BUG: DataFrame.loc/iloc fails to update object with new dtype GH24269
- [ ] closes #24269 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Hello @slnguyen! Thanks for updating the PR.

 - There are no PEP8 issues in the file [`pandas/core/indexing.py`](https://github.com/slnguyen/pandas/blob/1ea92e7b77b0292516e4138519c19d40aa5dd19c/pandas/core/indexing.py) !



##### Comment last updated on December 21, 2018 at 18:31 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24383?src=pr&el=h1) Report
> Merging [#24383](https://codecov.io/gh/pandas-dev/pandas/pull/24383?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/04a0eacfb2df7c42700badccbc3ea857d392ffb7?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/24383/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24383?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24383      +/-   ##
==========================================
- Coverage   92.29%   92.29%   -0.01%     
==========================================
  Files         162      162              
  Lines       51845    51849       +4     
==========================================
+ Hits        47852    47855       +3     
- Misses       3993     3994       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.7% <100%> (ø)` | :arrow_up: |
| #single | `42.98% <25%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24383?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24383/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhpbmcucHk=) | `93.89% <100%> (+0.02%)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24383/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.57% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24383?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/24383?src=pr&el=footer). Last update [04a0eac...49df896](https://codecov.io/gh/pandas-dev/pandas/pull/24383?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24383?src=pr&el=h1) Report
> Merging [#24383](https://codecov.io/gh/pandas-dev/pandas/pull/24383?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/04a0eacfb2df7c42700badccbc3ea857d392ffb7?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/24383/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24383?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24383      +/-   ##
==========================================
+ Coverage   92.29%    92.3%   +<.01%     
==========================================
  Files         162      162              
  Lines       51845    51879      +34     
==========================================
+ Hits        47852    47888      +36     
+ Misses       3993     3991       -2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.71% <100%> (ø)` | :arrow_up: |
| #single | `42.98% <25%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24383?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24383/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhpbmcucHk=) | `93.89% <100%> (+0.02%)` | :arrow_up: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/24383/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `96.62% <0%> (-0.04%)` | :arrow_down: |
| [pandas/core/indexes/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/24383/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZWxpa2UucHk=) | `97.28% <0%> (-0.01%)` | :arrow_down: |
| [pandas/core/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/24383/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYmFzZS5weQ==) | `97.66% <0%> (ø)` | :arrow_up: |
| [pandas/core/reshape/reshape.py](https://codecov.io/gh/pandas-dev/pandas/pull/24383/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9yZXNoYXBlLnB5) | `99.56% <0%> (ø)` | :arrow_up: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/24383/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `93.71% <0%> (ø)` | :arrow_up: |
| [pandas/core/arrays/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/24383/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lbGlrZS5weQ==) | `96.51% <0%> (+0.06%)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24383/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.84% <0%> (+0.18%)` | :arrow_up: |
| [pandas/core/dtypes/concat.py](https://codecov.io/gh/pandas-dev/pandas/pull/24383/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbmNhdC5weQ==) | `97.05% <0%> (+0.45%)` | :arrow_up: |
| [pandas/compat/numpy/function.py](https://codecov.io/gh/pandas-dev/pandas/pull/24383/diff?src=pr&el=tree#diff-cGFuZGFzL2NvbXBhdC9udW1weS9mdW5jdGlvbi5weQ==) | `87.91% <0%> (+0.77%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24383?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/24383?src=pr&el=footer). Last update [04a0eac...1ea92e7](https://codecov.io/gh/pandas-dev/pandas/pull/24383?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @slnguyen : Good start!  Going to need to add a new test + `whatsnew` entry as well. closing as stale if you want to continue working, please ping.