BUG: Fix Series.get() for ExtensionArray and Categorical
- [x] closes #20882
- [x] tests added / passed
  - pandas/tests/extension/base/getitem.py::test_get()
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
  - Not created - assume will go in v0.23 as part of ExtensionArray support

Taking a look now. OK, so something that confuses me here. I initially hacked this in with a `if isinstance(s, (ExtensionArray, Index)) and is_scalar(key):`.

That's mainly because this line failed:

```
            return self._engine.get_value(s, k,
                                          tz=getattr(series.dtype, 'tz', None))
 ```

`self._engine.get_value(s, k)` expects an ndarray, but we don't want to convert to an ndarray here, since that's expensive. It doesn't seem easy to let `self._engine.get_value` take an ndarray, but what if we do a bit of indirect indexing, by passing `np.arange(len(s)), k`? Something like


```diff
diff --git a/pandas/core/indexes/base.py b/pandas/core/indexes/base.py
index 2ceec1592..b5b175c11 100644
--- a/pandas/core/indexes/base.py
+++ b/pandas/core/indexes/base.py
@@ -36,6 +36,7 @@ from pandas.core.dtypes.common import (
     is_period_dtype,
     is_bool,
     is_bool_dtype,
+    is_extension_array_dtype,
     is_signed_integer_dtype,
     is_unsigned_integer_dtype,
     is_integer_dtype, is_float_dtype,
@@ -3068,21 +3069,21 @@ class Index(IndexOpsMixin, PandasObject):
         # if we have something that is Index-like, then
         # use this, e.g. DatetimeIndex
         s = getattr(series, '_values', None)
-        if isinstance(s, (ExtensionArray, Index)) and is_scalar(key):
-            try:
-                return s[key]
-            except (IndexError, ValueError):
+        is_extension = is_extension_array_dtype(series)
 
-                # invalid type as an indexer
-                pass
+        if is_extension:
+            s = np.arange(len(series))
+        else:
+            s = com._values_from_object(series)
 
-        s = com._values_from_object(series)
         k = com._values_from_object(key)
-
         k = self._convert_scalar_indexer(k, kind='getitem')
         try:
-            return self._engine.get_value(s, k,
-                                          tz=getattr(series.dtype, 'tz', None))
+            result = self._engine.get_value(
+                s, k, tz=getattr(series.dtype, 'tz', None))
+            if is_extension:
+                result = series._values[result]
+            return result
         except KeyError as e1:
             if len(self) > 0 and self.inferred_type in ['integer', 'boolean']:
                 raise
```

The basic idea is to index into the positions, and then do a secondary ExtensionArray.__getitem__ once you know the positions. Does that make any sense? FYI, I think this can go in after the RC. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20885?src=pr&el=h1) Report
> Merging [#20885](https://codecov.io/gh/pandas-dev/pandas/pull/20885?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/bd4332f4bff135d4119291f66e98f76cc5f9a80e?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20885/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/20885?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20885      +/-   ##
==========================================
+ Coverage   91.81%   91.81%   +<.01%     
==========================================
  Files         153      153              
  Lines       49481    49483       +2     
==========================================
+ Hits        45430    45434       +4     
+ Misses       4051     4049       -2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.21% <100%> (ø)` | :arrow_up: |
| #single | `41.84% <0%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20885?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/20885/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.65% <100%> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/20885/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `84.59% <0%> (+0.2%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20885?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/20885?src=pr&el=footer). Last update [bd4332f...65087a6](https://codecov.io/gh/pandas-dev/pandas/pull/20885?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 see also #14865 I believe this will also be solved by the same fix. If so, pls add that as a test (its for Categorical) @jreback with respect to #14865, as @jorisvandenbossche stated in the original issue #20885 for this PR, I think they are separate issues, so I will not add additional tests. @jreback with respect to your suggested change of using `s[self.get_loc[key]]` instead of `s[key]` for `Index`, it breaks two existing tests, so I think the fix as I've implemented it should be taken as is. (cc @TomAugspurger)

