Comment by woodruffw
15 hours ago
Python’s type system has union types (including Option), and using them is pretty ergonomic. As TFA points out the problem is that `None` gets contextually reused in all kinds of places where a proper sentinel type is more appropriate.
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)
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
But to type checkers this is equivalent to
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).
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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
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.
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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.