use == to test String equality
@jreback quick fix following f4330611ff5ac1cbb4a89c4a7dab3d0900f9e64a (see [lgtm report](https://lgtm.com/projects/g/pydata/pandas/rev/f4330611ff5ac1cbb4a89c4a7dab3d0900f9e64a))

Using `is` instead of `==` might work in simple cases such as this one due to the Python implementation being "efficient" but it checks for identity rather than value and will  produce likely un-intended results pretty quickly:

```
>>> a = "f"
>>> b = "f"
>>> a == b
True
>>> a is b
True
```
but:

```
>>> a = "f "
>>> b = "f "
>>> a == b
True
>>> a is b
False
```

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17171?src=pr&el=h1) Report
> Merging [#17171](https://codecov.io/gh/pandas-dev/pandas/pull/17171?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/929c66fd74da221078a67ea7fd3dbcbe21d642e0?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17171/graphs/tree.svg?token=eZ4WkYLtcO&width=650&src=pr&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/17171?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17171      +/-   ##
==========================================
- Coverage   90.99%   90.98%   -0.02%     
==========================================
  Files         162      162              
  Lines       49508    49508              
==========================================
- Hits        45052    45043       -9     
- Misses       4456     4465       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.75% <100%> (ø)` | :arrow_up: |
| #single | `40.27% <0%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17171?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/parquet.py](https://codecov.io/gh/pandas-dev/pandas/pull/17171?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnF1ZXQucHk=) | `65.75% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17171?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17171?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.66% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17171?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/17171?src=pr&el=footer). Last update [929c66f...46d95bd](https://codecov.io/gh/pandas-dev/pandas/pull/17171?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/17171?src=pr&el=h1) Report
> Merging [#17171](https://codecov.io/gh/pandas-dev/pandas/pull/17171?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/929c66fd74da221078a67ea7fd3dbcbe21d642e0?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17171/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/17171?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17171      +/-   ##
==========================================
- Coverage   90.99%   90.98%   -0.02%     
==========================================
  Files         162      162              
  Lines       49508    49508              
==========================================
- Hits        45052    45043       -9     
- Misses       4456     4465       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.75% <100%> (ø)` | :arrow_up: |
| #single | `40.27% <0%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17171?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/parquet.py](https://codecov.io/gh/pandas-dev/pandas/pull/17171?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnF1ZXQucHk=) | `65.75% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17171?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17171?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.66% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17171?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/17171?src=pr&el=footer). Last update [929c66f...46d95bd](https://codecov.io/gh/pandas-dev/pandas/pull/17171?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/17171?src=pr&el=h1) Report
> Merging [#17171](https://codecov.io/gh/pandas-dev/pandas/pull/17171?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/929c66fd74da221078a67ea7fd3dbcbe21d642e0?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17171/graphs/tree.svg?token=eZ4WkYLtcO&width=650&src=pr&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/17171?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17171      +/-   ##
==========================================
- Coverage   90.99%   90.98%   -0.02%     
==========================================
  Files         162      162              
  Lines       49508    49508              
==========================================
- Hits        45052    45043       -9     
- Misses       4456     4465       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.75% <100%> (ø)` | :arrow_up: |
| #single | `40.27% <0%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17171?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/parquet.py](https://codecov.io/gh/pandas-dev/pandas/pull/17171?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnF1ZXQucHk=) | `65.75% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17171?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17171?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.66% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17171?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/17171?src=pr&el=footer). Last update [929c66f...46d95bd](https://codecov.io/gh/pandas-dev/pandas/pull/17171?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!