Annotate DataFrame (Part 1)
- [x] Part of #26792 
- [x] tests passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26867?src=pr&el=h1) Report
> Merging [#26867](https://codecov.io/gh/pandas-dev/pandas/pull/26867?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/430f0fd04646a0ab44ccbfbad706dfe22e9fabfe?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/26867/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26867?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26867      +/-   ##
==========================================
- Coverage   91.88%   91.87%   -0.01%     
==========================================
  Files         179      179              
  Lines       50696    50701       +5     
==========================================
+ Hits        46581    46582       +1     
- Misses       4115     4119       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.46% <100%> (ø)` | :arrow_up: |
| #single | `41.09% <100%> (-0.08%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26867?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26867/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.89% <100%> (-0.11%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26867/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `88.88% <0%> (-11.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26867?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/26867?src=pr&el=footer). Last update [430f0fd...5e70e01](https://codecov.io/gh/pandas-dev/pandas/pull/26867?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/26867?src=pr&el=h1) Report
> Merging [#26867](https://codecov.io/gh/pandas-dev/pandas/pull/26867?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/ebcfee442b04c083b054461bb421edc031b0cb40?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/26867/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26867?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26867      +/-   ##
==========================================
- Coverage      93%      93%   -0.01%     
==========================================
  Files         182      182              
  Lines       50311    50316       +5     
==========================================
+ Hits        46793    46794       +1     
- Misses       3518     3522       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `91.67% <100%> (ø)` | :arrow_up: |
| #single | `42.4% <100%> (-0.09%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26867?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26867/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.15% <100%> (-0.12%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26867/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `88.88% <0%> (-11.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26867?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/26867?src=pr&el=footer). Last update [ebcfee4...fba0fcc](https://codecov.io/gh/pandas-dev/pandas/pull/26867?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Note to self: finish this. Hello @vaibhavhrt! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:





There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2019-09-12 15:57:32 UTC @WillAyd tests are passing, u can review now lgtm. @WillAyd and @simonjayhawkins  @vaibhavhrt ci should be fixed. can you merge master. ping on green. @WillAyd ok to merge? Thanks @vaibhavhrt  part 2 coming this weekend, this one took too long(3 months :cold_sweat: ), will try to speed up.