRIBUTE:
            value = self.get_value(obj, attr, accessor=accessor)
            if value is missing_ and hasattr(self, 'default'):
                default = self.default
                value = default() if callable(default) else default
            if value is missing_:
                return value
        else:
            value = None
        return self._serialize(value, attr, obj, **kwargs)

In 3.x self.default is used as a value that gets passed to self._serialize()

What do you think we should do about this?

[evanfwelch]

Actually, this 2.x vs 3.x changelog post captures the issue: https://marshmallow.readthedocs.io/en/3.0/upgrading.html#missing-and-default-field-parameters-are-passed-in-deserialized-form

[lovasoa]

@evanfwelch : I am not sure I understand why this would be a problem in marshmallow_dataclass ? The default values are simply passed to marshmallow. It is then up to the user to use whatever value is compatible with the marshmallow version they are using. That is, with marshmallow 3, they would write

@dataclass
class Params:
  date_i_need: date = field(default=date(2018,1,1))

and with marshmallow 2:

@dataclass
class Params:
  date_i_need: date = field(default="2018-01-01")

[evanfwelch]

@lovasoa you're right, this isn't an issue w/ marshmallow_dataclass. I initially misunderstood the fact that marshmallow expects missing and default to be in the same form (either serialized in 2.x or deserialized in 3.x) and I thought that marshmallow_dataclass was erroneously blending this distinction by assigning both to the default value of the dataclass.field. I'm going to close this, thank you for the attention to it.