BUG: Fix MultiIndex DataFrame to_csv() segfault
State: closed

- [x] closes #26303 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

This fix for #26303 would avoid an indexing issue caused by using MultiIndexes which results to a segfault in the example provided in #26303.

Labels:
name: IO CSV
description: read_csv, to_csv
name: MultiIndex

Comments:
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26355?src=pr&el=h1) Report
> Merging [#26355](https://codecov.io/gh/pandas-dev/pandas/pull/26355?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/681a22c154b84659e90507f7727dbec5cd65d01d?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/26355/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26355?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26355      +/-   ##
==========================================
- Coverage   92.04%   92.03%   -0.01%     
==========================================
  Files         175      175              
  Lines       52292    52292              
==========================================
- Hits        48132    48128       -4     
- Misses       4160     4164       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.59% <100%> (ø)` | :arrow_up: |
| #single | `40.71% <0%> (-0.15%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26355?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/multi.py](https://codecov.io/gh/pandas-dev/pandas/pull/26355/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9tdWx0aS5weQ==) | `95.62% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26355/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26355/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.01% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26355?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/26355?src=pr&el=footer). Last update [681a22c...17d04a5](https://codecov.io/gh/pandas-dev/pandas/pull/26355?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/26355?src=pr&el=h1) Report
> Merging [#26355](https://codecov.io/gh/pandas-dev/pandas/pull/26355?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/a6e43a43f2cb1b4b7d46b262be2efb825d033eb8?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/26355/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26355?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26355      +/-   ##
==========================================
- Coverage   91.68%   91.67%   -0.01%     
==========================================
  Files         174      174              
  Lines       50703    50703              
==========================================
- Hits        46488    46484       -4     
- Misses       4215     4219       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.18% <100%> (ø)` | :arrow_up: |
| #single | `41.17% <0%> (-0.18%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26355?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/multi.py](https://codecov.io/gh/pandas-dev/pandas/pull/26355/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9tdWx0aS5weQ==) | `95.62% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26355/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26355/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.01% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26355?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/26355?src=pr&el=footer). Last update [a6e43a4...d1d0565](https://codecov.io/gh/pandas-dev/pandas/pull/26355?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).

Hello @mahepe! 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 2019-05-13 18:15:51 UTC
thanks @mahepe