BUG: Add fillna at the beginning of _where not to fill NA. #60729

[sanggon6107]

• closes BUG: Series.mask incorrectly replaces positions of pd.NA in the cond argument #60729 (Replace xxxx with the GitHub issue number)
• 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.

Added fillna at the beginning of _where so that we can fill pd.NA.
Since this is my first PR, please correct me if I'm mistaken. Thanks!

[WillAyd]

I think this test can be simplified a bit to reduce the number of operations. Maybe something like:

ser = Series([None, 1, 2, None, 3, 4, None], dtype=pd.Int64Dtype())
result = ser.mask(ser <= 2, -99)
expected = Series([None, 1, 2, None, -99, -99, None], dtype=pd.Int64Dtype())

tm.assert_series_equal(result, expected)

[sanggon6107]

Thanks for the suggestion, just updated the code.

[WillAyd]

Is this trying to fill missing values when NaN is the missing value indicator? I don't think that is right either - the missing values should propogate for all types. We may just be missing coverage for the NaN case (which should be added to the test)

[sanggon6107]

Thanks for the feedback, @WillAyd .
I thought we could make the values propagate by filling cond with True, since _where() would finally keep the values in self alive where its cond is True.
Even if I don't fill those values here, _where would call fillna() using inplace at the below code. That's also why the result varies depending on whether inpalce=True or not.

pandas/pandas/core/generic.py

Lines 9695 to 9698 in e3b2de8

	# make sure we are boolean
	fill_value = bool(inplace)
	cond = cond.fillna(fill_value)
	cond = cond.infer_objects()

Could you explain in more detail what you mean by propagate for all type? Do you mean we need to keep NA as it is even after this line?