Adding test_map_missing_mixed to test_apply.py in pandas test suite series
Checklist for other PRs (remove this part if you are doing a PR for the pandas documentation sprint):

- [x] closes #20495
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry (n/a)

Alright, I think I have something here. Is there any convenient way for me to run just an individual test file rather than all tests in their entirety? `pytest path.to.the.module::ClassName::TestName` would be the most granular, though you can strip off any of those elements to move up a level and run more tests Hello @readyready15728! Thanks for updating the PR.



Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on April 03, 2018 at 00:03 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20574?src=pr&el=h1) Report
> Merging [#20574](https://codecov.io/gh/pandas-dev/pandas/pull/20574?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/4efb39f01f5880122fa38d91e12d217ef70fad9e?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20574/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/20574?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20574      +/-   ##
==========================================
+ Coverage   91.82%   91.82%   +<.01%     
==========================================
  Files         152      153       +1     
  Lines       49255    49256       +1     
==========================================
+ Hits        45226    45227       +1     
  Misses       4029     4029
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.2% <ø> (ø)` | :arrow_up: |
| #single | `41.9% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20574?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/resample.py](https://codecov.io/gh/pandas-dev/pandas/pull/20574/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzYW1wbGUucHk=) | `96.43% <0%> (ø)` | :arrow_up: |
| [pandas/core/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/20574/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS5weQ==) | | |
| [pandas/core/groupby/\_\_init\_\_.py](https://codecov.io/gh/pandas-dev/pandas/pull/20574/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9fX2luaXRfXy5weQ==) | `100% <0%> (ø)` | |
| [pandas/core/groupby/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/20574/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9ncm91cGJ5LnB5) | `92.55% <0%> (ø)` | |

------

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> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20574?src=pr&el=footer). Last update [4efb39f...e927be4](https://codecov.io/gh/pandas-dev/pandas/pull/20574?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 The parametrized test checks out. Now I have something that passes but I'm not sure it's idiomatic. Actually there is at least one last problem: I am calling pd.Series a bunch of times when I can just put it around exp in the assertion step. Alright, I just learned that supplying `-s` to pytest allows output from `print()` statements to be shown. With that knowledge I was able to check whether the values being generated made sense before committing this time. Sorry for jumping the gun. All set Thanks @readyready15728 !