sets in str.cat?!

[h-vetinari]

In #20347, I was asked to allow list-like inputs for the .str.cat methods for Series. However, I didn't know that is_listlike({'a', 'b', 'c'}) is True (for sets) until I stumbled over it in #22486.

These objects should imo clearly be disallowed, lest users shoot themselves in the foot massively:

>>> s = pd.Series(['a', 'b', 'c'])
>>> s.str.cat({'a', 'b', 'c'})
0    ac
1    bb
2    ca
dtype: object

I'm not even sure this should go through a deprecation cycle - IMO it's just a plain ol' bug that should be fixed ASAP.

[pambot]

@h-vetinari There's definitely ways that this can misbehave, but the docstring for is_listlike shows using sets as an example, so it looks like it was intended?

[h-vetinari]

@pambot
There was time pressure for #20347 before v0.23.0rc and a few things did not get fully discussed. Had I known that is_listlike(<set>) = True, I would have excluded it specifically.

In fact, I'm struggling to think of a use-case in pandas where the order is irrelevant - it's a different (and larger) discussion whether that aspect of is_listlike should maybe be changed, but it should definitely be changed for .str.cat