BUG: fix indexing.py to make a copy of the indices (#20852)
- [x] closes #20852 
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

WIP. Reviewing the test cases and would add a test case to validate this fix. 
Hello @subhrm! Thanks for updating the PR.



Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on May 07, 2018 at 16:55 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20855?src=pr&el=h1) Report
> Merging [#20855](https://codecov.io/gh/pandas-dev/pandas/pull/20855?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/563a6ad108a79b0192a25d51d743812a18e21b15?src=pr&el=desc) will **increase** coverage by `0.03%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20855/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/20855?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20855      +/-   ##
==========================================
+ Coverage   91.77%   91.81%   +0.03%     
==========================================
  Files         153      153              
  Lines       49313    49479     +166     
==========================================
+ Hits        45259    45430     +171     
+ Misses       4054     4049       -5
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.21% <ø> (+0.04%)` | :arrow_up: |
| #single | `41.85% <ø> (-0.04%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20855?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexing.py](https://codecov.io/gh/pandas-dev/pandas/pull/20855/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhpbmcucHk=) | `93.56% <ø> (+0.42%)` | :arrow_up: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/20855/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.13% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/indexes/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/20855/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy90aW1lZGVsdGFzLnB5) | `91.15% <0%> (-0.07%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/20855/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.73% <0%> (-0.04%)` | :arrow_down: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/20855/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `95.94% <0%> (ø)` | :arrow_up: |
| [pandas/core/reshape/pivot.py](https://codecov.io/gh/pandas-dev/pandas/pull/20855/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9waXZvdC5weQ==) | `96.97% <0%> (ø)` | :arrow_up: |
| [pandas/core/panel.py](https://codecov.io/gh/pandas-dev/pandas/pull/20855/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcGFuZWwucHk=) | `97.29% <0%> (ø)` | :arrow_up: |
| [pandas/io/formats/latex.py](https://codecov.io/gh/pandas-dev/pandas/pull/20855/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvbGF0ZXgucHk=) | `100% <0%> (ø)` | :arrow_up: |
| [pandas/core/resample.py](https://codecov.io/gh/pandas-dev/pandas/pull/20855/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzYW1wbGUucHk=) | `96.07% <0%> (ø)` | :arrow_up: |
| [pandas/core/reshape/concat.py](https://codecov.io/gh/pandas-dev/pandas/pull/20855/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9jb25jYXQucHk=) | `97.59% <0%> (ø)` | :arrow_up: |
| ... and [13 more](https://codecov.io/gh/pandas-dev/pandas/pull/20855/diff?src=pr&el=tree-more) | |

------

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> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20855?src=pr&el=footer). Last update [563a6ad...6b0a872](https://codecov.io/gh/pandas-dev/pandas/pull/20855?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @subhrm Can you update for @jreback 's questions:

* whatsnew note (a small note in v0.23.1.txt in the bug fixes section
* add a test (in tests/indexing/test_indexing.py). You can use the example in the issue, and then assert that the indexer array has not changed
 @jorisvandenbossche sorry I could not devote any time for this. 
I would do the suggested changes and update by this weekend (10th June). 
Thanks for reminding me.  Implemented via PR #21867 . Closing this one. 