If you guys want me to do anything else with this PR, let me know, but IMHO, you ought to just merge it in. > it breaks two existing tests

what does it break? I really dont' like treating ExtensionArray different from Index, if that's the case its a bug in the API (for indexing). @jreback The tests that fail are
 -  `pandas\tests\test_base.py` in `test_value_counts_unique_nunique`
 -  `tests/groupby/aggregate/test_other.py` in ` test_agg_timezone_round_trip`

I've investigated the first one and here is what I have found out. (I think the second one is the same problem).  If we use the way you propose, where we use `get_loc(key)` on the `Index`, the example below fails on the expression `s2[0]`.



```
In [1]: import pandas as pd

In [2]: def makeDateIndex(k=10, freq='B', name=None):
   ...:     dt = pd.datetime(2000, 1, 1)
   ...:     dr = pd.bdate_range(dt, periods=k, freq=freq, name=name)
   ...:     return pd.DatetimeIndex(dr, name=name)
   ...:
   ...: dt_tz_index = makeDateIndex(10, name='a').tz_localize(tz='US/Eastern')
   ...: s1 = pd.Series([i for i in range(len(dt_tz_index))], index=dt_tz_index)
   ...:
   ...: s2 = pd.Series(dt_tz_index, index=dt_tz_index)
   ...:

In [3]: s1
Out[3]:
a
2000-01-03 00:00:00-05:00    0
2000-01-04 00:00:00-05:00    1
2000-01-05 00:00:00-05:00    2
2000-01-06 00:00:00-05:00    3
2000-01-07 00:00:00-05:00    4
2000-01-10 00:00:00-05:00    5
2000-01-11 00:00:00-05:00    6
2000-01-12 00:00:00-05:00    7
2000-01-13 00:00:00-05:00    8
2000-01-14 00:00:00-05:00    9
Freq: B, dtype: int64

In [4]: s2
Out[4]:
a
2000-01-03 00:00:00-05:00   2000-01-03 00:00:00-05:00
2000-01-04 00:00:00-05:00   2000-01-04 00:00:00-05:00
2000-01-05 00:00:00-05:00   2000-01-05 00:00:00-05:00
2000-01-06 00:00:00-05:00   2000-01-06 00:00:00-05:00
2000-01-07 00:00:00-05:00   2000-01-07 00:00:00-05:00
2000-01-10 00:00:00-05:00   2000-01-10 00:00:00-05:00
2000-01-11 00:00:00-05:00   2000-01-11 00:00:00-05:00
2000-01-12 00:00:00-05:00   2000-01-12 00:00:00-05:00
2000-01-13 00:00:00-05:00   2000-01-13 00:00:00-05:00
2000-01-14 00:00:00-05:00   2000-01-14 00:00:00-05:00
Freq: B, Name: a, dtype: datetime64[ns, US/Eastern]

In [5]: s1[0]
Out[5]: 0

In [6]: s2[0]
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\core\indexes\base.py in get_loc(self, key, method, tolerance)
   3062             try:
-> 3063                 return self._engine.get_loc(key)
   3064             except KeyError:

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\_libs\index.pyx in pandas._libs.index.DatetimeEngine.get_loc (pandas\_libs\index.c:10784)()
    429
--> 430     cpdef get_loc(self, object val):
    431         if is_definitely_invalid_key(val):

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\_libs\index.pyx in pandas._libs.index.DatetimeEngine.get_loc (pandas\_libs\index.c:10616)()
    465             val = maybe_datetimelike_to_i8(val)
--> 466             return self.mapping.get_item(val)
    467         except (TypeError, ValueError):

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\_libs\hashtable_class_helper.pxi in pandas._libs.hashtable.Int64HashTable.get_item (pandas\_libs\hashtable.c:15389)()
    957
--> 958     cpdef get_item(self, int64_t val):
    959         cdef khiter_t k

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\_libs\hashtable_class_helper.pxi in pandas._libs.hashtable.Int64HashTable.get_item (pandas\_libs\hashtable.c:15333)()
    963         else:
--> 964             raise KeyError(val)
    965

KeyError: 0

During handling of the above exception, another exception occurred:

KeyError                                  Traceback (most recent call last)
C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\core\indexes\datetimes.py in get_loc(self, key, method, tolerance)
   1609         try:
-> 1610             return Index.get_loc(self, key, method, tolerance)
   1611         except (KeyError, ValueError, TypeError):

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\core\indexes\base.py in get_loc(self, key, method, tolerance)
   3064             except KeyError:
-> 3065                 return self._engine.get_loc(self._maybe_cast_indexer(key))
   3066

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\_libs\index.pyx in pandas._libs.index.DatetimeEngine.get_loc (pandas\_libs\index.c:10784)()
    429
--> 430     cpdef get_loc(self, object val):
    431         if is_definitely_invalid_key(val):

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\_libs\index.pyx in pandas._libs.index.DatetimeEngine.get_loc (pandas\_libs\index.c:10616)()
