How can I make a field wrapped in Optional without a default value to be required during deserialization?

[Yura52]

Hi! Thank you for the great library!

Let's consider this snippet:

@serde.serde(type_check=serde.Strict)
@dataclass
class A:
    b: Optional[int]

Here, I require the field b to be explicitly passed to the constructor of A. However, serde allows not passing the field b during deserialization:

serde.from_dict(A, {})
# returns A(b=None)

Knowing that dacite made the same design choice, I assume this is an intentional thing, not a bug. Is there any available workaround to require being explicit whenever a field is missing a default value?

Also, I would like to share why, to me personally, the current behavior is surprising.

First, because of the mismatch between Python and serde: I cannot create the object without passing the required field in Python, but I can do it via serde. In other words, for serde, Optional[int] and Optional[int] = None are equivalent, which is not intuitive to me.

Second, because of the following inconsistency:

serde.to_dict(A(None))
# returns {'b': None}

What do you think?

[yukinarit]

Hi @Yura52!

Do you mean that

With this serde class,

@serde.serde(type_check=serde.Strict)
@dataclass
class A:
    b: Optional[int]

This should raise an error because the dict has no key of "b"? 🤔

serde.from_dict(A, {})

[Yura52]

Hi @yukinarit , yes, this is what I would expect, since there is no default value for the b field, and I want users to explicitly pass None.

It seems to me that this kind of behavior (implicit Optional filling) is a popular design choice: I have already mentioned dacite, but an even more prominent example is TOML, where null was not added to the format, because "you don't need nil or null, just leave out that assignment".

That said, having a flag to make a required field truly required would be nice!

[yukinarit]

Understood, I am okay to have that flag in pyserde, as long as we don't change the default behaviour!