Custom class serializer doesn't work for nested data?

[AustinScola]

Hey @yukinarit,

I'm running into an issue with a custom class level serializer. I have a dataclass with a field that is a dictionary and I want all datetimes in the dict to be serialized using .timestamp(). This seems to work for top level values, but not for anything nested ? I'm not sure exactly what the expected behavior is here? Is there a different way to accomplish this or is it a bug?

Example

from dataclasses import dataclass
from datetime import datetime
from typing import Any

from serde import SerdeSkip, serde
from serde.json import to_json


def serializer(cls, o):
    if cls is datetime or isinstance(o, datetime):
        return o.timestamp()
    else:
        raise SerdeSkip()


@serde(serializer=serializer)
@dataclass
class Foo:
    bar: dict[str, Any]


def main():
    d = datetime(2021, 1, 1, 0, 0, 0)
    foo = Foo(bar={"d": d})
    print(to_json(foo))  # prints {"bar":{"d":1609477200.0}}

    # The nested datetime here isn't serialized as I would expect it to be?
    d = datetime(2021, 1, 1, 0, 0, 0)
    foo = Foo(bar={"a": {"d": d}})
    print(to_json(foo))  # prints {"bar":{"a":{"d":"2021-01-01T00:00:00"}}}



if __name__ == '__main__':
    main()

[yukinarit]

Hi @AustinScola

pyserde doesn't inspect the nested type at Runtime if the type is declared as Any. That is because of the performance.
The library like dataclasses-json does runtime type inspection therefore it was said to be really slow.

---

I wanted to provide this code as a workaround

@serde(serializer=serializer)
@dataclass
class Foo:
    bar: dict[str, Union[datetime, dict[str, datetime]]]

But it seems custom serializer isn't supported for Union yet 😄
Let me check if I can quickly implement it.