Marshmallow: Confused by fields.Function(serialize=f) behavior

[Quidge]

(marshmallow 2.xx) (Yes, I do recognize that fields.DateTime is built for my example case. My questions below still holds though.)

I don't follow how the serialize=function kwarg is supposed to work for fields.Function. Digging around the code for _serialize, I found this comment which leads me to believe I don't understand the how this is supposed to be used:

    def _serialize(self, value, attr, obj):
        try:
            return self._call_or_raise(self.serialize_func, obj, attr)
        except AttributeError:  # the object is not expected to have the attribute
            pass
        return missing_

# the object is not expected to have the attribute really? Then what should I do if I want to have a serialized version of an attribute?

Say I have this schema:

class SomeSchema(Schema):
    created_at = fields.Function(
        serialize=lambda value: value['created_at'].toisoformat(),
        deserialize=lambda value: datetime.fromisoformat(value)
    )

What can should the serialize function be passed if it isn't expected to be passed an object with the attribute? The following result from a load + dump is baffling to me:

In [2]: from datetime import datetime, timezone

In [3]: from upstreamapi.activities import SomeSchema

In [4]: d = SomeSchema().load({'created_at': datetime.now().isoformat()}).data

In [5]: d
Out[5]: {'created_at': datetime.datetime(2020, 2, 5, 9, 9, 50, 847165)}

In [6]: SomeSchema().dump(d)
Out[6]: MarshalResult(data={}, errors={})

The key point being that the data is missing a created_at field, even though it was present in the input to dump.

[sloria]

It looks like an AttributeError is being raised because toisoformat is not a method of datetime.datetime. It should work if you change it to isoformat.

Note: marshmallow 3 no longer swallows AttributeError, which makes bugs like this much easier to find.

[Quidge]

That was it, thank you! Had one of those 'I'm losing my mind' experiences this morning with this.