Pretty sure Python's union types don't nest, though. If you could nest optionals then I feel that reduces the need for separate sentinel types since e.g., you can distinguish between "value not present" (None) and "null value present" (Some(None))

Indeed. Option[Foo] really just means Foo | None - you can’t have an Option[Foo] at runtime. Fundamentally, Python’s static typing system describes a language that is a sort of mediocre match for Python.

Another fun one is List[Foo]. Runtime Python has lists but you can’t ask a list what type of list it is, and you can’t write isinstance(a, List[Foo]).

Can you explain or give an example of nested optional types? I don't quite see how that makes sense

The idea is that optional values are either some value or no value, but sometimes the inner "some value" bit can itself be an optional value (in Rust this would be Option<Option<T>>, in C++ it'd be std::optional<std::optional<T>>, in Python it'd hypothetically be Option[Option[T]], etc.). Such a situation gives you three possible states:

- You have an inner value and the inner value is something non-null (an integer, a string, another optional, etc.)

- You have an inner value and the inner value is null

- You do not have an inner value

The problem, from what I understand, is that Option[T] in Python is equivalent to the union type T | None, and union types are flattened (e.g., (T | U) | V is equivalent to T | U | V) and duplicates are ignored (e.g., T | U | T is equivalent to T | U). As a result, Option[Option[T]] is equivalent to (T | None) | None, which is equivalent to T | None | None, which is finally equivalent to T | None, which means you lose the distinction between "No inner value" and "Inner value is present but is None".

As a more concrete (albeit somewhat contrived) example, say there's some API you can query for the owner of a particular document ID where the document ID may or may not refer to an actual document (e.g., maybe you got it from a cache and the document has since been deleted). One way to express this would be (using Rust syntax)

    fn get_owner(id: DocumentId) -> Option<Option<UserId>> { ... }

So if you get None, the document does not exist; if you get Some(None), the document exists but it has no owner; and if you get Some(<id>) the document exists and it has an owner.

(This can probably be done better by creating new enums, but it's an example so give me a bit of slack)

A naive port of this signature to Python would be

    def get_owner(id: DocumentId) -> Option[Option[UserId]]: pass

But to type checkers this is equivalent to

    def get_owner(id: DocumentId) -> UserId | None: pass

So now you're in the pickle the OP describes: you can end up with a None, but you no longer knonw whether it means "no document" or "no owner". The Python solution is to basically create a different name for either the outer None (e.g., MISSING as in the OP) or the inner one (e.g., NO_OWNER or the like).

This seems really silly to me. Why does get_owner need to differentiate between the document exists and maybe has an owner or the document doesn’t exist and thus has no owner. In both cases getting no owner should suffice? Why are you using get_owner to try and figure out if the document exists? Can you give an example where this distinction actually matters?

> In both cases getting no owner should suffice?

Depends on what exactly you plan on doing with the information, I'd imagine. I don't think it's necessarily a given that "no document" and "no owner" should be handled identically (e.g., searching for the updated document to try again vs. returning an operation-not-allowed error).

> Why are you using get_owner to try and figure out if the document exists?

Potential TOCTOU issues, perhaps? As I mentioned the document could have vanished out from under you so detecting this situation could let handle things appropriately (though to be fair that argument could probably extend to future operations as well).

Maybe a versioned filesystem would have made a bit more sense, where None would indicate a new version is available? Another example I've seen mentioned is trying to get a key-value pair from a JSON where the key may be missing.

But in any case, I did say it was a bit of a contrived example, so cut me a bit of slack :P It was just the first thing that came to mind.

The examples sibling comments give are probably more practical.

> I don't think it's necessarily a given that "no document" and "no owner" should be handled identically (e.g., searching for the updated document to try again vs. returning an operation-not-allowed error).

If you care whether a document exists or not, you should use an API that exclusively tells you that instead of expecting everything related to documents telling you if the document exists or not. Otherwise you’re overloading the meaning. A non existent document has no owner. An API to fetch the owner of the document should not care if it exists or not, it does not matter.

As I said, that sounds like a recipe for potential TOCTOU errors.

But again, this was an off-the-cuff example that I explicitly described as contrived from the start. I could probably flesh it out to make more sense, but I think it'd be easier just to refer to sibling comments rather than bikeshed this to death.

