add type hint for core/computation/eval
- [x] closes #26792
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

I am not fully sure  about target variable type and return type. If anyone could take a look and give some feedback, that will be great. Thanks Hello @xcz011! 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 2019-07-31 02:58:32 UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26992?src=pr&el=h1) Report
> Merging [#26992](https://codecov.io/gh/pandas-dev/pandas/pull/26992?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/171615a35574a1fc9c4e8260edca7d1e08e9c302?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/26992/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26992?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26992      +/-   ##
==========================================
- Coverage   91.99%   91.99%   -0.01%     
==========================================
  Files         180      180              
  Lines       50774    50777       +3     
==========================================
- Hits        46711    46710       -1     
- Misses       4063     4067       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.63% <100%> (ø)` | :arrow_up: |
| #single | `41.83% <100%> (-0.08%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26992?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/computation/eval.py](https://codecov.io/gh/pandas-dev/pandas/pull/26992/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tcHV0YXRpb24vZXZhbC5weQ==) | `97.16% <100%> (+0.08%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26992/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `88.88% <0%> (-11.12%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26992/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.89% <0%> (-0.12%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/26992/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.84% <0%> (-0.11%)` | :arrow_down: |
| [pandas/core/dtypes/missing.py](https://codecov.io/gh/pandas-dev/pandas/pull/26992/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL21pc3NpbmcucHk=) | `93.93% <0%> (+0.6%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26992?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/26992?src=pr&el=footer). Last update [171615a...8998a0b](https://codecov.io/gh/pandas-dev/pandas/pull/26992?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/26992?src=pr&el=h1) Report
> Merging [#26992](https://codecov.io/gh/pandas-dev/pandas/pull/26992?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/143bc34aa8f068b18e7137df7ca91b9929cc1389?src=pr&el=desc) will **increase** coverage by `0.3%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26992/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26992?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##           master   #26992     +/-   ##
=========================================
+ Coverage   91.68%   91.99%   +0.3%     
=========================================
  Files         182      180      -2     
  Lines       50246    50779    +533     
=========================================
+ Hits        46069    46713    +644     
+ Misses       4177     4066    -111
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.63% <100%> (-1.06%)` | :arrow_down: |
| #single | `41.85% <83.33%> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26992?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/computation/eval.py](https://codecov.io/gh/pandas-dev/pandas/pull/26992/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tcHV0YXRpb24vZXZhbC5weQ==) | `97.22% <100%> (+0.13%)` | :arrow_up: |
| [pandas/io/excel/\_openpyxl.py](https://codecov.io/gh/pandas-dev/pandas/pull/26992/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2V4Y2VsL19vcGVucHl4bC5weQ==) | `84.71% <0%> (-3.23%)` | :arrow_down: |
| [pandas/util/\_decorators.py](https://codecov.io/gh/pandas-dev/pandas/pull/26992/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvX2RlY29yYXRvcnMucHk=) | `92.13% <0%> (-3.22%)` | :arrow_down: |
| [pandas/core/dtypes/cast.py](https://codecov.io/gh/pandas-dev/pandas/pull/26992/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2Nhc3QucHk=) | `90.72% <0%> (-2.34%)` | :arrow_down: |
| [pandas/io/formats/printing.py](https://codecov.io/gh/pandas-dev/pandas/pull/26992/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvcHJpbnRpbmcucHk=) | `86.72% <0%> (-2.11%)` | :arrow_down: |
| [pandas/core/internals/blocks.py](https://codecov.io/gh/pandas-dev/pandas/pull/26992/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzL2Jsb2Nrcy5weQ==) | `94.38% <0%> (-1.56%)` | :arrow_down: |
| [pandas/core/indexing.py](https://codecov.io/gh/pandas-dev/pandas/pull/26992/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhpbmcucHk=) | `93.48% <0%> (-1.55%)` | :arrow_down: |
| [pandas/core/dtypes/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/26992/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbW1vbi5weQ==) | `96.35% <0%> (-1.44%)` | :arrow_down: |
| [pandas/core/arrays/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/26992/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2NhdGVnb3JpY2FsLnB5) | `95.92% <0%> (-1.14%)` | :arrow_down: |
| [pandas/core/groupby/ops.py](https://codecov.io/gh/pandas-dev/pandas/pull/26992/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9vcHMucHk=) | `96% <0%> (-0.92%)` | :arrow_down: |
| ... and [164 more](https://codecov.io/gh/pandas-dev/pandas/pull/26992/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26992?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/26992?src=pr&el=footer). Last update [143bc34...8a1ed2b](https://codecov.io/gh/pandas-dev/pandas/pull/26992?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Removing from 0.25 milestone - @xcz011 if you can merge master and address comments can keep with this though > Removing from 0.25 milestone - @xcz011 if you can merge master and address comments can keep with this though

Sorry about leaving this open, I think I will try to finish this soon. Thanks Can you merge master? Couldn't see old build failures closing as stale, if you'd like to continue pls ping to reopen