TST: Add the default separator test for PythonParser
- [x] closes #17333
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17822?src=pr&el=h1) Report
> Merging [#17822](https://codecov.io/gh/pandas-dev/pandas/pull/17822?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/9f0ee53dad3bd14a3f0475a29d326c89c6b6aced?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17822/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/17822?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17822      +/-   ##
==========================================
- Coverage   91.26%   91.24%   -0.02%     
==========================================
  Files         163      163              
  Lines       49980    49980              
==========================================
- Hits        45613    45604       -9     
- Misses       4367     4376       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.04% <ø> (ø)` | :arrow_up: |
| #single | `40.24% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17822?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17822?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17822?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.74% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17822?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/17822?src=pr&el=footer). Last update [9f0ee53...a787c20](https://codecov.io/gh/pandas-dev/pandas/pull/17822?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/17822?src=pr&el=h1) Report
> Merging [#17822](https://codecov.io/gh/pandas-dev/pandas/pull/17822?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/727ea20bdb72ff4a5b8380d7f613d4aff058e28a?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/17822/graphs/tree.svg?height=150&width=650&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/17822?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17822      +/-   ##
==========================================
+ Coverage   91.22%   91.23%   +<.01%     
==========================================
  Files         163      163              
  Lines       50014    50014              
==========================================
+ Hits        45627    45630       +3     
+ Misses       4387     4384       -3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.04% <ø> (+0.02%)` | :arrow_up: |
| #single | `40.24% <ø> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17822?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17822?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17822?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.77% <0%> (-0.1%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/17822?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.2% <0%> (+1.81%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17822?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/17822?src=pr&el=footer). Last update [727ea20...c273d9b](https://codecov.io/gh/pandas-dev/pandas/pull/17822?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 so this is just validating what we already do? no actual change / bug?

@gfyoung @Licht-T  > so this is just validating what we already do? no actual change / bug?

Yep, just validation also does this warrant a small note on the doc-string as well? > also does this warrant a small note on the doc-string as well?

Maybe: we could mention that we use Python's builtin sniffer tool from the `csv` library. @gfyoung Added the comment.
 @jreback @gfyoung Should I also add the document? > Should I also add the document?

I think so.  Look at the documentation for `sep` and add that we use Python's builtin sniffer for the Python engine (need to add both in `parsers.py` and `io.rst`). @gfyoung Fixed & Added the document. Same issue with the matplotlib failures that'll be fixed by testing the merge commit.

Thanks again @Licht-T 