Save the name of undefined variables in expressions.
This can be useful information for downstream error-handling code.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20637?src=pr&el=h1) Report
> Merging [#20637](https://codecov.io/gh/pandas-dev/pandas/pull/20637?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6d610a4d9393c0d0335267dc3252ccabb9e51e43?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20637/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/20637?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20637      +/-   ##
==========================================
+ Coverage   91.82%   91.84%   +0.02%     
==========================================
  Files         153      153              
  Lines       49256    49258       +2     
==========================================
+ Hits        45227    45243      +16     
+ Misses       4029     4015      -14
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.23% <100%> (+0.02%)` | :arrow_up: |
| #single | `41.91% <100%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20637?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/computation/ops.py](https://codecov.io/gh/pandas-dev/pandas/pull/20637/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tcHV0YXRpb24vb3BzLnB5) | `95.63% <100%> (+0.03%)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/20637/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `84.73% <0%> (+0.2%)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/20637/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `66.81% <0%> (+1.73%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20637?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/20637?src=pr&el=footer). Last update [6d610a4...7223823](https://codecov.io/gh/pandas-dev/pandas/pull/20637?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 would need a test to show the use of this @kalekundert can you make a test case out of some of the downstream code that motivated this?  Once this is tested this will be about ready. @kalekundert ping re implementing a test. Closing as stale. Ping if you'd like to reopen