API: should allow setting with enlargement with list-likes

[jreback]

from #7867

In [1]: s = pd.Series([1,2,3])

In [2]: s
Out[2]: 
0    1
1    2
2    3
dtype: int64

# this is de-facto: s.reindex([[1,2,3]])
In [3]: s.loc[[1,2,3]]
Out[3]: 
1     2
2     3
3   NaN
dtype: float64

should this work?

In [4]: s.loc[[1,2,3]] = 4
KeyError: '[3] not in index'

(Doing this with only included labels works)

[jreback]

cc @immerrr

[hgilde]

did this turn out to be harder than it seems?

[jreback]

@hgilde no-one has worked on this (and we might eliminate this behavior going forward anyhow).

[hgilde]

@jreback hmm, link to the discussion about changing indexing behavior?

[jreback]

#15747

[jbrockmendel]

having spent most of the last couple days on the relevant code in ILoc._setitem_with_indexer, I lean towards "no". figuring out when we need to do expansion is a PITA

[jreback]

even better we should deprecate this entirely

[mroeschke]

Sounds like generally we dont want to do this anymore. Closing