    465             val = maybe_datetimelike_to_i8(val)
--> 466             return self.mapping.get_item(val)
    467         except (TypeError, ValueError):

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\_libs\hashtable_class_helper.pxi in pandas._libs.hashtable.Int64HashTable.get_item (pandas\_libs\hashtable.c:15389)()
    957
--> 958     cpdef get_item(self, int64_t val):
    959         cdef khiter_t k

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\_libs\hashtable_class_helper.pxi in pandas._libs.hashtable.Int64HashTable.get_item (pandas\_libs\hashtable.c:15333)()
    963         else:
--> 964             raise KeyError(val)
    965

KeyError: 0

During handling of the above exception, another exception occurred:

KeyError                                  Traceback (most recent call last)
C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\_libs\index.pyx in pandas._libs.index.DatetimeEngine.get_loc (pandas\_libs\index.c:10434)()
    457         try:
--> 458             return self.mapping.get_item(val.value)
    459         except KeyError:

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\_libs\hashtable_class_helper.pxi in pandas._libs.hashtable.Int64HashTable.get_item (pandas\_libs\hashtable.c:15389)()
    957
--> 958     cpdef get_item(self, int64_t val):
    959         cdef khiter_t k

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\_libs\hashtable_class_helper.pxi in pandas._libs.hashtable.Int64HashTable.get_item (pandas\_libs\hashtable.c:15333)()
    963         else:
--> 964             raise KeyError(val)
    965

KeyError: 0

During handling of the above exception, another exception occurred:

KeyError                                  Traceback (most recent call last)
C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\core\indexes\base.py in get_loc(self, key, method, tolerance)
   3062             try:
-> 3063                 return self._engine.get_loc(key)
   3064             except KeyError:

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\_libs\index.pyx in pandas._libs.index.DatetimeEngine.get_loc (pandas\_libs\index.c:10784)()
    429
--> 430     cpdef get_loc(self, object val):
    431         if is_definitely_invalid_key(val):

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\_libs\index.pyx in pandas._libs.index.DatetimeEngine.get_loc (pandas\_libs\index.c:10521)()
    459         except KeyError:
--> 460             raise KeyError(val)
    461         except AttributeError:

KeyError: Timestamp('1969-12-31 19:00:00-0500', tz='US/Eastern')

During handling of the above exception, another exception occurred:

KeyError                                  Traceback (most recent call last)
C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\_libs\index.pyx in pandas._libs.index.DatetimeEngine.get_loc (pandas\_libs\index.c:10434)()
    457         try:
--> 458             return self.mapping.get_item(val.value)
    459         except KeyError:

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\_libs\hashtable_class_helper.pxi in pandas._libs.hashtable.Int64HashTable.get_item (pandas\_libs\hashtable.c:15389)()
    957
--> 958     cpdef get_item(self, int64_t val):
    959         cdef khiter_t k

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\_libs\hashtable_class_helper.pxi in pandas._libs.hashtable.Int64HashTable.get_item (pandas\_libs\hashtable.c:15333)()
    963         else:
--> 964             raise KeyError(val)
    965

KeyError: 0

During handling of the above exception, another exception occurred:

KeyError                                  Traceback (most recent call last)
C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\core\indexes\datetimes.py in get_loc(self, key, method, tolerance)
   1618                 stamp = Timestamp(key, tz=self.tz)
-> 1619                 return Index.get_loc(self, stamp, method, tolerance)
   1620             except KeyError:

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\core\indexes\base.py in get_loc(self, key, method, tolerance)
   3064             except KeyError:
-> 3065                 return self._engine.get_loc(self._maybe_cast_indexer(key))
   3066

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\_libs\index.pyx in pandas._libs.index.DatetimeEngine.get_loc (pandas\_libs\index.c:10784)()
    429
--> 430     cpdef get_loc(self, object val):
    431         if is_definitely_invalid_key(val):

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\_libs\index.pyx in pandas._libs.index.DatetimeEngine.get_loc (pandas\_libs\index.c:10521)()
