COMPAT: Iteration should always yield a python scalar
xref #10904
closes #13236
closes #13258
xref #14216

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17491?src=pr&el=h1) Report
> Merging [#17491](https://codecov.io/gh/pandas-dev/pandas/pull/17491?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/e6aed2ebb7374ed2a6a7c284750d47728aec285e?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `94.73%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17491/graphs/tree.svg?width=650&height=150&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/17491?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17491      +/-   ##
==========================================
+ Coverage   91.15%   91.15%   +<.01%     
==========================================
  Files         163      163              
  Lines       49534    49540       +6     
==========================================
+ Hits        45153    45160       +7     
+ Misses       4381     4380       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.94% <94.73%> (+0.02%)` | :arrow_up: |
| #single | `40.22% <57.89%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17491?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/17491?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `94.92% <ø> (-0.03%)` | :arrow_down: |
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/17491?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.28% <ø> (-0.01%)` | :arrow_down: |
| [pandas/core/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/17491?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYmFzZS5weQ==) | `96.01% <100%> (+0.05%)` | :arrow_up: |
| [pandas/core/indexes/category.py](https://codecov.io/gh/pandas-dev/pandas/pull/17491?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9jYXRlZ29yeS5weQ==) | `98.54% <100%> (ø)` | :arrow_up: |
| [pandas/core/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/17491?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY2F0ZWdvcmljYWwucHk=) | `95.51% <100%> (+0.01%)` | :arrow_up: |
| [pandas/core/sparse/array.py](https://codecov.io/gh/pandas-dev/pandas/pull/17491?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3BhcnNlL2FycmF5LnB5) | `91.3% <85.71%> (-0.12%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17491?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17491?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.72% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/17491?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.43% <0%> (-0.1%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/17491?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.05% <0%> (+1.81%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17491?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/17491?src=pr&el=footer). Last update [e6aed2e...2ebcbfc](https://codecov.io/gh/pandas-dev/pandas/pull/17491?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/17491?src=pr&el=h1) Report
> Merging [#17491](https://codecov.io/gh/pandas-dev/pandas/pull/17491?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/9a8427404efb3df5deda12f76352725d628adf5e?src=pr&el=desc) will **increase** coverage by `0.01%`.
> The diff coverage is `94.73%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17491/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/17491?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17491      +/-   ##
==========================================
+ Coverage   91.15%   91.16%   +0.01%     
==========================================
  Files         163      163              
  Lines       49534    49543       +9     
==========================================
+ Hits        45153    45168      +15     
+ Misses       4381     4375       -6
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.95% <94.73%> (+0.03%)` | :arrow_up: |
| #single | `40.21% <57.89%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17491?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/17491?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `94.92% <ø> (-0.03%)` | :arrow_down: |
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/17491?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.28% <ø> (-0.01%)` | :arrow_down: |
| [pandas/core/indexes/category.py](https://codecov.io/gh/pandas-dev/pandas/pull/17491?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9jYXRlZ29yeS5weQ==) | `98.54% <100%> (ø)` | :arrow_up: |
| [pandas/core/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/17491?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY2F0ZWdvcmljYWwucHk=) | `95.51% <100%> (+0.01%)` | :arrow_up: |
| [pandas/core/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/17491?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYmFzZS5weQ==) | `96.01% <100%> (+0.05%)` | :arrow_up: |
| [pandas/core/sparse/array.py](https://codecov.io/gh/pandas-dev/pandas/pull/17491?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3BhcnNlL2FycmF5LnB5) | `91.3% <85.71%> (-0.12%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17491?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17491?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.77% <0%> (-0.05%)` | :arrow_down: |
| [pandas/core/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/17491?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS5weQ==) | `92.22% <0%> (+0.01%)` | :arrow_up: |
| [pandas/core/reshape/pivot.py](https://codecov.io/gh/pandas-dev/pandas/pull/17491?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9waXZvdC5weQ==) | `96.35% <0%> (+0.99%)` | :arrow_up: |
| ... and [1 more](https://codecov.io/gh/pandas-dev/pandas/pull/17491?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17491?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/17491?src=pr&el=footer). Last update [9a84274...05f8a6f](https://codecov.io/gh/pandas-dev/pandas/pull/17491?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
