BUG: RangeIndex accepting length-1 arrays as argument

[jorisvandenbossche]

• closes BUG: groupby.transform fails with incompatible size #13542
• tests added / passed
• passes git diff upstream/master | flake8 --diff
• whatsnew entry

[codecov-io]

Current coverage is 85.27% (diff: 100%)

Merging #13765 into master will decrease coverage by <.01%

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+ Hits          42819      42820     +1   
- Misses         7392       7394     +2   
  Partials          0          0          

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[jorisvandenbossche]

@jreback OK?

[jreback]

can you add the orginal issue example somewhere (and assert it works)

[jorisvandenbossche]

That's what I added in test_concat.py (next to the addition in the tests in test_range.py) I think?

[jreback]

is it a simplified example?

the original one is on a panel

[jorisvandenbossche]

Well, looking back at the original issue #13542, this is indeed not the original example I included in the tests. But I found the reported example rather convoluted/difficult to comprehend what was going on. And the example I added, is a simplified (whithout using panels) example that experience exactly the same issue (it is what happened inside the groupby).

But I can add the original panel example as well if you like

[jreback]

it's fine
a simplified example is often better anyhow
so lgtm