    459         except KeyError:
--> 460             raise KeyError(val)
    461         except AttributeError:

KeyError: Timestamp('1969-12-31 19:00:00-0500', tz='US/Eastern')

During handling of the above exception, another exception occurred:

KeyError                                  Traceback (most recent call last)
C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\core\indexes\datetimes.py in get_value(self, series, key)
   1559         try:
-> 1560             return com._maybe_box(self, Index.get_value(self, series, key),
   1561                                   series, key)

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\core\indexes\base.py in get_value(self, series, key)
   3087 #                    return s[key]
-> 3088                     return s[self.get_loc(key)]
   3089                 except (IndexError, ValueError):

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\core\indexes\datetimes.py in get_loc(self, key, method, tolerance)
   1620             except KeyError:
-> 1621                 raise KeyError(key)
   1622             except ValueError as e:

KeyError: 0

During handling of the above exception, another exception occurred:

KeyError                                  Traceback (most recent call last)
C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\_libs\index.pyx in pandas._libs.index.DatetimeEngine.get_loc (pandas\_libs\index.c:10434)()
    457         try:
--> 458             return self.mapping.get_item(val.value)
    459         except KeyError:

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\_libs\hashtable_class_helper.pxi in pandas._libs.hashtable.Int64HashTable.get_item (pandas\_libs\hashtable.c:15389)()
    957
--> 958     cpdef get_item(self, int64_t val):
    959         cdef khiter_t k

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\_libs\hashtable_class_helper.pxi in pandas._libs.hashtable.Int64HashTable.get_item (pandas\_libs\hashtable.c:15333)()
    963         else:
--> 964             raise KeyError(val)
    965

KeyError: 0

During handling of the above exception, another exception occurred:

KeyError                                  Traceback (most recent call last)
C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\core\indexes\datetimes.py in get_value(self, series, key)
   1569             try:
-> 1570                 return self.get_value_maybe_box(series, key)
   1571             except (TypeError, ValueError, KeyError):

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\core\indexes\datetimes.py in get_value_maybe_box(self, series, key)
   1580         values = self._engine.get_value(com._values_from_object(series),

-> 1581                                         key, tz=self.tz)
   1582         return com._maybe_box(self, values, series, key)

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\_libs\index.pyx in pandas._libs.index.IndexEngine.get_value (pandas\_libs\index.c:4847)()
    104
--> 105     cpdef get_value(self, ndarray arr, object key, object tz=None):
    106         """

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\_libs\index.pyx in pandas._libs.index.IndexEngine.get_value (pandas\_libs\index.c:4530)()
    112
--> 113         loc = self.get_loc(key)
    114         if PySlice_Check(loc) or cnp.PyArray_Check(loc):

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\_libs\index.pyx in pandas._libs.index.DatetimeEngine.get_loc (pandas\_libs\index.c:10521)()
    459         except KeyError:
--> 460             raise KeyError(val)
    461         except AttributeError:

KeyError: Timestamp('1969-12-31 19:00:00-0500', tz='US/Eastern')

During handling of the above exception, another exception occurred:

KeyError                                  Traceback (most recent call last)
<ipython-input-6-ab7c8e26b0d3> in <module>()
----> 1 s2[0]

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\core\series.py in __getitem__(self, key)
    764         key = com._apply_if_callable(key, self)
    765         try:
--> 766             result = self.index.get_value(self, key)
    767
    768             if not is_scalar(result):

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\core\indexes\datetimes.py in get_value(self, series, key)
   1570                 return self.get_value_maybe_box(series, key)
   1571             except (TypeError, ValueError, KeyError):
-> 1572                 raise KeyError(key)
   1573
   1574     def get_value_maybe_box(self, series, key):

KeyError: 0
```

