BUG: RecursionError with CategoricalIndex.get_indexer

[jbrockmendel]

• closes CI: categorical failures #42088
• tests added / passed
• Ensure all linting tests pass, see here for how to run them
• whatsnew entry

[jreback]

cool. merge & backport on green.

[jreback]

@meeseeksdev backport 1.3.x

[lumberbot-app]

Something went wrong ... Please have a look at my logs.

[jorisvandenbossche]

This fix seems to have caused a large slowdown on the get_indexer benchmarks: https://pandas.pydata.org/speed/pandas/#indexing.CategoricalIndexIndexing.time_get_indexer_list?python=3.8&Cython=0.29.21&p-index='monotonic_incr'&commits=cf5852bf-fce7f9eb

The regression overview (https://pandas.pydata.org/speed/pandas/#regressions?sort=1&dir=desc) lists it as a 1000x slowdown, but that's only because #42042 first improved the performance a lot (which might be a bit suspicious?). Compared to the timing before that, it's only 4-5x slowdown. With the below code, I see locally a ~9x slowdown on master compared to 1.2.5.

import string, itertools
data_unique = pd.CategoricalIndex(
            ["".join(perm) for perm in itertools.permutations(string.printable, 3)]
)
cat_list = ["a", "c"]

%timeit data_unique.get_indexer(cat_list)
52.8 ms ± 5.56 ms per loop (mean ± std. dev. of 7 runs, 10 loops each)  # <-- pandas 1.2.5
417 ms ± 22.1 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)  # <-- master

I think it has to do with the fact that before we called the Engine.get_indexer on the codes, while now in the base class version we do that with the .categories, which means in this case that both self and target are cast to object dtype and thus use the Engine.get_indexer for object dtype.

[jorisvandenbossche]

I opened #42249 to keep track of this.