Fix _naxis dimensionality - #18419

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Would these changes also fix #18405, @ayshih ?

Yes, it fixes the primary issue. I'll point out that in #18405 I additionally chose to hide the NAXIS: 0... line from the repr when the shape is undefined.

@ayshih I don't think we need to hide NAXIS although I am not against it.

Since @astrofrog did a lot of fixes in his PR + unit tests, would you like to implement my suggested fix in your PR? or will you rebase your PR after this (simplified PR without your ellipses fixes and tests) is merged?

I don't think we need to hide NAXIS although I am not against it.

I guess I'm ambivalent now. It was problematic when the line was always NAXIS: 0 0 regardless of dimensionality.

I guess I'm ambivalent. It was problematic when the line was always NAXIS: 0 0 regardless of dimensionality.

I agree. I do not often work with 3D WCS but I noticed this now and didn't like it. I think it is OK to remove it since the code already prints

Number of WCS axes: 3

and also the number of axes can be inferred from the shape of the PC or CD matrix or CRPIX. That is,

NAXIS: 0 0 0

does not really provide any new information.

Eh, after I wrote this, I think it does make it easier to see 0 0 0 and then think that there is no info on image dimensionality. I wonder if not seeing NAXIS at all would make this more difficult to see.

@astrofrog Feel free to port this fix to your PR and then close this or accept/merge this and rebase yours.