Here's an easier-to-read stack trace of calls for `s2[0]` in terms of where it bombs out:
```
pandas\core\indexes\base.py in get_loc(self, key, method, tolerance)
--> 3063                 return self._engine.get_loc(key)
pandas\core\indexes\datetimes.py in get_loc(self, key, method, tolerance)
-->1610             return Index.get_loc(self, key, method, tolerance)
pandas\core\indexes\base.py in get_value(self, series, key)
--> 3088                     return s[self.get_loc(key)]
pandas\core\indexes\datetimes.py in get_value(self, series, key)
--> 1560             return com._maybe_box(self, Index.get_value(self, series, key),
    1561                                   series, key)
pandas\core\series.py in __getitem__(self, key)
--> 766             result = self.index.get_value(self, key)
```

So what may be the error here is that if the values of a `Series` containing TZ-aware values end up being a `DateTimeIndex` and `get_loc(0)` isn't defined for an `Index`.  Note that in the code below, if you remove the `.tz_localize()` part, it works fine. 

Maybe when `DateTimeIndex` is based on `ExtensionArray`, then this code as I wrote it could change so that `ExtensionArray` and `Index` are handled the same way.

Or maybe you know how to fix things for `DateTimeIndex`, but it's not clear what is the thing I should be testing to be sure that this issue is fixed. 
 this https://github.com/pandas-dev/pandas/pull/20885#issuecomment-386438422

is a bug and should be fixed as a pre-cursor to this PR, @Dr-Irv pls make a separate issue (I thought we had one, but couldn't fin it). @jreback  I'm not sure how to report the issue.  The only way to replicate it is *if* we implement this PR as you suggested.  Should I create the issue that way?  

I can't figure out how to demonstrate the bad behavior on 0.23rc2, or, equivalently, what test I could write that would verify that the bug illustrated above exists (since the bug only occurs if the PR is implemented in a different way). @jreback Above, I mentioned that the change you want also breaks `pandas\core\indexes\datetimes.py` in `test_agg_timezone_round_trip` .  I think that is related to the bug reported in #20949 .  @jreback I've pushed a new commit here, as I found another bug in how I did things, so the new implementation looks more different for `Index` and `ExtensionArray` because of how the exceptions are handled.  I added another test that captured this difference.

In the case of `ExtensionArray`, if the key is a scalar, we have a `Series` backed by an `ExtensionArray`, so we first have to try to translate that key into an integer index to pass down to `ExtensionArray.__getitem__`.  If we can't do that, then if the scalar is an integer, we pass that integer down.  If that fails, we pass back whatever exception is raised by the `ExtensionArray.__getitem__()` implementation..

