CLN: prepare unifying hashtable.factorize and .unique; add doc-strings
For adding a `return_inverse`-kwarg to `unique` (#21357 / #21645), I originally didn't want to have to add something to the templated cython code, preferring to add a solution using `numpy.unique` as a first step (with the idea of improving the performance later).

@jorisvandenbossche then remarked:
> Which led me think: `pd.unique` with a `return_inverse` argument is actually basically the same as `pd.factorize`?

I didn't know what `factorize` was doing, so I had no idea about this connection. From looking at the cython code in `hashtable_class_helper.pxi.in`, I found that there's substantial overlap between `unique` and `get_labels` (the core of `factorize`) - the only difference is the handling of `NaN/None/etc.`

I think that these could/should be unified for less code duplication. Alternatively, the first commit of this PR could be split off into a separate PR - it is *just* adding a `return_inverse`-kwarg to `unique` -- though effectively by duplicating the `factorize` code.

Both variants pass the the test suite locally, the only question is whether there's appreciable differences in the ASVs. Since the only extra complexity of unifying the code is reading the value of some `bint` within the loop, I don't think that it'll be relevant. I got some noisy data the first time 'round, will let it run again when I have the possibility.

So long story short, this PR prepares the hashtable-backend to support `return_inverse=True`, which plays into #21357 #21645 #22824, and will also allow to easily solve #21720.

Side note: this already builds on #22978 to be able to run the ASVs at all.
Hello @h-vetinari! Thanks for updating the PR.

 - There are no PEP8 issues in the file [`pandas/core/algorithms.py`](https://github.com/h-vetinari/pandas/blob/a267d4a2e872a48986b35c771814fdf8617b0792/pandas/core/algorithms.py) !

 - There are no PEP8 issues in the file [`pandas/tests/test_algos.py`](https://github.com/h-vetinari/pandas/blob/a267d4a2e872a48986b35c771814fdf8617b0792/pandas/tests/test_algos.py) !



##### Comment last updated on October 11, 2018 at 20:09 Hours UTC Here's the full ASV run on a cleanly restarted machine with nothing else running.

