DEPR: Error with ambiguous groupby strings
Title is self-explanatory.

xref #14432.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22415?src=pr&el=h1) Report
> Merging [#22415](https://codecov.io/gh/pandas-dev/pandas/pull/22415?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/8bb2cc19ca3aa347d086b4fb5704ca9a3723172f?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/22415/graphs/tree.svg?src=pr&width=650&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/22415?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #22415      +/-   ##
==========================================
- Coverage   92.05%   92.04%   -0.01%     
==========================================
  Files         169      169              
  Lines       50713    50708       -5     
==========================================
- Hits        46683    46675       -8     
- Misses       4030     4033       +3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.45% <100%> (-0.01%)` | :arrow_down: |
| #single | `42.25% <20%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22415?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/reshape/merge.py](https://codecov.io/gh/pandas-dev/pandas/pull/22415/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9tZXJnZS5weQ==) | `94.15% <100%> (-0.01%)` | :arrow_down: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/22415/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `96.44% <100%> (-0.01%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/22415/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.24% <100%> (-0.01%)` | :arrow_down: |
| [pandas/core/groupby/grouper.py](https://codecov.io/gh/pandas-dev/pandas/pull/22415/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9ncm91cGVyLnB5) | `98.16% <100%> (-0.01%)` | :arrow_down: |
| [pandas/core/reshape/pivot.py](https://codecov.io/gh/pandas-dev/pandas/pull/22415/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9waXZvdC5weQ==) | `96.55% <0%> (-0.63%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/22415/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `85.75% <0%> (-0.11%)` | :arrow_down: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22415?src=pr&el=footer). Last update [8bb2cc1...5374ba4](https://codecov.io/gh/pandas-dev/pandas/pull/22415?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback : Any further thoughts on this PR?  I answered all of your questions. lgtm. @jorisvandenbossche @TomAugspurger 

 Running dask's test suite on this branch quick. I suspect this will break some things (because dask has unhandled warnings). @TomAugspurger if might make sense to expand the test_dask test that we have (which basically just tests imports). Perhaps, but this would have been a minor edge case.

Things seem good on dask's end, +1

On Wed, Aug 22, 2018 at 7:28 AM Jeff Reback <notifications@github.com>
wrote:

> @TomAugspurger <https://github.com/TomAugspurger> if might make sense to
> expand the test_dask test that we have (which basically just tests imports).
>
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 thanks @gfyoung 