BUG: Groupby.sum, DataFrame.sum and Series.sum for object type should be NA instead of 0 for all-nan values

[snitish]

• partly addresses BUG/API: sum of a string column with all-NaN or empty #60229
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added note to whatsnew/v3.0.0.rst

[github-actions]

This pull request is stale because it has been open for thirty days with no activity. Please update and respond to this comment if you're still interested in working on this.

[rhshadrach]

@snitish - This looks good to me if you'd like to reopen. I'm handling the string case in #60936

[snitish]

@rhshadrach sure I'd be happy to. There was some debate on the original thread on whether the default sum value for object dtype should be NA or "" (empty string). Any thoughts on this?

[rhshadrach]

There was some debate on the original thread on whether the default sum value for object dtype should be NA or "" (empty string). Any thoughts on this?

For object dtype, I'm only seeing support for NA in that thread. Am I missing any contrary opinions? The only hesitation I see is @Dr-Irv on how this impacts users, but I do not see an easy deprecation path and I am comfortable with listing this in the breaking changes for 3.0

When I say NA here, I mean "the NA value for the given dtype". For object dtype, the NA value is np.nan.

[snitish]

Thanks for approving, @WillAyd. This PR still doesn't address the issue for DataFrame.sum() and Series.sum(). I'm working on that.