Bit surprised at the spread (both positive and negative):
```
      before           after         ratio
     [640162fa]       [9918d52b]
+        488±30μs         967±70μs     1.98  inference.NumericInferOps.time_subtract(<class 'numpy.uint16'>)
+         622±0μs         1.14±0ms     1.83  inference.NumericInferOps.time_multiply(<class 'numpy.int16'>)
+        579±30μs         977±80μs     1.69  inference.NumericInferOps.time_multiply(<class 'numpy.uint16'>)
+      25.7±0.5μs         40.6±3μs     1.58  indexing.CategoricalIndexIndexing.time_getitem_list_like('monotonic_decr')
+      1.98±0.1ms       3.08±0.2ms     1.55  indexing.CategoricalIndexIndexing.time_get_loc_scalar('monotonic_decr')
+         110±8μs          171±0μs     1.55  indexing.DataFrameNumericIndexing.time_iloc
+      2.08±0.1ms       3.12±0.4ms     1.51  groupby.RankWithTies.time_rank_ties('int64', 'max')
+      9.94±0.7μs         14.6±2μs     1.47  indexing.DataFrameStringIndexing.time_getitem_scalar
+      1.95±0.1ms       2.86±0.2ms     1.47  groupby.RankWithTies.time_rank_ties('int64', 'min')
+        2.67±0ms       3.91±0.8ms     1.46  indexing.DataFrameNumericIndexing.time_loc_dups
+         613±0μs          893±0μs     1.46  indexing.DataFrameNumericIndexing.time_bool_indexer
+     4.20±0.05μs       6.10±0.1μs     1.45  indexing.CategoricalIndexIndexing.time_getitem_scalar('monotonic_incr')
+          2.48ms           3.57ms     1.44  index_object.Indexing.time_get_loc_non_unique_sorted('Float')
+        1.23±0ms       1.75±0.1ms     1.42  indexing.CategoricalIndexIndexing.time_get_loc_scalar('monotonic_incr')
+        26.0±2μs         36.6±4μs     1.41  indexing.CategoricalIndexIndexing.time_getitem_list_like('non_monotonic')
+        1.83±0μs       2.57±0.3μs     1.41  indexing.GetItemSingleColumn.time_frame_getitem_single_column_int
+        977±70μs      1.36±0.03ms     1.39  indexing.CategoricalIndexIndexing.time_getitem_list('non_monotonic')
+        220±10μs         306±20μs     1.39  indexing.DataFrameNumericIndexing.time_iloc_dups
+          7.50ms           10.4ms     1.39  index_object.Indexing.time_boolean_array('Float')
+        97.1±0μs          135±8μs     1.39  indexing.DataFrameNumericIndexing.time_loc
+        4.24±0μs       5.86±0.2μs     1.38  indexing.CategoricalIndexIndexing.time_getitem_scalar('non_monotonic')
+         268±0μs         368±20μs     1.37  indexing.DataFrameStringIndexing.time_boolean_rows_object
+        14.3±1μs       19.5±0.9μs     1.36  indexing.DataFrameStringIndexing.time_get_value
+        24.4±0μs         33.3±2μs     1.36  indexing.DataFrameStringIndexing.time_ix
+        1.07±0ms      1.46±0.03ms     1.36  indexing.CategoricalIndexIndexing.time_getitem_list('monotonic_decr')
+     4.20±0.05μs       5.69±0.1μs     1.36  indexing.CategoricalIndexIndexing.time_getitem_scalar('monotonic_decr')
+          1.83μs           2.44μs     1.33  index_object.Indexing.time_get('Float')
+        28.2±2μs         36.6±2μs     1.30  indexing.CategoricalIndexIndexing.time_getitem_list_like('monotonic_incr')
+        13.4±0μs         17.2±1μs     1.29  indexing.CategoricalIndexIndexing.time_getitem_slice('non_monotonic')
+        12.2±0ms         15.6±1ms     1.29  algorithms.Duplicated.time_duplicated_int(False)
+         368±0μs         467±60μs     1.27  groupby.GroupByMethods.time_dtype_as_group('int', 'mean', 'direct')
+         342±0μs         431±30μs     1.26  indexing.CategoricalIndexIndexing.time_getitem_bool_array('monotonic_decr')
+     1.07±0.08ms         1.35±0ms     1.26  indexing.CategoricalIndexIndexing.time_getitem_list('monotonic_incr')
+     1.09±0.06ms      1.37±0.02ms     1.25  indexing.CategoricalIndexIndexing.time_getitem_bool_array('non_monotonic')
+         342±0μs          426±8μs     1.25  indexing.CategoricalIndexIndexing.time_getitem_bool_array('monotonic_incr')
+      18.3±0.4μs       22.6±0.4μs     1.23  indexing.NonNumericSeriesIndexing.time_getitem_scalar('datetime', 'nonunique_monotonic_inc')
+      8.01±0.4μs         9.77±1μs     1.22  indexing.DataFrameStringIndexing.time_loc
+        2.14±0ms       2.60±0.1ms     1.21  indexing.CategoricalIndexIndexing.time_get_loc_scalar('non_monotonic')
+        14.1±0μs       17.1±0.3μs     1.21  indexing.CategoricalIndexIndexing.time_getitem_slice('monotonic_decr')
+           832ns            994ns     1.19  index_object.Indexing.time_get('String')
+        2.68±0ms         3.12±0ms     1.17  index_object.SetOperations.time_operation('datetime', 'intersection')
+        3.12±0ms         3.65±0ms     1.17  io.csv.ReadCSVFloatPrecision.time_read_csv(',', '_', 'high')
+          7.06μs           8.07μs     1.14  categoricals.CategoricalSlicing.time_getitem_list_like('non_monotonic')
+        27.3±0ms         31.2±0ms     1.14  binary_ops.Timeseries.time_timestamp_ops_diff('US/Eastern')
+          13.7ms           15.6ms     1.14  categoricals.Isin.time_isin_categorical('int64')
+           109ms            125ms     1.14  index_object.IndexAppend.time_append_obj_list
+         109±0ms          125±0ms     1.14  index_object.SetOperations.time_operation('strings', 'union')
+           518μs            587μs     1.13  categoricals.CategoricalSlicing.time_getitem_list('non_monotonic')
+        62.5±0ms         70.3±0ms     1.12  eval.Eval.time_and('numexpr', 'all')
+          4.58μs           5.13μs     1.12  categoricals.CategoricalSlicing.time_getitem_slice('monotonic_incr')
+          85.4μs           95.6μs     1.12  frame_methods.XS.time_frame_xs(0)
+           141ms            156ms     1.11  index_object.Indexing.time_get_loc_non_unique('String')
+        85.2±0μs         94.7±6μs     1.11  indexing.NumericSeriesIndexing.time_iloc_array(<class 'pandas.core.indexes.numeric.Float64Index'>, 'unique_monotonic_inc')
+      3.29±0.2ms         3.65±0ms     1.11  io.csv.ReadCSVFloatPrecision.time_read_csv(',', '.', None)
+         383±4ms          422±0ms     1.10  io.stata.Stata.time_read_stata('tq')
-          52.1ms           46.9ms     0.90  eval.Query.time_query_with_boolean_selection
-         375±8ms          336±4ms     0.90  io.excel.Excel.time_write_excel('xlsxwriter')
-          22.0μs           19.5μs     0.89  ctors.SeriesDtypesConstructors.time_index_from_array_floats
-        440±40μs          391±0μs     0.89  groupby.GroupByMethods.time_dtype_as_field('int', 'sum', 'direct')
-      2.44±0.2ms         2.14±0ms     0.88  replace.FillNa.time_replace(True)
-          15.6μs           13.6μs     0.87  index_object.Indexing.time_slice_step('Float')
-          4.84μs           4.20μs     0.87  frame_methods.XS.time_frame_xs(1)
-          5.13μs           4.39μs     0.86  categoricals.CategoricalSlicing.time_getitem_slice('monotonic_decr')
-         183±0μs         155±10μs     0.85  groupby.GroupByMethods.time_dtype_as_field('int', 'shift', 'transformation')
-           1.33s            1.12s     0.85  groupby.GroupByMethods.time_dtype_as_group('float', 'mad', 'direct')
-         406±8ms          344±0ms     0.85  indexing.NonNumericSeriesIndexing.time_getitem_scalar('string', 'unique_monotonic_inc')
-     1.16±0.07ms          977±0μs     0.84  groupby.GroupByMethods.time_dtype_as_field('int', 'rank', 'direct')
-         188±8ms          156±0ms     0.83  groupby.GroupByMethods.time_dtype_as_field('int', 'skew', 'direct')
-         188±4ms          156±0ms     0.83  groupby.GroupByMethods.time_dtype_as_field('int', 'skew', 'transformation')
-        188±70ms          156±8ms     0.83  groupby.GroupByMethods.time_dtype_as_field('int', 'unique', 'direct')
-          5.86μs           4.86μs     0.83  index_object.Indexing.time_slice_step('String')
-          10.1μs           8.32μs     0.83  categoricals.CategoricalSlicing.time_getitem_list_like('monotonic_incr')
-        419±30μs         344±40μs     0.82  groupby.GroupByMethods.time_dtype_as_field('int', 'bfill', 'direct')
-         574±0μs          469±0μs     0.82  frame_methods.Iteration.time_iteritems_cached
