ENH: Processing of .mask() for pd.NA #56844

[mafaldam]

• closes ENH: Processing of .mask() for pd.NA #56844
• 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 an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

This commit addresses an issue where using NA in conjunction with the mask() method that resulted in unexpected behavior. The problem arose when comparing a Series containing NA as a condition within mask(), causing inconsistencies in the output. This fix ensures that NA behaves consistently as False in logical operations within mask().

[WillAyd]

Rather than a fillna like this do you see where the logic is breaking down that causes .mask to treat pd.NA the same was as True? We shouldn't have to mess around with pd.NA equality semantics for this fix

[jorisvandenbossche]

For indexing this goes through check_array_indexer which essentially also converts NA to False (but lower in the stack):

pandas/pandas/core/indexers/utils.py

Lines 532 to 534 in 2aa155a

	if is_bool_dtype(dtype):
	if isinstance(dtype, ExtensionDtype):
	indexer = indexer.to_numpy(dtype=bool, na_value=False)

[jbrockmendel]

@phofl @jorisvandenbossche this seems related to part of the ice cream agreement. Are we ready to move forward with that part? I’d expect to do the indexing bits all at once

[jorisvandenbossche]

@jbrockmendel I think we can simply treat this as a bug fix of the current implementation. We do handle the nullable boolean dtype specifically in indexing already (considering NA as False, a behaviour which has been discussed in the past already, #31591), so fixing that for mask as well seems like a bug fix to me / fix to make things consistent.