If the `Series` is backed by an `Index`, then we need to let the existing machinery for `Index` return the result, and that machinery uses deep pandas internals to get the right value whether it is a loc-type key or an integer.
 @Dr-Irv ok this change looks ok. can you run the indexing asv's and report if anything is changing? this is a central piece of code so need to check. @jreback I hope I did this right.  I'm doing this on my 16GB 4 core laptop on Windows.  I compared upstream/master (last commit on May 5) to my latest commit. Here are the results:
```
      before           after         ratio
     [bd4332f4]       [65087a6d]
+        30.8±0μs         44.0±5μs     1.43  indexing.NumericSeriesIndexing.time_iloc_slice(<class 'pandas.core.indexes.numeric.Float64Index'>)
-         516±0ms          453±0ms     0.88  indexing.NumericSeriesIndexing.time_loc_list_like(<class 'pandas.core.indexes.numeric.Float64Index'>)
-        83.3±5μs         71.7±0μs     0.86  indexing.NumericSeriesIndexing.time_getitem_slice(<class 'pandas.core.indexes.numeric.Int64Index'>)
-         109±2μs         93.9±0μs     0.86  indexing.NumericSeriesIndexing.time_loc_scalar(<class 'pandas.core.indexes.numeric.Float64Index'>)
-     1.09±0.04ms         934±10μs     0.85  indexing.NumericSeriesIndexing.time_loc_list_like(<class 'pandas.core.indexes.numeric.Int64Index'>)
-        170±10μs          141±0μs     0.83  indexing.IntervalIndexing.time_getitem_list
-         427±0μs         351±30μs     0.82  indexing.MultiIndexing.time_series_ix
-         516±8ms          406±0ms     0.79  indexing.NumericSeriesIndexing.time_loc_array(<class 'pandas.core.indexes.numeric.Float64Index'>)
-        498±50ns          382±0ns     0.77  indexing.MethodLookup.time_lookup_loc
-        97.7±6μs         70.7±0μs     0.72  indexing.NumericSeriesIndexing.time_loc_slice(<class 'pandas.core.indexes.numeric.Float64Index'>)
-         107±0μs         75.0±0μs     0.70  indexing.NumericSeriesIndexing.time_ix_scalar(<class 'pandas.core.indexes.numeric.Float64Index'>)
-        562±20ms          383±4ms     0.68  indexing.NumericSeriesIndexing.time_ix_list_like(<class 'pandas.core.indexes.numeric.Float64Index'>)
-         103±6μs         67.2±2μs     0.65  indexing.NumericSeriesIndexing.time_loc_scalar(<class 'pandas.core.indexes.numeric.Int64Index'>)
```
So many things got faster.  I'm not surprised by this, as we're not falling through exceptions any more when indexing by a scalar.

I also reran just the one that got slower, and when I did that, it flipped to 62.3±4μs for master and 55.6±0μs for the new version, showing an improvement.

I then ran the benchmarks again to better understand variability:
```
      before           after         ratio
     [bd4332f4]       [65087a6d]
+         567±0μs         758±50μs     1.34  indexing.DataFrameNumericIndexing.time_bool_indexer
+        72.1±6μs         88.8±0μs     1.23  indexing.NumericSeriesIndexing.time_loc_slice(<class 'pandas.core.indexes.numeric.Float64Index'>)
```

And once more:
```
      before           after         ratio
     [bd4332f4]       [65087a6d]
+        2.15±0ms         62.5±4ms    29.09  indexing.NumericSeriesIndexing.time_loc_array(<class 'pandas.core.indexes.numeric.Int64Index'>)
-        92.5±0μs         83.8±0μs     0.91  indexing.NonNumericSeriesIndexing.time_getitem_label_slice('datetime')
```
Given the inconsistent results (probably affected by other background processes happening on my laptop), I think we're OK.



 @Dr-Irv IIRC there is a flag for affinity to help prevent this kind of variable, also I thin we have some asv options to run this many times. but close enough for now. thanks!