-        426±20μs          342±0μs     0.80  groupby.GroupByMethods.time_dtype_as_field('int', 'mean', 'transformation')
-        426±30μs          342±0μs     0.80  groupby.GroupByMethods.time_dtype_as_field('int', 'prod', 'transformation')
-        133±10μs          107±6μs     0.80  groupby.GroupByMethods.time_dtype_as_field('int', 'size', 'direct')
-         334±0μs         267±20μs     0.80  groupby.GroupByMethods.time_dtype_as_field('float', 'bfill', 'direct')
-           1.41s            1.12s     0.80  groupby.Datelike.time_sum('period_range')
-        488±30μs          391±0μs     0.80  groupby.GroupByMethods.time_dtype_as_field('int', 'min', 'direct')
-          26.0ms           20.8ms     0.80  eval.Query.time_query_datetime_column
-          30.3μs           24.2μs     0.80  index_object.Indexing.time_get_loc('Float')
-        91.6±0μs         72.9±3μs     0.80  indexing.NonNumericSeriesIndexing.time_getitem_pos_slice('datetime', 'nonunique_monotonic_inc')
-        537±50μs          426±0μs     0.79  groupby.GroupByMethods.time_dtype_as_field('int', 'tail', 'direct')
-         536±0μs         426±20μs     0.79  groupby.GroupByMethods.time_dtype_as_field('int', 'nunique', 'transformation')
-        513±30μs         402±20μs     0.78  groupby.GroupByMethods.time_dtype_as_field('int', 'tail', 'transformation')
-         141±0μs          110±0μs     0.78  groupby.GroupByMethods.time_dtype_as_field('int', 'size', 'transformation')
-        868±30μs         671±40μs     0.77  groupby.GroupByMethods.time_dtype_as_field('int', 'pct_change', 'transformation')
-        868±30μs          671±0μs     0.77  groupby.GroupByMethods.time_dtype_as_field('int', 'sem', 'direct')
-        509±20μs          391±0μs     0.77  groupby.GroupByMethods.time_dtype_as_field('int', 'min', 'transformation')
-         407±5μs         311±20μs     0.76  groupby.GroupByMethods.time_dtype_as_field('int', 'std', 'transformation')
-         107±8μs         80.1±0μs     0.75  groupby.GroupByMethods.time_dtype_as_field('float', 'all', 'direct')
-        458±10μs          344±0μs     0.75  groupby.GroupByMethods.time_dtype_as_field('int', 'mean', 'direct')
-          4.29ms           3.22ms     0.75  index_object.Indexing.time_get_loc_non_unique('Float')
-        897±50μs         671±40μs     0.75  groupby.GroupByMethods.time_dtype_as_field('int', 'pct_change', 'direct')
-      3.57±0.2ms         2.67±0ms     0.75  groupby.Datelike.time_sum('date_range_tz')
-      2.88±0.2ms       2.14±0.2ms     0.75  groupby.Datelike.time_sum('date_range')
-      8.88±0.9ms         6.58±0ms     0.74  io.hdf.HDFStoreDataFrame.time_query_store_table
-        556±30μs          407±0μs     0.73  groupby.GroupByMethods.time_dtype_as_field('int', 'median', 'transformation')
-        446±20μs          326±0μs     0.73  groupby.GroupByMethods.time_dtype_as_field('int', 'ffill', 'transformation')
-        340±50μs         244±10μs     0.72  groupby.GroupByMethods.time_dtype_as_field('int', 'var', 'transformation')
-     1.20±0.09ms         862±60μs     0.72  groupby.GroupByMethods.time_dtype_as_field('int', 'cummax', 'direct')
-        119±10μs         85.4±5μs     0.72  groupby.GroupByMethods.time_dtype_as_field('int', 'any', 'transformation')
-        570±30μs         407±20μs     0.71  groupby.GroupByMethods.time_dtype_as_field('int', 'head', 'direct')
-        570±40μs         407±40μs     0.71  groupby.GroupByMethods.time_dtype_as_field('int', 'last', 'direct')
-        951±70μs          679±0μs     0.71  inference.NumericInferOps.time_add(<class 'numpy.int16'>)
-        586±50μs          407±0μs     0.69  groupby.GroupByMethods.time_dtype_as_field('int', 'median', 'direct')
-     1.22±0.09ms         837±60μs     0.69  groupby.GroupByMethods.time_dtype_as_field('int', 'cumprod', 'direct')
-        536±70μs          367±0μs     0.68  groupby.GroupByMethods.time_dtype_as_field('int', 'last', 'transformation')
-           719ms          492±4ms     0.68  groupby.GroupByMethods.time_dtype_as_field('int', 'mad', 'transformation')
-         471±0μs          322±0μs     0.68  groupby.GroupByMethods.time_dtype_as_field('int', 'bfill', 'transformation')
-         114±7μs         77.7±0μs     0.68  groupby.GroupByMethods.time_dtype_as_field('int', 'any', 'direct')
-        575±50μs          388±5μs     0.67  groupby.GroupByMethods.time_dtype_as_field('int', 'max', 'direct')
-        586±70μs          391±0μs     0.67  groupby.GroupByMethods.time_dtype_as_field('int', 'max', 'transformation')
-      1.29±0.1ms         854±60μs     0.66  groupby.GroupByMethods.time_dtype_as_field('int', 'cummin', 'transformation')
-           1.94s            1.28s     0.66  groupby.GroupByMethods.time_dtype_as_field('int', 'describe', 'transformation')
-      1.22±0.1ms         804±60μs     0.66  groupby.GroupByMethods.time_dtype_as_field('int', 'cummin', 'direct')
-        20.8±0ms         13.7±0ms     0.66  groupby.Float32.time_sum
-     1.25±0.08ms         804±40μs     0.64  groupby.GroupByMethods.time_dtype_as_field('int', 'cumprod', 'transformation')
-         220±0μs          141±8μs     0.64  groupby.GroupByMethods.time_dtype_as_field('int', 'count', 'direct')
-      1.71±0.1ms         1.09±0ms     0.64  groupby.GroupByMethods.time_dtype_as_field('int', 'value_counts', 'transformation')
-         382±0μs         244±10μs     0.64  groupby.GroupByMethods.time_dtype_as_field('int', 'var', 'direct')
-           2.03s            1.28s     0.63  groupby.GroupByMethods.time_dtype_as_field('int', 'describe', 'direct')
-        467±30μs         293±20μs     0.63  groupby.GroupByMethods.time_dtype_as_field('int', 'ffill', 'direct')
-         250±8ms          156±8ms     0.62  groupby.GroupByMethods.time_dtype_as_field('int', 'unique', 'transformation')
-        613±30μs          382±0μs     0.62  groupby.GroupByMethods.time_dtype_as_field('int', 'first', 'transformation')
-     1.34±0.08ms         822±60μs     0.61  groupby.GroupByMethods.time_dtype_as_field('int', 'cumsum', 'transformation')
-           844ms          500±0ms     0.59  groupby.GroupByMethods.time_dtype_as_field('int', 'mad', 'direct')
-        710±40μs         420±20μs     0.59  groupby.GroupByMethods.time_dtype_as_field('int', 'head', 'transformation')
-        239±10μs          141±0μs     0.59  groupby.GroupByMethods.time_dtype_as_field('int', 'count', 'transformation')
-        1.82±0ms         1.07±0ms     0.59  groupby.GroupByMethods.time_dtype_as_field('int', 'value_counts', 'direct')
-        1.44±0ms         839±60μs     0.58  groupby.GroupByMethods.time_dtype_as_field('int', 'cummax', 'transformation')
-     1.52±0.08ms          854±0μs     0.56  groupby.GroupByMethods.time_dtype_as_field('int', 'cumsum', 'direct')
-         153±0μs         80.1±0μs     0.53  groupby.GroupByMethods.time_dtype_as_field('int', 'all', 'transformation')
-         168±0μs         77.7±0μs     0.46  groupby.GroupByMethods.time_dtype_as_field('float', 'all', 'transformation')
-         171±9μs         73.2±6μs     0.43  groupby.GroupByMethods.time_dtype_as_field('float', 'any', 'direct')
-         171±9μs         73.2±6μs     0.43  groupby.GroupByMethods.time_dtype_as_field('float', 'any', 'transformation')
-     1.17±0.08ms          412±0μs     0.35  frame_ctor.FromRecords.time_frame_from_records_generator(1000)

SOME BENCHMARKS HAVE CHANGED SIGNIFICANTLY.
```

Since #22978 wasn't merged yet 10h ago, I tested against master+#22978 see 
https://github.com/pandas-dev/pandas/pull/22986/commits/640162fab9498f56ab0c93748ef4655cd0fc449f
https://github.com/pandas-dev/pandas/pull/22986/commits/9918d52b96f722495fb1a72135e991d725ba3cda I'm guessing that the perf decrease for some indexing methods will be a dealbreaker, and I don't believe that always returning the inverse will be faster than reading the `bint`. Might have to have separate  codes for unique without inverse vs. factorize/unique with inverse... > Might have to have separate codes for unique without inverse vs. factorize/unique with inverse...

