ENH: allow EA to register types for is_scalar

[jbrockmendel]

#27461 (comment)

i think we need. way for EA to hook into this for an EA scalar
eg an IPaddress from cyberpandas could register a scalar i think

Before we move on this, I think we need to clarify in which situations we care about lib.is_scalar(x) vs the simpler np.ndim(x) == 0

[TomAugspurger]

One example is for nested data. In this case we need something like scalar_for_dtype(value, dtype), since the ndim of a "scalar" for a nested data type would be > 0.

[jorisvandenbossche]

Alternative for registering, could be a method on the dtype/array that can check if a value is a valid scalar?

[sterlinm]

Hi! I think I've run into this issue in my own attempt at building an ExtensionArray and I was curious if there'd been any changes on this or if it was something I could potentially contribute on.

I've been working on an extension array where the na_value I want to return for the ExtensionDtype is not recognized as a scalar by is_scalar. That seems to cause issues with some methods that aren't part of the ExtensionArray interface that I can't figure out how to fix (e.g. Series.where).

Is there another workaround for this that I haven't found yet? Thanks!

[jbrockmendel]

Is there another workaround for this that I haven't found yet?

Only thought that comes to mind is trying to replace is_scalar checks with not is_listlike checks. Last time I checked (worth double-checking since this was a while ago) is_listlike was faster than is_scalar anyway, and should be more robust to this problem.

[sterlinm]

Thanks very much! It looks like that change has already been made in a number of places in the most recent versions of Pandas (I was testing on 1.3).

Thanks for your help and sorry to bother you!