Not really? It returns no owner if the document is deleted or just does not exist, which is accurate. Why do you need get_owner to tell you if the document exists or not? If anything it should be an error if the document doesn’t exist because you’ve violated a precondition.

> It returns no owner if the document is deleted or just does not exist, which is accurate.

"I don't know" is not the same as "definitely no owner". You may handle the two answers the same for a particular scenario, but then again you may not.

Again, consider the case where get_owner is working on some kind of cache (in which case sure, the function name could stand to be improved). If the document is no longer in the cache it could be because it was actually deleted or because it was simply evicted to make room for other entries. In general you cannot assume that a missing cache entry is equivalent to the document existing and having no owner.

Or similarly (and also yet again), consider the case where get_owner is working on a versioned filesystem, in which case the outer optional could indicate that the (generational) document ID is out of date. In such a case treating "I don't know" and "no owner" the same is incorrect because the owner could have been updated from "no owner" to something else (such a case would likely be more realistically handled by returning a Result<Option<UserId>, SomeErrorKind>, though, given the myriad other ways a file system handle can go stale)

> Why do you need get_owner to tell you if the document exists or not?

Because sometimes there is no good alternative. Asking a cache whether it has an entry and then trying to get the entry value in a separate operation (particularly in a concurrent environment) is a classic setup for TOCTOU errors. You can say something similar for the versioned file system example.

> If anything it should be an error if the document doesn’t exist because you’ve violated a precondition.

I mean, enabling that kind of handling is precisely the point of the outer optional?

Maybe a different example works better for your mental model:

A user can set custom settings, and those settings may or may not have a value assigned to them. For example, one setting is optimization level, another option is a debug flag.

You want to have a unified interface to get information about settings, queried through some intermediate system. (lets just say it is limited to querying specific settings, rather than getting the entire list, due to legacy constraints in this case) Because for some settings you just care about them existing and for others you care about the actual value, you have to differentiate between a setting that contains something, a setting that contains nothing, and one that doesnt exist.

It's analogous to how we use undefined and null in JS.

Undefined = No value known. Think 0 rows found, the 1st row's column is undefined.

Null = Value known. It is void of value. The 1st row is populated, and this column has no value.

You run into this issue any time you do a LEFT JOIN in sql and the field you care about is nullable.

The right side value could be null because it was missing, or because it was literally null. You resolve by checking the join key, and if that is itself nullable, you’re forced to introduce a sentinel

Most languages can do this if it's what you want. In Python it would be:

    class Some[T]:
        value: T

    type Option[T] = Some[T] | None

    def get_owner(id: DocumentId) -> Option[Option[UserId]]: ...

    match get_owner(document_id):
        case None:
            ...
        case Some(None):
            ...
        case Some(Some(x)):
            ...

Yeah, that's more like what you would do if you wanted to translate the entire concept to Python instead of just the signature, and I probably should have mentioned that alternative as well (or should have said "A Python solution"). Can't say I've seen this approach used in practice, interestingly, though I'm probably not nearly as familiar with the Python that exists out there as most other commenters here.

The name clash is unfortunate, but luckily naming things is famously one of the easiest problems in CS so that should be trivial to solve :P

I see, thanks. Though this feels like something that would always have a better alternative than returning Some(None). Probably a new enum, or in Python's case, exceptions (e.g. instead of returning a value if the document is missing, you'd raise DocumentMissingError or something)

Say you select nullable column into vec, Vec<Option<T>>. vec.get returns Option<Option<T>>, which distinguishes null from out of bounds

I'd like nested Nullable in ClickHouse to use CoalescingMergeTree to support postgres unchanged TOAST values easily without getting mixed up on nulls

If you have a map, get(key) returns Option[ValueT]: None if the key is missing, Some(value) otherwise. If the value type itself is, say, Optional[String], your return type is Optional[Optional[String]]. Here None still always means the key is missing, while Some(None) means the value is just None.

The answer is they don't in practical code but they come up in generic code, where you want to wrap something that is already returning an optional in your optional. In languages that do support optional[optional[whatever]], if you every actually write that you know you've messed up your API.

They absolutely do in practical non-generic code. A case I personally encounter regularly is json APIs which distinguish between a key being missing and one being set to null in a structure.