Yes (which is basically what it was before? but with some additions to get_labels to satisfy the behaviour of unique)

But first, in addition to asv, I would recommend to run a few targetted benchmarks (eg unique and factorize on a single array of some dtype) manually using `%timeit` before / after. I think that will give you a better idea of the performance cost. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22986?src=pr&el=h1) Report
> Merging [#22986](https://codecov.io/gh/pandas-dev/pandas/pull/22986?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/1546c35dc839b3f6c094ad41d14cda24e9e13a0c?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22986/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22986?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #22986      +/-   ##
==========================================
- Coverage   92.19%   92.19%   -0.01%     
==========================================
  Files         169      169              
  Lines       50954    50952       -2     
==========================================
- Hits        46975    46973       -2     
  Misses       3979     3979
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.61% <100%> (-0.01%)` | :arrow_down: |
| #single | `42.27% <100%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22986?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/algorithms.py](https://codecov.io/gh/pandas-dev/pandas/pull/22986/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYWxnb3JpdGhtcy5weQ==) | `95.11% <100%> (-0.02%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22986?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22986?src=pr&el=footer). Last update [1546c35...6d0e86b](https://codecov.io/gh/pandas-dev/pandas/pull/22986?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 After trying to force compilation of the different `bint` code paths (unsuccessfully...) I now added the original (no-inverse-)unique code back in.

> But first, in addition to asv, I would recommend to run a few targetted benchmarks (eg unique and factorize on a single array of some dtype)

Following the advice of @jorisvandenbossche, I ran the following benchmarks for the commits in question (first is master + #22978; then the fully unified function; then the different functions for different `bint`, and reduplicating the no-inverse code, and finally, adding `@cython.wraparound(False)`). The hashes are the same as in this PR

Relative to master:
```
                   640162fab9  9918d52b96  52ae84e7f0  dbe4e0ed81  8481e19619
StringIndex               1.0    0.987317    0.977104    0.983180    0.970452
CategoricalIndex          1.0    1.049310    1.050426    0.983520    0.988915
IntIndex                  1.0    1.127943    1.143860    1.013043    0.986293
UIntIndex                 1.0    1.149187    1.144719    1.003053    0.989413
RangeIndex                1.0    1.126137    1.137957    0.996779    0.990026
FloatIndex                1.0    1.127896    1.097702    1.026405    1.026518
TimedeltaIndex            1.0    1.075186    1.078420    1.006917    0.988882
StringSeries              1.0    0.959835    0.983661    0.968425    0.954817
CategoricalSeries         1.0    1.074586    1.084046    1.023071    1.032467
IntSeries                 1.0    1.045707    1.043808    0.951452    0.944722
UIntSeries                1.0    1.100339    1.117216    1.001963    0.983911
RangeSeries               1.0    1.099110    1.107786    0.991575    0.980506
FloatSeries               1.0    1.085278    1.123008    0.994891    1.018042
TimedeltaSeries           1.0    1.067808    1.099913    0.996012    0.992843
```
Absolute values:
```
                        640162fab9       9918d52b96       52ae84e7f0       dbe4e0ed81       8481e19619
StringIndex        16.08 ± 0.19 ms  15.88 ± 0.34 ms  15.71 ± 0.20 ms  15.81 ± 0.21 ms  15.61 ± 0.14 ms
CategoricalIndex    1.28 ± 0.05 ms   1.35 ± 0.07 ms   1.35 ± 0.07 ms   1.26 ± 0.06 ms   1.27 ± 0.06 ms
IntIndex            2.75 ± 0.03 ms   3.10 ± 0.04 ms   3.14 ± 0.05 ms   2.78 ± 0.08 ms   2.71 ± 0.05 ms
UIntIndex           2.71 ± 0.03 ms   3.12 ± 0.03 ms   3.11 ± 0.08 ms   2.72 ± 0.03 ms   2.69 ± 0.02 ms
RangeIndex          2.75 ± 0.04 ms   3.10 ± 0.03 ms   3.13 ± 0.05 ms   2.74 ± 0.06 ms   2.72 ± 0.08 ms
FloatIndex          4.62 ± 0.08 ms   5.21 ± 0.33 ms   5.07 ± 0.08 ms   4.74 ± 0.43 ms   4.74 ± 0.32 ms
TimedeltaIndex      4.29 ± 0.04 ms   4.61 ± 0.09 ms   4.63 ± 0.06 ms   4.32 ± 0.08 ms   4.24 ± 0.08 ms
StringSeries       34.96 ± 0.50 ms  33.56 ± 0.36 ms  34.39 ± 1.19 ms  33.86 ± 0.31 ms  33.38 ± 0.51 ms
CategoricalSeries   1.22 ± 0.03 ms   1.31 ± 0.08 ms   1.32 ± 0.05 ms   1.25 ± 0.02 ms   1.26 ± 0.07 ms
IntSeries           3.41 ± 0.17 ms   3.57 ± 0.04 ms   3.56 ± 0.05 ms   3.24 ± 0.05 ms   3.22 ± 0.06 ms
UIntSeries          3.24 ± 0.03 ms   3.56 ± 0.05 ms   3.62 ± 0.07 ms   3.24 ± 0.04 ms   3.19 ± 0.04 ms
RangeSeries         3.25 ± 0.03 ms   3.57 ± 0.07 ms   3.60 ± 0.08 ms   3.22 ± 0.03 ms   3.19 ± 0.03 ms
FloatSeries         4.74 ± 0.06 ms   5.14 ± 0.11 ms   5.32 ± 0.47 ms   4.71 ± 0.05 ms   4.82 ± 0.37 ms
TimedeltaSeries     4.17 ± 0.04 ms   4.45 ± 0.10 ms   4.59 ± 0.11 ms   4.15 ± 0.04 ms   4.14 ± 0.03 ms
```
Code:
```
import pandas as pd
import pandas.util.testing as tm
import timeit
hash = pd.__git_version__[:10]

def pps(mean, std):
    system = [(1, 's'), (1e-3, 'ms'), (1e-6, 'μs'), (1e-9, 'ns'), (1e-12, 'ps')]
    for factor, suffix in system:
        if mean >= factor:
            break
    amount = round(mean / factor, 2)
    return f'{mean / factor:.2f} ± {std / factor:.2f} {suffix}'

k = 10 ** 5
rep = 10
number = 100
tic = timeit.default_timer()
tags = ['String', 'Categorical', 'Int', 'UInt',
         'Range', 'Float', 'Timedelta']
df = pd.DataFrame(index = [x+'Index' for x in tags], columns = ['mean', 'std'])
for tag in tags:
    idx = getattr(tm, f'make{tag}Index')(k=k)
    t = timeit.repeat('idx.unique()', setup='from __main__ import idx', repeat = rep+1, number = number)[1:]
    df.loc[tag+'Index', 'mean'] = pd.Series(t).mean() / number
    df.loc[tag+'Index', 'std'] = pd.Series(t).std() / number
    
    s = pd.Series(idx).sample(frac=1.1, replace=True)
    t = timeit.repeat('s.unique()', setup='from __main__ import s', repeat = rep+1, number = number)[1:]
    df.loc[tag+'Series', 'mean'] = pd.Series(t).mean() / number
    df.loc[tag+'Series', 'std'] = pd.Series(t).std() / number

res = df.apply(lambda r: pps(*tuple(r)), axis=1)
```


 Here's the ASV run with of https://github.com/pandas-dev/pandas/pull/22986/commits/8481e19619c8847b0bdb7dcb61726497f552b230 against master + #22978. I ran it with `-f 1.05` because ~10% for indexing ops would still be sensitive.

The spread still indicates to me that the results are noisy, but in any case, it's clearly skewed towards being faster now. I'll probably rerun against just `-b ^index` to see how much of the indexing stuff remains after a rerun.

