BUG: Groupby not keeping string dtype for empty objects

[phofl]

• closes #xxxx (Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

cc @rhshadrach

[rhshadrach]

What happens on an empty list of categoricals with observed=False? I think this is the only case where len(obj) == 0 but len(result) > 0.

[phofl]

I didn't track his down specifically, but the test that was mentioned in the comment is not passing by here anymore

[rhshadrach]

Running on this PR as-is:

func = "min"
dtype = "string[pyarrow_numpy]"
df = DataFrame({"a": ["a"], "b": "a", "c": "a"}, dtype=dtype)
df["a"] = pd.Categorical([1], categories=[1, 2, 3])
df = df.iloc[:0]
result = getattr(df.groupby("a", observed=False), func)()

print(result.dtypes)
# b    float64
# c    float64
# dtype: object

If you remove the condition len(result) == 0 in the line highlighted above, you get string[pyarrow_numpy] for the dtypes instead. However, the values in the DataFrame are still the float NaN. I didn't realize this was possible?

[phofl]

The new string dtype uses nan as missing value representation to keep numpy semantics, so it will work if you feed it an array with all NaN

[rhshadrach]

Ah - thanks

[rhshadrach]

I think this still needs fixed (but okay not for 2.1.2)

[lithomas1]

thanks @phofl.

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

git checkout 2.1.x
git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

git cherry-pick -x -m1 8afd868106dec889df89e3abed36af09bb7ddf8c

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

git commit -am 'Backport PR #55619: BUG: Groupby not keeping string dtype for empty objects'

4. Push to a named branch:

git push YOURFORK 2.1.x:auto-backport-of-pr-55619-on-2.1.x

5. Create a PR against branch 2.1.x, I would have named this PR:

"Backport PR #55619 on branch 2.1.x (BUG: Groupby not keeping string dtype for empty objects)"

And apply the correct labels and milestones.

Congratulations — you did some good work! Hopefully your backport PR will be tested by the continuous integration and merged soon!

Remember to remove the Still Needs Manual Backport label once the PR gets merged.

If these instructions are inaccurate, feel free to suggest an improvement.