REF: stricter checks in _simple_new, avoid shallow_copy in EAs
State: closed

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry


Labels:
name: Datetime
description: Datetime data dtype
name: API Design
name: ExtensionArray
description: Extending pandas with custom dtypes or arrays.

Comments:
Hello @jbrockmendel! Thanks for updating the PR.

 - There are no PEP8 issues in the file [`pandas/core/arrays/datetimelike.py`](https://github.com/jbrockmendel/pandas/blob/777ddffac81134e7a8e3c8399f09fe18b2b70097/pandas/core/arrays/datetimelike.py) !

 - There are no PEP8 issues in the file [`pandas/core/arrays/datetimes.py`](https://github.com/jbrockmendel/pandas/blob/777ddffac81134e7a8e3c8399f09fe18b2b70097/pandas/core/arrays/datetimes.py) !

 - There are no PEP8 issues in the file [`pandas/core/arrays/timedeltas.py`](https://github.com/jbrockmendel/pandas/blob/777ddffac81134e7a8e3c8399f09fe18b2b70097/pandas/core/arrays/timedeltas.py) !

 - There are no PEP8 issues in the file [`pandas/core/indexes/datetimelike.py`](https://github.com/jbrockmendel/pandas/blob/777ddffac81134e7a8e3c8399f09fe18b2b70097/pandas/core/indexes/datetimelike.py) !

 - There are no PEP8 issues in the file [`pandas/core/indexes/datetimes.py`](https://github.com/jbrockmendel/pandas/blob/777ddffac81134e7a8e3c8399f09fe18b2b70097/pandas/core/indexes/datetimes.py) !

 - There are no PEP8 issues in the file [`pandas/core/indexes/timedeltas.py`](https://github.com/jbrockmendel/pandas/blob/777ddffac81134e7a8e3c8399f09fe18b2b70097/pandas/core/indexes/timedeltas.py) !


# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23426?src=pr&el=h1) Report
> Merging [#23426](https://codecov.io/gh/pandas-dev/pandas/pull/23426?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7191af9b47f6f57991abdb20625d9563877370a2?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `95.58%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23426/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23426?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #23426      +/-   ##
==========================================
+ Coverage   92.19%   92.19%   +<.01%     
==========================================
  Files         161      161              
  Lines       51192    51220      +28     
==========================================
+ Hits        47197    47223      +26     
- Misses       3995     3997       +2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.63% <95.58%> (ø)` | :arrow_up: |
| #single | `42.24% <58.82%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23426?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/arrays/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/23426/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lbGlrZS5weQ==) | `94.04% <100%> (+0.03%)` | :arrow_up: |
| [pandas/core/indexes/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/23426/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZWxpa2UucHk=) | `98.05% <100%> (+0.03%)` | :arrow_up: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/23426/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `96.3% <100%> (-0.14%)` | :arrow_down: |
| [pandas/core/arrays/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/23426/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL3RpbWVkZWx0YXMucHk=) | `94.21% <87.5%> (-0.09%)` | :arrow_down: |
| [pandas/core/indexes/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/23426/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy90aW1lZGVsdGFzLnB5) | `90.71% <93.75%> (+0.09%)` | :arrow_up: |
| [pandas/core/arrays/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/23426/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lcy5weQ==) | `98.62% <96.29%> (+0.71%)` | :arrow_up: |
| [pandas/io/feather\_format.py](https://codecov.io/gh/pandas-dev/pandas/pull/23426/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2ZlYXRoZXJfZm9ybWF0LnB5) | `77.14% <0%> (-8.58%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23426?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/23426?src=pr&el=footer). Last update [7191af9...777ddff](https://codecov.io/gh/pandas-dev/pandas/pull/23426?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).

@jbrockmendel : I see two different types of changes in a single PR (as your title also indicates).  Is there a reason they have to be together instead of in separate PR's (for reviewing purposes)?
> you are adding some more usages of shallow_copy and removing others

I'll double-check, but the intent was to only remove shallow_copy, particularly in the EA mixins.

> where is the simplification here?

main simplification is in being strict/clear about what goes in to _simple_new (though the "clear" part evidently was not so successful).

Will update/split.
Closing in favor of #23430, #23431, #23433.