BTW, is it possible to do an ASV run for a PR on a server? Or could someone else rerun it locally? Not sure how representative my machine is.
```
      before           after         ratio
     [640162fa]       [8481e196]
!      13.9±0.9ms           failed      n/a  algorithms.Duplicated.time_duplicated_float('first')
!      13.9±0.2ms           failed      n/a  algorithms.Duplicated.time_duplicated_float('last')
!        10.9±0ms           failed      n/a  binary_ops.Ops.time_frame_add(False, 'default')
+        536±40μs          744±0μs     1.39  inference.NumericInferOps.time_add(<class 'numpy.int16'>)
+          52.1ms           70.3ms     1.35  eval.Query.time_query_with_boolean_selection
+        13.7±0ms         18.2±1ms     1.33  stat_ops.Rank.time_rank('DataFrame', False)
+        13.7±0ms         18.2±0ms     1.33  stat_ops.Rank.time_rank('DataFrame', True)
+          26.6μs           34.1μs     1.28  index_object.Indexing.time_get_loc_sorted('Float')
+        85.9±0ms          109±4ms     1.27  io.csv.ReadCSVCategorical.time_convert_post
+         477±4ms          602±4ms     1.26  groupby.GroupByMethods.time_dtype_as_field('float', 'mad', 'transformation')
+          2.60ms           3.28ms     1.26  index_object.Indexing.time_get_loc_non_unique('Float')
+        78.1±1ms         97.7±4ms     1.25  binary_ops.Ops.time_frame_comparison(False, 1)
+           562ms            703ms     1.25  gil.ParallelReadCSV.time_read_csv('float')
+          2.03μs           2.44μs     1.20  index_object.Indexing.time_get('Float')
+          10.4ms           12.5ms     1.20  categoricals.CategoricalSlicing.time_getitem_bool_array('non_monotonic')
+      5.76±0.4ms         6.84±0ms     1.19  frame_methods.Fillna.time_frame_fillna(True, 'pad')
+        93.8±6ms            109ms     1.17  io.sas.SAS.time_read_msgpack('sas7bdat')
+        1.08±0ms       1.26±0.2ms     1.16  indexing.CategoricalIndexIndexing.time_getitem_list('non_monotonic')
+           2.34s            2.70s     1.15  gil.ParallelGroups.time_get_groups(4)
+          13.4μs           15.3μs     1.14  ctors.SeriesDtypesConstructors.time_dtindex_from_series
+           109ms            125ms     1.14  index_object.IndexAppend.time_append_obj_list
+        36.5±0ms         41.7±0ms     1.14  join_merge.MergeAsof.time_on_int
+         281±0ms         320±10ms     1.14  io.stata.Stata.time_write_stata('th')
+     1.71±0.06ms         1.93±0ms     1.13  reindex.DropDuplicates.time_frame_drop_dups_int(True)
+        41.7±0ms         46.9±0ms     1.12  frame_methods.Iteration.time_itertuples
+          4.58μs           5.13μs     1.12  index_object.Indexing.time_slice_step('String')
+        28.1±0ms         31.2±0ms     1.11  index_object.SetOperations.time_operation('date_string', 'symmetric_difference')
+        18.8±0ms         20.8±0ms     1.11  io.hdf.HDFStoreDataFrame.time_read_store
+           4.52s            5.00s     1.11  gil.ParallelGroups.time_get_groups(8)
+           103μs            114μs     1.11  frame_methods.XS.time_frame_xs(0)
+        68.7±0μs         72.9±0μs     1.06  indexing.NumericSeriesIndexing.time_getitem_slice(<class 'pandas.core.indexes.numeric.Float64Index'>, 'unique_monotonic_inc')
+        73.2±0μs         77.7±0μs     1.06  indexing.NumericSeriesIndexing.time_ix_scalar(<class 'pandas.core.indexes.numeric.Float64Index'>, 'unique_monotonic_inc')
+          4.40μs           4.66μs     1.06  categoricals.CategoricalSlicing.time_getitem_slice('monotonic_decr')
+        397±20μs          419±0μs     1.05  stat_ops.SeriesOps.time_op('mean', 'float', False)
+           1.19s            1.25s     1.05  join_merge.I8Merge.time_i8merge('left')
-        20.7±0μs       19.7±0.4μs     0.95  indexing.NonNumericSeriesIndexing.time_getitem_scalar('datetime', 'nonunique_monotonic_inc')
-         312±0ms          297±0ms     0.95  io.json.ToJSON.time_delta_int_tstamp_lines('index')
-         312±0ms          297±0ms     0.95  io.json.ToJSON.time_delta_int_tstamp_lines('split')
-      2.82±0.2μs         2.67±0μs     0.95  categoricals.Contains.time_categorical_index_contains
-          8.27ms           7.81ms     0.94  index_object.Indexing.time_boolean_array('Float')
-        4.24±0μs         4.01±0μs     0.94  io.hdf.HDFStoreDataFrame.time_store_repr
-         365±0μs         344±20μs     0.94  indexing.CategoricalIndexIndexing.time_getitem_bool_array('monotonic_decr')
-          7.77μs           7.30μs     0.94  categoricals.CategoricalSlicing.time_getitem_list_like('non_monotonic')
-        83.3±3ms         78.1±0ms     0.94  rolling.Quantile.time_quantile('DataFrame', 1000, 'float', 0.5, 'nearest')
-        1.23±0ms      1.15±0.08ms     0.93  categoricals.Constructor.time_datetimes_with_nat
-        9.77±1ms       9.11±0.2ms     0.93  strings.Cat.time_cat(0, None, '-', 0.001)
-        4.52±0ms       4.21±0.3ms     0.93  algorithms.Hashing.time_series_int
-     1.34±0.08μs       1.24±0.1μs     0.93  period.PeriodProperties.time_property('min', 'dayofweek')
-        372±30ns          344±0ns     0.92  timestamp.TimestampProperties.time_is_quarter_start(None, None)
-         102±0ms         93.8±0ms     0.92  frame_methods.Count.time_count_level_multi(1)
-        4.56±0ms       4.21±0.3ms     0.92  algorithms.Hashing.time_series_timedeltas
-         188±0ms          172±0ms     0.92  io.hdf.HDFStoreDataFrame.time_write_store_table_dc
-         671±0μs          613±0μs     0.91  indexing.DataFrameNumericIndexing.time_bool_indexer
-           1.23s            1.12s     0.91  groupby.GroupByMethods.time_dtype_as_group('float', 'mad', 'transformation')
-      2.29±0.2ms         2.08±0ms     0.91  stat_ops.SeriesOps.time_op('var', 'float', True)
-        7.81±0ms       7.10±0.2ms     0.91  index_object.Ops.time_add('float')
-        61.0±0μs         55.5±1μs     0.91  indexing.NumericSeriesIndexing.time_loc_scalar(<class 'pandas.core.indexes.numeric.Int64Index'>, 'nonunique_monotonic_inc')
-         123±0μs          111±1μs     0.90  indexing.NumericSeriesIndexing.time_loc_slice(<class 'pandas.core.indexes.numeric.Int64Index'>, 'nonunique_monotonic_inc')
-      4.32±0.3ms       3.91±0.1ms     0.90  rolling.Quantile.time_quantile('DataFrame', 1000, 'float', 1, 'lower')
-         156±0ms          141±6ms     0.90  strings.Split.time_split(False)
-      9.38±0.6ms         8.41±0ms     0.90  timeseries.DatetimeIndex.time_normalize('repeated')
-        383±30ns         343±10ns     0.90  timestamp.TimestampProperties.time_quarter(None, None)
-        4.74±0μs         4.24±0μs     0.89  indexing.CategoricalIndexIndexing.time_getitem_scalar('monotonic_decr')
-          13.7ms           12.2ms     0.89  index_object.Indexing.time_boolean_array('String')
-          13.7ms           12.2ms     0.89  index_object.Indexing.time_boolean_series('String')
-         141±0ms          125±0ms     0.89  io.excel.Excel.time_read_excel('openpyxl')
-      93.8±0.7ms         83.3±0ms     0.89  rolling.Quantile.time_quantile('DataFrame', 1000, 'float', 0.5, 'linear')
-      93.8±0.7ms         83.3±3ms     0.89  rolling.Quantile.time_quantile('DataFrame', 1000, 'float', 0.5, 'midpoint')
-        46.9±4ms         41.7±0ms     0.89  strings.Methods.time_title
-         541±0μs          478±0μs     0.88  indexing.NumericSeriesIndexing.time_ix_list_like(<class 'pandas.core.indexes.numeric.Int64Index'>, 'unique_monotonic_inc')
-        88.5±5ms         78.1±0ms     0.88  rolling.Quantile.time_quantile('DataFrame', 1000, 'float', 0.5, 'higher')
-        88.5±5ms         78.1±3ms     0.88  rolling.Quantile.time_quantile('DataFrame', 1000, 'float', 0.5, 'lower')
-         121±0μs          107±0μs     0.88  panel_methods.PanelMethods.time_shift('items')
-        391±20ns         343±20ns     0.88  timestamp.TimestampProperties.time_quarter(<DstTzInfo 'Europe/Amsterdam' LMT+0:20:00 STD>, None)
-          4.88ms           4.29ms     0.88  index_object.Indexing.time_get_loc_non_unique_sorted('Float')
-          20.8ms           18.2ms     0.88  eval.Query.time_query_datetime_column
-      10.4±0.9ms         9.11±0ms     0.88  strings.Cat.time_cat(0, ',', '-', 0.0)
-           125ms            109ms     0.88  strings.Repeat.time_repeat('int')
-        15.6±1ms       13.7±0.7ms     0.88  timeseries.AsOf.time_asof('DataFrame')
-      3.91±0.2ms         3.42±0ms     0.88  timeseries.ToDatetimeCache.time_dup_string_dates(True)
-        458±10ns         401±30ns     0.87  timestamp.TimestampProperties.time_dayofyear(None, None)
-        78.7±0μs         68.7±0μs     0.87  indexing.NumericSeriesIndexing.time_iloc_list_like(<class 'pandas.core.indexes.numeric.Int64Index'>, 'unique_monotonic_inc')
-        48.6±3μs         42.4±2μs     0.87  timestamp.TimestampProperties.time_is_year_start(<DstTzInfo 'Europe/Amsterdam' LMT+0:20:00 STD>, 'B')
-      4.51±0.1ms      3.91±0.09ms     0.87  rolling.Quantile.time_quantile('DataFrame', 1000, 'float', 0, 'midpoint')
-         406±0ms          352±4ms     0.87  timeseries.ToDatetimeISO8601.time_iso8601_tz_spaceformat
-         117±6μs          101±2μs     0.86  indexing.NumericSeriesIndexing.time_getitem_slice(<class 'pandas.core.indexes.numeric.Float64Index'>, 'nonunique_monotonic_inc')
-        473±50μs          407±0μs     0.86  groupby.GroupByMethods.time_dtype_as_group('float', 'sum', 'transformation')
-        54.7±1ms         46.9±0ms     0.86  rolling.Quantile.time_quantile('Series', 10, 'int', 0.5, 'linear')
-        54.7±0ms         46.9±3ms     0.86  strings.Methods.time_center
-        54.7±0ms         46.9±3ms     0.86  strings.Methods.time_pad
-        36.5±0ms         31.2±0ms     0.86  strings.Methods.time_strip
-        13.7±3ms         11.7±0ms     0.86  timedelta.ToTimedelta.time_convert_string_seconds
-      4.56±0.2ms         3.91±0ms     0.86  timeseries.ToDatetimeCache.time_dup_string_tzoffset_dates(True)
-      3.12±0.3ms       2.67±0.2ms     0.86  timeseries.DatetimeIndex.time_add_timedelta('tz_naive')
-        49.1±5μs       42.0±0.5μs     0.86  timestamp.TimestampProperties.time_is_quarter_end(<DstTzInfo 'Europe/Amsterdam' LMT+0:20:00 STD>, 'B')
-        313±30ns          267±0ns     0.85  timedelta.DatetimeAccessor.time_dt_accessor
-        50.0±6μs         42.4±0μs     0.85  timestamp.TimestampProperties.time_is_year_end(<DstTzInfo 'Europe/Amsterdam' LMT+0:20:00 STD>, 'B')
-      7.40±0.2ms       6.25±0.1ms     0.84  timeseries.ResampleSeries.time_resample('period', '5min', 'mean')
-           1.17s            984ms     0.84  groupby.Apply.time_copy_function_multi_col
-      4.65±0.1ms       3.91±0.1ms     0.84  rolling.Quantile.time_quantile('DataFrame', 1000, 'float', 1, 'midpoint')
-         466±0ns         391±20ns     0.84  timestamp.TimestampProperties.time_week(None, None)
-      7.25±0.6ms       6.08±0.4ms     0.84  timeseries.DatetimeIndex.time_timeseries_is_month_start('tz_naive')
-        21.9±0ms         18.2±0ms     0.83  inference.DateInferOps.time_timedelta_plus_datetime
-        10.9±0ms       9.11±0.7ms     0.83  strings.Cat.time_cat(0, None, '-', 0.0)
-      10.9±0.8ms         9.11±0ms     0.83  strings.Cat.time_cat(0, None, None, 0.001)
-      10.9±0.6ms       9.11±0.5ms     0.83  timeseries.DatetimeAccessor.time_dt_accessor_normalize
