EHN: to_csv compression accepts file-like object
State: closed

- [x] closes #21227 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Handle an unsupported case when a file-like object instead of path passed into to_csv with compression. According to documentation, compression keyword requires it to be a filename.

At the moment, when a handle is passed, it appears to be uncompressed.

Tentative enhancement.

Labels:
name: Output-Formatting
description: __repr__ of pandas objects, to_string
name: IO CSV
description: read_csv, to_csv

Comments:
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21249?src=pr&el=h1) Report
> Merging [#21249](https://codecov.io/gh/pandas-dev/pandas/pull/21249?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/c85ab083919b59ce84c220d5baf7d34ff4a0bcf2?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21249/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/21249?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21249      +/-   ##
==========================================
+ Coverage   91.84%   91.84%   +<.01%     
==========================================
  Files         153      153              
  Lines       49538    49540       +2     
==========================================
+ Hits        45499    45501       +2     
  Misses       4039     4039
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.24% <100%> (ø)` | :arrow_up: |
| #single | `41.87% <0%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21249?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/21249/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.22% <ø> (ø)` | :arrow_up: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/21249/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `94.12% <ø> (ø)` | :arrow_up: |
| [pandas/io/formats/csvs.py](https://codecov.io/gh/pandas-dev/pandas/pull/21249/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvY3N2cy5weQ==) | `98.13% <100%> (ø)` | :arrow_up: |
| [pandas/util/\_decorators.py](https://codecov.io/gh/pandas-dev/pandas/pull/21249/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvX2RlY29yYXRvcnMucHk=) | `82.25% <0%> (ø)` | :arrow_up: |
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/21249/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `97% <0%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21249?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/21249?src=pr&el=footer). Last update [c85ab08...8d0c45b](https://codecov.io/gh/pandas-dev/pandas/pull/21249?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).

thanks @minggli nice patch!

note - we might be able to use this compression_only fixture in some of the parser compression tests (eg. where we would skip on None)