ENH: Non-nullable extension types

[gsakkis]

As far as I can tell, all custom extension types are nullable. It would be useful to provide a way to create non-nullable extensions. One way to support this could be by setting the na_value attribute to NotImplemented. Thoughts?

[dsaxton]

Sounds like you want something like a database constraint but for DataFrames? That's an interesting idea, although I suspect this might be easiest to accomplish in end-user code by simply asserting that a given column contains no null values, rather than creating whole new data types.

[gsakkis]

IMO nullability is part of the type definition, it's not some extra constraint to be tested by the end-user code. Checking the constraint in the extension code itself (e.g. in __init__ and/or _from_sequence) would be an improvement but even that is not sufficient for my use case. Ideally nullability should be an inspectable property of the type, discoverable by a new pd.api.types.is_nullable_dtype(arr_or_dtype) function.

[jbrockmendel]

xref #40574

[jbrockmendel]

A 3rd party EA could just raise if any NA-like is passed to the relevant constructor or __setitem__. I guess we could document this? Otherwise not clear that this is actionable.

[mroeschke]

Looks like there's not much support for this feature from the core team so closing