-        93.8±0ms         78.1±1ms     0.83  strings.Cat.time_cat(3, ',', '-', 0.15)
-         375±8ms          312±0ms     0.83  strings.Split.time_split(True)
-         188±4ms          156±8ms     0.83  timeseries.DatetimeIndex.time_timeseries_is_month_start('tz_aware')
-           703ms          586±4ms     0.83  timeseries.ToDatetimeFormat.time_no_exact
-        17.0±1μs         14.1±0μs     0.83  timestamp.TimestampProperties.time_is_year_end(None, 'B')
-           1.56s            1.30s     0.83  groupby.GroupByMethods.time_dtype_as_field('int', 'describe', 'transformation')
-           734ms            609ms     0.83  timeseries.ToDatetimeFormat.time_exact
-        52.7±2ms         43.7±2ms     0.83  rolling.Quantile.time_quantile('Series', 10, 'int', 0.5, 'higher')
-        23.6±1μs       19.5±0.9μs     0.83  timeseries.AsOf.time_asof_single('Series')
-         180±4ms          148±4ms     0.83  frame_methods.Iteration.time_iteritems_indexing
-        486±30ns          401±0ns     0.82  timestamp.TimestampProperties.time_week(<DstTzInfo 'Europe/Amsterdam' LMT+0:20:00 STD>, 'B')
-        66.4±4ms         54.7±4ms     0.82  strings.Cat.time_cat(3, None, '-', 0.001)
-      7.81±0.8ms       6.43±0.1ms     0.82  timeseries.ToDatetimeYYYYMMDD.time_format_YYYYMMDD
-        568±30μs         467±30μs     0.82  multiindex_object.Values.time_datetime_level_values_sliced
-        343±30ns          282±0ns     0.82  timestamp.TimestampProperties.time_microsecond(None, None)
-        172±20μs          142±0μs     0.82  groupby.GroupByMethods.time_dtype_as_field('float', 'count', 'transformation')
-        378±30ns         311±20ns     0.82  timestamp.TimestampProperties.time_tz(None, None)
-      3.51±0.2ms         2.88±0ms     0.82  timeseries.DatetimeIndex.time_add_timedelta('repeated')
-      10.4±0.9ms       8.52±0.5ms     0.82  strings.Cat.time_cat(0, None, None, 0.0)
-      8.37±0.6ms       6.84±0.4ms     0.82  algorithms.Hashing.time_series_categorical
-        491±60ns          401±0ns     0.82  timestamp.TimestampProperties.time_week(None, 'B')
-         211±8ms          172±0ms     0.81  timeseries.ToDatetimeCache.time_dup_string_tzoffset_dates(False)
-           1.09s            891ms     0.81  timedelta.TimedeltaOps.time_add_td_ts
-           250ms            203ms     0.81  gil.ParallelKth.time_kth_smallest
-     1.35±0.07ms         1.09±0ms     0.81  timeseries.InferFreq.time_infer_freq(None)
-      4.84±0.2ms       3.91±0.1ms     0.81  rolling.Quantile.time_quantile('DataFrame', 1000, 'float', 0, 'higher')
-        10.4±0ms       8.41±0.5ms     0.81  strings.Cat.time_cat(0, ',', '-', 0.001)
-      5.02±0.3ms       4.04±0.1ms     0.80  rolling.Quantile.time_quantile('DataFrame', 1000, 'float', 0, 'nearest')
-           797ms          641±0ms     0.80  inference.MaybeConvertNumeric.time_convert
-      4.62±0.4ms       3.70±0.2ms     0.80  timeseries.AsOf.time_asof_nan_single('DataFrame')
-      3.76±0.1ms       3.01±0.1ms     0.80  rolling.Quantile.time_quantile('DataFrame', 1000, 'int', 1, 'midpoint')
-      2.74±0.2ms         2.20±0ms     0.80  stat_ops.SeriesOps.time_op('var', 'int', True)
-        458±60ns         366±20ns     0.80  timestamp.TimestampProperties.time_days_in_month(None, None)
-        78.1±4ms         62.5±2ms     0.80  rolling.Quantile.time_quantile('DataFrame', 1000, 'int', 0.5, 'midpoint')
-        58.6±4ms         46.9±0ms     0.80  rolling.Quantile.time_quantile('Series', 10, 'int', 0.5, 'midpoint')
-        78.1±0ms         62.5±4ms     0.80  strings.Cat.time_cat(3, None, '-', 0.15)
-        305±20ns         244±10ns     0.80  timedelta.TimedeltaProperties.time_timedelta_seconds
-        31.2±2ms         25.0±0ms     0.80  timeseries.DatetimeIndex.time_to_date('repeated')
-        54.7±0ms         43.7±2ms     0.80  rolling.Quantile.time_quantile('Series', 10, 'int', 0.5, 'lower')
-        27.3±2ms         21.9±0ms     0.80  strings.Contains.time_contains(False)
-      4.63±0.1ms      3.70±0.05ms     0.80  timeseries.ResampleSeries.time_resample('period', '1D', 'mean')
-      3.98±0.1ms      3.17±0.09ms     0.80  rolling.Quantile.time_quantile('DataFrame', 1000, 'int', 1, 'nearest')
-           4.09s            3.27s     0.80  timeseries.ToDatetimeNONISO8601.time_different_offset
-        1.33±0ms         1.06±0ms     0.80  timeseries.InferFreq.time_infer_freq('D')
-        430±20ns         342±20ns     0.79  timestamp.TimestampProperties.time_is_quarter_start(<DstTzInfo 'Europe/Amsterdam' LMT+0:20:00 STD>, None)
-        44.3±5ms         35.2±1ms     0.79  strings.Methods.time_endswith
-        375±10ms          297±2ms     0.79  strings.Methods.time_extract
-           24.8s            19.6s     0.79  strings.Dummies.time_get_dummies
-      3.76±0.2ms      2.97±0.02ms     0.79  rolling.Quantile.time_quantile('DataFrame', 1000, 'int', 1, 'lower')
-        477±10ms          375±4ms     0.79  groupby.Groups.time_series_groups('int64_large')
-        219±10ms          172±0ms     0.79  timeseries.DatetimeIndex.time_to_date('tz_aware')
-      5.07±0.2ms       3.98±0.1ms     0.79  rolling.Quantile.time_quantile('DataFrame', 1000, 'float', 0, 'linear')
-        16.4±1μs       12.8±0.9μs     0.79  timestamp.TimestampOps.time_replace_None('US/Eastern')
-           2.17s            1.70s     0.78  timeseries.ToDatetimeNONISO8601.time_same_offset
-        342±20ns          267±0ns     0.78  timestamp.TimestampProperties.time_microsecond(None, 'B')
-        31.1±0μs         24.3±0μs     0.78  timestamp.TimestampAcrossDst.time_replace_across_dst
-        2.50±0μs         1.95±0μs     0.78  timedelta.TimedeltaConstructor.time_from_missing
-          4.39μs           3.44μs     0.78  categoricals.CategoricalSlicing.time_getitem_scalar('monotonic_decr')
-      7.81±0.5μs         6.10±0μs     0.78  timedelta.TimedeltaConstructor.time_from_unit
-        938±60ns         732±60ns     0.78  timestamp.TimestampOps.time_to_pydatetime(None)
-         343±0ns          267±0ns     0.78  timestamp.TimestampProperties.time_microsecond(<DstTzInfo 'Europe/Amsterdam' LMT+0:20:00 STD>, None)
-          17.6ms           13.7ms     0.78  categoricals.Isin.time_isin_categorical('int64')
-        70.3±5ms         54.7±0ms     0.78  rolling.Quantile.time_quantile('DataFrame', 1000, 'int', 0.5, 'higher')
-        6.87±0μs       5.34±0.4μs     0.78  timedelta.TimedeltaConstructor.time_from_int
-        80.7±3ms         62.5±0ms     0.77  rolling.Quantile.time_quantile('DataFrame', 1000, 'int', 0.5, 'linear')
-        484±10ms          375±4ms     0.77  groupby.Apply.time_copy_overhead_single_col
-         442±0ns          342±0ns     0.77  timestamp.TimestampProperties.time_is_quarter_end(<DstTzInfo 'Europe/Amsterdam' LMT+0:20:00 STD>, None)
-        674±40μs          521±0μs     0.77  indexing.NumericSeriesIndexing.time_ix_list_like(<class 'pandas.core.indexes.numeric.Int64Index'>, 'nonunique_monotonic_inc')
-        443±30ns          342±0ns     0.77  timestamp.TimestampProperties.time_is_year_start(<DstTzInfo 'Europe/Amsterdam' LMT+0:20:00 STD>, None)
-      5.36±0.2ms         4.09±0ms     0.76  rolling.Quantile.time_quantile('Series', 10, 'int', 1, 'linear')
-        13.0±1ms       9.94±0.7ms     0.76  strings.Cat.time_cat(0, None, None, 0.15)
-        1.91±0ms         1.46±0ms     0.76  timeseries.InferFreq.time_infer_freq('B')
-      3.98±0.1ms       3.01±0.1ms     0.76  rolling.Quantile.time_quantile('DataFrame', 1000, 'int', 1, 'higher')
-      4.52±0.1ms         3.42±0ms     0.76  timeseries.AsOf.time_asof_single('DataFrame')
-        487±30μs         366±20μs     0.75  timeseries.DatetimeIndex.time_add_timedelta('dst')
-        31.2±1ms         23.4±0ms     0.75  algorithms.Hashing.time_frame
-        93.8±4ms         70.3±4ms     0.75  strings.Cat.time_cat(3, ',', '-', 0.0)
-        62.5±8ms         46.9±5ms     0.75  strings.Cat.time_cat(3, ',', None, 0.15)
-        41.7±3ms         31.2±2ms     0.75  strings.Methods.time_len
-        46.9±3ms         35.2±2ms     0.75  strings.Methods.time_startswith
-      5.58±0.5ms       4.15±0.1ms     0.74  rolling.Quantile.time_quantile('Series', 10, 'int', 1, 'higher')
-      7.35±0.5ms       5.47±0.2ms     0.74  timeseries.ToDatetimeISO8601.time_iso8601_format
-      4.11±0.2ms       3.05±0.1ms     0.74  rolling.Quantile.time_quantile('DataFrame', 1000, 'int', 0, 'midpoint')
-        70.3±0ms         52.1±0ms     0.74  strings.Cat.time_cat(3, None, None, 0.0)
-      14.1±0.8ms         10.4±0ms     0.74  strings.Cat.time_cat(0, None, '-', 0.15)
-      2.47±0.1ms       1.82±0.1ms     0.74  stat_ops.SeriesOps.time_op('var', 'float', False)
-      5.21±0.3ms       3.83±0.1ms     0.74  rolling.Quantile.time_quantile('DataFrame', 1000, 'float', 0, 'lower')
-        75.5±3ms         54.7±0ms     0.72  rolling.Quantile.time_quantile('DataFrame', 1000, 'int', 0.5, 'lower')
-      4.12±0.2ms      2.98±0.09ms     0.72  rolling.Quantile.time_quantile('DataFrame', 1000, 'int', 1, 'linear')
-        204±30ns          147±0ns     0.72  timeseries.DatetimeAccessor.time_dt_accessor
-      3.47±0.4ms         2.49±0ms     0.72  timeseries.ToDatetimeCache.time_dup_string_dates_and_format(False)
-        21.9±2ms         15.6±0ms     0.71  algorithms.Hashing.time_series_string
-      6.49±0.4μs         4.58±0μs     0.71  timeseries.DatetimeIndex.time_get('tz_naive')
-        488±20ns          343±0ns     0.70  timestamp.TimestampProperties.time_is_year_end(None, None)
-          52.1ms           36.5ms     0.70  eval.Query.time_query_datetime_index
-      2.69±0.4μs       1.83±0.1μs     0.68  timestamp.TimestampConstruction.time_parse_iso8601_no_tz
-      7.29±0.3ms         4.97±0ms     0.68  timeseries.ToDatetimeISO8601.time_iso8601_format_no_sep
-        46.9±8ms         31.2±0ms     0.67  strings.Methods.time_lstrip
-      7.49±0.6ms       4.84±0.5ms     0.65  rolling.Quantile.time_quantile('Series', 10, 'float', 0, 'nearest')
-      2.67±0.2ms       1.71±0.2ms     0.64  timedelta.DatetimeAccessor.time_timedelta_nanoseconds
-      2.68±0.2ms       1.66±0.1ms     0.62  timedelta.DatetimeAccessor.time_timedelta_days
-       997±200μs          613±0μs     0.61  inference.NumericInferOps.time_multiply(<class 'numpy.int16'>)
-      2.60±0.1ms         1.59±0ms     0.61  timedelta.DatetimeAccessor.time_timedelta_seconds
-           1.05s          609±0ms     0.58  timeseries.Iteration.time_iter(<function date_range at 0x000001C81A7DB7B8>)
-        195±70μs          111±0μs     0.57  series_methods.SeriesConstructor.time_constructor(None)
-         977±0μs          518±0μs     0.53  inference.NumericInferOps.time_subtract(<class 'numpy.int16'>)
-        1.09±0ms          570±0μs     0.52  inference.NumericInferOps.time_multiply(<class 'numpy.uint16'>)
-         117±8ms         33.9±0ms     0.29  join_merge.Align.time_series_align_left_monotonic 
```  And here's the one with `@cython.wraparound(False)`:
On some calls, this can completely avoid python interaction, it seems - there's a ~180'000 speed-up for `multiindex_object.Integer.time_is_monotonic`, for example. The outliers on the upper end seem more harmless to me - and less index-related. 

Purely from what's happening in the code:
* `hashtable.unique` goes through one more call layer to `hashtable._unique_no_inverse` (which is unchanged from before), with the layer just forwarding the arguments.
* `hashtable.get_labels` goes through `hashtable._unique_with_inverse` and has a tiny overhead for evaluating the `bint ignore_na` in the loop (which should however be small compared to `(val != val or (use_na_value and val == na_value2)` in the exact same if-condition).

To me, these changes shouldn't amount to much difference at all - I'm guessing the spread should mostly be the result of randomness (i.e. what else is happening on the computer in that exact  moment), although the mean is (luckily) skewed towards being faster.

What do you all think? Also repeating my previous point:
> BTW, is it possible to do an ASV run for a PR on a server? Or could someone else rerun it locally? Not sure how representative my machine is.

```
      before           after         ratio
     [640162fa]       [27ceb4d6]
!        9.11±0ms           failed      n/a  algorithms.Duplicated.time_duplicated_int('first')
!      9.11±0.3ms           failed      n/a  algorithms.Duplicated.time_duplicated_int('last')
!        258±10ms           failed      n/a  io.json.ToJSON.time_floats_with_dt_index_lines('index')
+         105±9μs        434±100μs     4.13  series_methods.SeriesConstructor.time_constructor(None)
+        133±10μs         244±20μs     1.83  groupby.GroupByMethods.time_dtype_as_field('float', 'count', 'direct')
+          36.5ms           62.5ms     1.71  eval.Query.time_query_datetime_index
+         142±8μs         229±30μs     1.61  groupby.GroupByMethods.time_dtype_as_field('float', 'count', 'transformation')
+        17.1±0μs         27.5±3μs     1.61  offset.OffestDatetimeArithmetic.time_apply(<BusinessMonthEnd>)
+         484±0μs        767±100μs     1.59  inference.NumericInferOps.time_add(<class 'numpy.uint16'>)
+          46.9ms           70.3ms     1.50  eval.Query.time_query_with_boolean_selection
+      1.46±0.1ms       2.20±0.1ms     1.50  timedelta.DatetimeAccessor.time_timedelta_days
+        23.4±0ms       33.9±0.7ms     1.44  io.sql.WriteSQLDtypes.time_to_sql_dataframe_column('sqlite', 'float')
+     1.49±0.09ms         2.14±0ms     1.44  timedelta.DatetimeAccessor.time_timedelta_microseconds
+         243±0μs          344±0μs     1.42  groupby.GroupByMethods.time_dtype_as_field('float', 'cumcount', 'transformation')
+          4.26ms           5.95ms     1.40  categoricals.CategoricalSlicing.time_getitem_bool_array('monotonic_decr')
+        26.1±2μs         35.5±2μs     1.36  offset.OffestDatetimeArithmetic.time_add_10(<SemiMonthEnd: day_of_month=15>)
+         250±0ms          328±4ms     1.31  io.json.ToJSON.time_floats_with_dt_index_lines('columns')
+           486μs            592μs     1.22  categoricals.CategoricalSlicing.time_getitem_list('monotonic_decr')
+         500±8ms         594±20ms     1.19  groupby.GroupByMethods.time_dtype_as_field('int', 'mad', 'direct')
+      6.58±0.4ms         7.81±0ms     1.19  index_object.Ops.time_add('float')
+          4.39μs           5.13μs     1.17  categoricals.CategoricalSlicing.time_getitem_slice('monotonic_incr')
+          3.12ms           3.65ms     1.17  index_object.Indexing.time_get_loc_non_unique('Float')
+          13.7ms           15.6ms     1.14  categoricals.Isin.time_isin_categorical('int64')
+          12.2ms           13.9ms     1.14  index_object.Indexing.time_boolean_series('String')
+        15.6±0ms         17.9±0ms     1.14  index_object.SetOperations.time_operation('date_string', 'union')
+      9.23±0.4ms           10.4ms     1.13  index_object.Indexing.time_boolean_array('Float')
+        31.2±0ms         35.2±0ms     1.12  eval.Eval.time_chained_cmp('numexpr', 1)
+           125ms            141ms     1.12  index_object.Indexing.time_get_loc_sorted('String')
+          10.2ms         11.4±0ms     1.11  index_object.Indexing.time_boolean_series('Float')
+         828±8ms          906±0ms     1.09  gil.ParallelGroups.time_get_groups(2)
+         102±0μs          111±0μs     1.09  indexing.NumericSeriesIndexing.time_ix_scalar(<class 'pandas.core.indexes.numeric.Float64Index'>, 'nonunique_monotonic_inc')
+          6.46μs           7.06μs     1.09  categoricals.CategoricalSlicing.time_getitem_list_like('non_monotonic')
+        404±20ns         442±30ns     1.09  timestamp.TimestampProperties.time_freqstr(None, 'B')
+        91.6±1μs         99.4±0μs     1.09  indexing.NumericSeriesIndexing.time_iloc_array(<class 'pandas.core.indexes.numeric.Float64Index'>, 'unique_monotonic_inc')
+          4.30μs           4.66μs     1.08  categoricals.CategoricalSlicing.time_getitem_slice('non_monotonic')
+          28.3μs           30.5μs     1.08  ctors.SeriesDtypesConstructors.time_dtindex_from_index_with_series
+        47.5±0μs         50.9±1μs     1.07  indexing.NumericSeriesIndexing.time_loc_scalar(<class 'pandas.core.indexes.numeric.Float64Index'>, 'unique_monotonic_inc')
+          11.7ms           12.5ms     1.07  index_object.Indexing.time_boolean_array('String')
+           547ms            578ms     1.06  gil.ParallelReadCSV.time_read_csv('float')
+        10.7±0ms         11.3±0ms     1.06  rolling.Methods.time_rolling('Series', 10, 'int', 'kurt')
+          6.72μs           7.06μs     1.05  categoricals.CategoricalSlicing.time_getitem_list_like('monotonic_incr')
-         160±8μs          153±0μs     0.95  indexing.NumericSeriesIndexing.time_ix_slice(<class 'pandas.core.indexes.numeric.Float64Index'>, 'nonunique_monotonic_inc')
-          4.82μs           4.58μs     0.95  frame_methods.XS.time_frame_xs(1)
-     1.19±0.02ms         1.13±0ms     0.95  frame_methods.Isnull.time_isnull
-         430±4ms          406±0ms     0.95  io.json.ReadJSONLines.time_read_json_lines('int')
-        8.27±0ms       7.81±0.4ms     0.94  indexing.NumericSeriesIndexing.time_getitem_lists(<class 'pandas.core.indexes.numeric.Int64Index'>, 'unique_monotonic_inc')
-        3.17±0ms      2.97±0.02ms     0.94  rolling.Quantile.time_quantile('DataFrame', 10, 'int', 1, 'lower')
-      4.63±0.1ms       4.32±0.2ms     0.93  reindex.DropDuplicates.time_frame_drop_dups_na(True)
-      22.9±0.5μs         21.4±0μs     0.93  indexing.NumericSeriesIndexing.time_getitem_scalar(<class 'pandas.core.indexes.numeric.Float64Index'>, 'unique_monotonic_inc')
-           1.12s            1.05s     0.93  stat_ops.FrameMultiIndexOps.time_op([0, 1], 'kurt')
-         219±0ms          203±0ms     0.93  frame_methods.Iteration.time_iterrows
-           219ms            203ms     0.93  gil.ParallelKth.time_kth_smallest
-          7.86μs           7.28μs     0.93  categoricals.CategoricalSlicing.time_getitem_list_like('monotonic_decr')
-         528±0μs          488±0μs     0.92  frame_methods.Iteration.time_iteritems_cached
-         432±0μs          398±0μs     0.92  frame_ctor.FromRecords.time_frame_from_records_generator(1000)
-        4.51±0ms         4.15±0ms     0.92  rolling.Methods.time_rolling('DataFrame', 10, 'int', 'skew')
-        4.97±0ms         4.56±0ms     0.92  stat_ops.SeriesMultiIndexOps.time_op(0, 'var')
-         172±0ms          156±0ms     0.91  indexing.NonNumericSeriesIndexing.time_getitem_list_like('datetime', 'nonunique_monotonic_inc')
-          3.81μs           3.43μs     0.90  categoricals.CategoricalSlicing.time_getitem_scalar('monotonic_decr')
-           156ms            141ms     0.90  index_object.Indexing.time_get_loc_non_unique('String')
-        13.7±0ms         12.2±0ms     0.89  replace.FillNa.time_replace(False)
-         211±4ms          188±0ms     0.89  frame_methods.Dropna.time_dropna_axis_mixed_dtypes('any', 0)
-        46.9±0ms         41.7±0ms     0.89  io.hdf.HDFStoreDataFrame.time_write_store_table
-          35.5μs           31.5μs     0.89  index_object.Indexing.time_get_loc_sorted('Float')
-         346±0μs          306±5μs     0.88  categoricals.Constructor.time_from_codes_all_int8
-          4.98μs           4.39μs     0.88  categoricals.CategoricalSlicing.time_getitem_slice('monotonic_decr')
-           117μs            103μs     0.88  frame_methods.XS.time_frame_xs(0)
-         108±0μs         95.4±0μs     0.88  indexing.NumericSeriesIndexing.time_ix_slice(<class 'pandas.core.indexes.numeric.Float64Index'>, 'unique_monotonic_inc')
-          14.6μs           12.9μs     0.88  ctors.SeriesDtypesConstructors.time_dtindex_from_series
-      32.6±0.7μs         28.6±0μs     0.88  indexing.NumericSeriesIndexing.time_getitem_scalar(<class 'pandas.core.indexes.numeric.Float64Index'>, 'nonunique_monotonic_inc')
-          2.91ms           2.55ms     0.88  index_object.Indexing.time_get_loc_non_unique_sorted('Float')
-         122±0μs          105±0μs     0.86  indexing.NumericSeriesIndexing.time_loc_slice(<class 'pandas.core.indexes.numeric.Float64Index'>, 'nonunique_monotonic_inc')
-      7.25±0.8μs       6.22±0.3μs     0.86  timedelta.TimedeltaConstructor.time_from_unit
-          37.9μs           32.3μs     0.85  index_object.Indexing.time_get_loc('Float')
-         903±0μs         766±10μs     0.85  indexing.NumericSeriesIndexing.time_getitem_list_like(<class 'pandas.core.indexes.numeric.Int64Index'>, 'nonunique_monotonic_inc')
-      7.72±0.4μs       6.48±0.4μs     0.84  timedelta.TimedeltaConstructor.time_from_string
-         135±5μs          113±4μs     0.84  index_object.SetOperations.time_operation('datetime', 'union')
-          5.54μs           4.58μs     0.83  index_object.Indexing.time_slice_step('String')
-        17.7±0μs         14.6±0μs     0.82  timedelta.TimedeltaConstructor.time_from_components
-          15.3μs           12.5μs     0.82  index_object.Indexing.time_slice('Float')
-         179±4μs          147±3μs     0.82  indexing.NumericSeriesIndexing.time_ix_slice(<class 'pandas.core.indexes.numeric.Int64Index'>, 'nonunique_monotonic_inc')
-      6.25±0.4ms       5.08±0.4ms     0.81  io.csv.ReadCSVFloatPrecision.time_read_csv_python_engine(',', '_', None)
-        39.1±3ms         31.2±0ms     0.80  strings.Methods.time_strip
-        27.3±3ms         21.9±0ms     0.80  timedelta.ToTimedelta.time_convert_string_days
-        62.5±4ms         49.5±3ms     0.79  strings.Cat.time_cat(3, None, None, 0.15)
-        93.8±0ms         74.2±4ms     0.79  series_methods.IsInFloat64.time_isin_nan_values
-        182±10μs         144±10μs     0.79  timedelta.ToTimedelta.time_convert_int
-      13.9±0.9ms         10.9±0ms     0.79  algorithms.Duplicated.time_duplicated_float('last')
-           1.16s            906ms     0.78  timedelta.TimedeltaOps.time_add_td_ts
-        3.51±0ms         2.73±0ms     0.78  timeseries.DatetimeIndex.time_add_timedelta('tz_naive')
-      5.73±0.4μs       4.41±0.2μs     0.77  timedelta.TimedeltaConstructor.time_from_np_timedelta
-        15.8±0μs       12.1±0.9μs     0.77  timedelta.TimedeltaConstructor.time_from_iso_format
-        7.21±0μs       5.50±0.4μs     0.76  timedelta.TimedeltaConstructor.time_from_int
-      9.38±0.6μs         7.06±0μs     0.75  timedelta.TimedeltaConstructor.time_from_datetime_timedelta
-       93.8±20ms         70.3±0ms     0.75  strings.Cat.time_cat(3, None, '-', 0.15)
-        41.7±3ms         31.2±0ms     0.75  strings.Methods.time_len
-        62.5±1ms         46.9±3ms     0.75  strings.Methods.time_pad
-      12.2±0.7ms       9.11±0.5ms     0.75  timeseries.DatetimeAccessor.time_dt_accessor_normalize
-        691±70μs          518±0μs     0.75  inference.NumericInferOps.time_subtract(<class 'numpy.uint16'>)
-      3.91±0.3ms       2.90±0.2ms     0.74  timeseries.DatetimeIndex.time_add_timedelta('tz_aware')
-        70.3±2ms         52.1±1ms     0.74  strings.Cat.time_cat(3, ',', None, 0.0)
-        70.3±0ms       52.1±0.7ms     0.74  strings.Cat.time_cat(3, ',', None, 0.001)
-         422±0ms          312±0ms     0.74  strings.Split.time_split(True)
-         234±8ms          172±8ms     0.73  strings.Slice.time_vector_slice
-         109±4ms         78.1±0ms     0.71  strings.Cat.time_cat(3, ',', '-', 0.15)
-      2.77±0.1μs         1.94±0μs     0.70  timedelta.TimedeltaConstructor.time_from_missing
-        78.1±1ms         54.7±0ms     0.70  strings.Cat.time_cat(3, None, '-', 0.0)
-        78.1±3ms         54.7±0ms     0.70  strings.Methods.time_get
-        39.1±5ms         27.3±0ms     0.70  strings.Methods.time_slice
-           156ms            109ms     0.70  strings.Repeat.time_repeat('int')
-         102±6ms         70.3±4ms     0.69  timedelta.ToTimedeltaErrors.time_convert('ignore')
-        14.0±2μs         9.71±0μs     0.69  timeseries.AsOf.time_asof_single_early('Series')
-        74.2±7ms         50.8±4ms     0.68  strings.Cat.time_cat(3, None, '-', 0.001)
-      5.76±0.7ms         3.91±0ms     0.68  timeseries.AsOf.time_asof_nan_single('Series')
-        54.7±8ms         36.5±3ms     0.67  strings.Methods.time_replace
-        188±20ms          125±2ms     0.67  strings.Methods.time_findall
-         188±8ms          125±8ms     0.67  strings.Split.time_split(False)
-        41.7±3ms         27.3±0ms     0.66  strings.Methods.time_upper
-       886±100μs          570±0μs     0.64  inference.NumericInferOps.time_multiply(<class 'numpy.uint16'>)
-         109±8ms         70.3±0ms     0.64  strings.Methods.time_count
-        221±30ns          141±0ns     0.64  timeseries.DatetimeAccessor.time_dt_accessor
-        46.9±5ms         29.3±2ms     0.62  strings.Methods.time_rstrip
-        21.9±2ms         13.7±0ms     0.62  timeseries.AsOf.time_asof('DataFrame')
-        21.9±0ms         13.7±0ms     0.62  timeseries.AsOf.time_asof_nan('DataFrame')
-        403±60μs         248±20μs     0.62  strings.Encode.time_encode_decode
-        17.9±2ms       10.9±0.8ms     0.61  timedelta.ToTimedelta.time_convert_string_seconds
-           31.2s            19.1s     0.61  strings.Dummies.time_get_dummies
-        31.2±3ms         18.8±1ms     0.60  stat_ops.FrameOps.time_op('mad', 'int', 0, True)
-        93.8±8ms         54.7±0ms     0.58  strings.Contains.time_contains(True)
-        9.94±0ms         5.76±0ms     0.58  timeseries.AsOf.time_asof('Series')
-      9.94±0.5ms       5.76±0.3ms     0.58  timeseries.AsOf.time_asof_nan('Series')
-        516±20ms          297±0ms     0.58  strings.Methods.time_extract
-        82.0±6ms         46.9±1ms     0.57  strings.Cat.time_cat(3, ',', None, 0.15)
-        41.7±3ms         23.4±1ms     0.56  strings.Contains.time_contains(False)
-        6.08±0ms       3.42±0.2ms     0.56  timeseries.AsOf.time_asof_nan_single('DataFrame')
-         453±4ms          250±8ms     0.55  multiindex_object.Values.time_datetime_level_values_copy
-      6.51±0.3ms       3.50±0.2ms     0.54  timeseries.AsOf.time_asof_single('DataFrame')
-        117±20ms         62.5±4ms     0.53  strings.Methods.time_match
-         125±8ms         66.4±4ms     0.53  strings.Cat.time_cat(3, ',', '-', 0.001)
-        70.3±8ms         36.5±3ms     0.52  strings.Methods.time_endswith
-         255±0μs          114±7μs     0.45  timeseries.AsOf.time_asof_single_early('DataFrame')
-        51.1±4μs         20.8±1μs     0.41  timeseries.AsOf.time_asof_single('Series')
-        93.8±0ms          418±0μs     0.00  frame_ctor.FromRecords.time_frame_from_records_generator(None)
-        93.8±6ms          525±0ns     0.00  multiindex_object.Integer.time_is_monotonic
``` @jreback:
The unifying aspect has been weakened for perf reasons (and because it didn't work to force cython to compile the different `bint` paths in https://github.com/pandas-dev/pandas/commit/52ae84e7f05d86ca228448b412fff76b99a0a1b0 ).

However, this still gives `hashtable.unique` a working `return_inverse` kwarg, and does so with minimal code additions by sharing the implementation of `hashtable.get_labels`. As such, it will allow fixing #21720 and proceed on all the `return_inverse` stuff. > However, this still gives hashtable.unique a working return_inverse kwarg, and does so with minimal code additions by sharing the implementation of hashtable.get_labels. As such, it will allow fixing #21720 and proceed on all the return_inverse stuff.

and how does this differ from factorize? which requires no code changes > and how does this differ from factorize? which requires no code changes

The difference between `.factorize` and `.unique` is the reason for #21720 - `.factorize` puts all flavours of `NaN` in one bucket, while `unique` keeps them separate. You can see the difference between them by looking for the `ignore_na`-kwarg that I introduced for `_unique_with_inverse`.

And for implementing a `return_inverse` kwarg on python level, it'd be nice (and much clearer) to have a cython-level `.unique` that supports that as well (rather than complicated wrappers around `get_labels`). > The difference between .factorize and .unique is the reason for #21720 - .factorize puts all flavours of NaN in one bucket, while unique keeps them separate. You can see the difference between them by looking for the ignore_na-kwarg that I introduced for _unique_with_inverse.

then ok with adding the keyword to the existing functions. I just see a giant moving around code with new names for little gain here.

let's try to minimize the changes.
 > then ok with adding the keyword to the existing functions. I just see a giant moving around code with new names for little gain here.

I don't want to implement this within `get_labels`. It's clunky to have to hand over your own instance of a HashVector (corresponding to the right dtype), not get the uniques returned, etc. This might be needed somewhere else, but is wholly useless for the purpose of `.unique`. I can "unmove" the existing `unique` implementations, just tried to group the functions together logically. @gfyoung @jreback 

Added a test for the different flavours of `hashtable.unique`. There were some `_lib.hashtable` tests in `test_algos`, so I added them there. I also cleaned up `test_vector_resize` because it was in a very outdated state (no parametrization depite being heavily parametrized), and then used that as a jumping off point for the test I added. Since this is a clean-up PR, this should hopefully not be an issue (plus it's a small test and the clean-up easy to follow)

In summary:
* `hashtable._unique_with_inverse(ignore_na=True)` tested through all tests for `.factorize`
* `hashtable._unique_with_inverse(ignore_na=False)` tested through the tests I just added (the part using `return_inverse=True`)
* `hashtable._unique_no_inverse` tested by existing tests for `.unique` plus the test I added using (the part using `return_inverse=False`) For some reason I can't comment anymore without closing the PR (even after reloading the page). Very strange... pls don't add to this PR, except for doc-strings. remove ignore_na for now. each PR needs to be a single change @h-vetinari Thanks for the extensive timings! 

Can you show a small python example illustrating the need for `ignore_na` (illustrating the difference you mention between unique and factorize)?
 @jorisvandenbossche 
Factorize leaves out all flavours of NaNs from the output of `uniques`, putting `na_sentinel` in their place of the labels. This makes reconstruction impossible on two fronts: wrong indexing, and mixing different kinds of NaNs.

So, for working towards `.unique(return_inverse=True)`, it's essential to switch off the `ignore_na` - since it's so close to what `factorize` does, I propose to unify those functions and differentiate via the kwarg.

For an illustration see: #21720 (`Series.unique` directly goes to `hashtable.unique`, while `DataFrame.unique` calls `.factorize` on the columns first). @jreback I don't understand why you insist on removing `ignore_na`. That was the whole point of this PR, as otherwise `unique` cannot use the same code as `factorize` to return the inverse values. But OK, it can also simply be done in a next PR .. > @jreback I don't understand why you insist on removing ignore_na. That was the whole point of this PR, as otherwise unique cannot use the same code as factorize to return the inverse values. But OK, it can also simply be done in a next PR ..

well aside from the fact that it has no tests whatsoever, and the reason for it is not obvious at all (and likely I will have a whole set of comments on that issue).

Change of highly used code need very strict review and preformance tests.  >  the reason for it is not obvious at all 

Well, then please try to *understand* the reason, debate the reason or suggest alternatives ways to sovle it, or ask for clarification. @h-vetinari already opened an issue about that some time ago and referenced that issue multiple times above as the reason why a different behaviour between unique and factorize is needed: https://github.com/pandas-dev/pandas/issues/21720

 @h-vetinari I think the following is an easier illustration of the "different handling of NA values" (compared to #21720, as the below directly handles with unique/factorize) that necessitates the `ignore_na` keyword you wanted to introduce. Is my understanding correct here?

```
In [1]: s = pd.Series([np.nan, 3, 3, None, np.nan], dtype=object)

In [2]: pd.unique(s)
Out[2]: array([nan, 3, None], dtype=object)

In [4]: pd.factorize(s)
Out[4]: (array([-1,  0,  0, -1, -1]), Int64Index([3], dtype='int64'))
```

So we have a difference between `unique` and `factorize` how they handle missing values (in case of multiple missing values sentinels for object dtype). Factorize treats them all as identical (since it needs to substitute all missing values with -1), while unique treats them as separate values. 

 @jorisvandenbossche 

I DO understand the debate and that is exactly why I don't want the argument in this PR. There is a history of adding many things to PR's which make them very complicated. PR's need to be focused on a single issue,, otherwise they will have an extended review time and/or not be merged. @jreback 
* there was test coverage, see https://github.com/pandas-dev/pandas/pull/22986#issuecomment-427691752
* this PR has now morphed into it's own precursor, which is OK I guess, but more work than a separate precursor with a clear (partial) goal
* > There is a history [...]
  
  As far as that is directed at me, I'll just note that I'd find that an unfair characterisation, but opinions obviously differ. In any case, this PR should be ready. The follow-up is ready to go as well (and can be reviewed then & there), so no need to spend too much time discussing the process. ;-) > I DO understand the debate and that is exactly why I don't want the argument in this PR.

Above you said you "the reason for it [to have the ignore_na keyword] is not obvious at all", so I interpreted this that you might not like adding the keyword at all, not that you just don't want it here. 

@jreback I just want to be sure we are on the same page about the end goal here, which is still adding a `ignore_na` keyword (or an equivalent way to implement a "unique with return_inverse"). 
Because otherwise, IMO this PR is not very useful, if we don't do this next step. This PR doesn't mention ``ignore_na`` at all in the top. sorry @jorisvandenbossche don't buy that argument. And even if it DID mention it, this was already at the limit of what a PR should do. This was moving lots of code around.

Much much much easier to move things in a single PR, then add in another, with appropriate tests.

Btw. I actually will require extensive testing for ``ignore_na``. This is NOT how we typicall handles nulls, which are treated the same. You would need to show a really good use case where this actually matters. @jorisvandenbossche @jreback 
Ok, let's take a breather here.

I'm fine with splitting off this first round (which shifts the implementation of `.get_labels` and `.factorize` to an underlying `._unique_with_inverse(...)` and prepares for the introduction of `.unique(return_inverse=True)`.
> Because otherwise, IMO this PR is not very useful, if we don't do this next step.

As I said, the follow-up is ready.

> This PR doesn't mention `ignore_na` at all in the top. sorry @jorisvandenbossche don't buy that argument. And even if it DID mention it, this was already at the limit of what a PR should do. This was moving lots of code around.

I wrote:
> I found that there's substantial overlap between `unique` and `get_labels` (the core of `factorize`) - the only difference is the handling of `NaN/None/etc.` I think that these could/should be unified for less code duplication.

IMO you're really splitting hairs here. It says to unify two slightly different functions (and how they differ) - which obviously needs a way to switch between the two, in this case the `ignore_na`-kwarg.

> Btw. I actually will require extensive testing for `ignore_na`. This is NOT how we typicall handles nulls, which are treated the same. You would need to show a really good use case where this actually matters.

This is documented behaviour for factorize (https://pandas.pydata.org/pandas-docs/stable/generated/pandas.factorize.html): 
> Note: Even if there’s a missing value in values, uniques will not contain an entry for it.

and, as I've been writing above about 5x already: All existing tests for `.factorize` **are** testing `ignore_na=False`, because they're calling `get_labels`, which in this PR only wraps `.unique_with_inverse(ignore_na=False)`. And once I add back in the tests for `.unique(return_inverse=True)` in the follow-up PR, then `ignore_na=True` will also be tested extensively. > This is NOT how we typicall handles nulls, which are treated the same. You would need to show a really good use case where this actually matters.

See above https://github.com/pandas-dev/pandas/pull/22986#issuecomment-427817793

If you don't find this a good use case, please respond to it.

> This PR doesn't mention ignore_na at all in the top

@jreback Maybe not in the top post (although as @h-vetinari says, it does give the motivation), but there has been ample discussion above about it, where at a certain point you even say "then ok with adding the keyword to the existing functions". So it is not that it has not been discussed.

> >  Because otherwise, IMO this PR is not very useful, if we don't do this next step.
>
> As I said, the follow-up is ready.

@h-vetinari Yep, I don't really care whether it is here or in a next PR. So maybe we should move the discussion to an issue / the next PR, since it is strictly not about this PR any more. I just wanted to get on the same page with @jreback on the way forward, otherwise you do a next PR that maybe again would not be welcomed. But maybe it is more productive here to focus on the current, slimmed down diff. @jreback Can we get this over the line? All the open questions about `ignore_na` have been "saved" for the follow-up.