BUG: DataFrameGroupBy.transform with engine='numba' reorder output by index

[havakv]

Pandas version checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• I have confirmed this bug exists on the main branch of pandas.

Reproducible Example

import pandas as pd
df = pd.DataFrame({'vals': [0, 1, 2, 3], 'group': [0, 1, 0, 1]})
df.index = df.index.values[::-1] #reverse index

def foo(values, index):
    return values 

df.groupby('group')['vals'].transform(foo, engine='numba')
# wrong output:
# 3    3.0
# 2    2.0
# 1    1.0
# 0    0.0
# Name: vals, dtype: float64

Issue Description

DataFrameGroupBy.transform with engine='numba' gives results that are ordered by the index values (and type has changed which is less important). This gives the wrong order unless the index is monotonically increasing. From what I can see we get the correct values and index in the foo function (same ordering as the dataframe), but this are ordered by the index before concatenating the results to the final series. This differs from the behaviour when called without engine='numba' (see below).

Expected Behavior

The example above should produce the same results as

df.groupby('group')['vals'].transform(lambda x: x)

with output

3    0
2    1
1    2
0    3
Name: vals, dtype: int64

Installed Versions

INSTALLED VERSIONS

commit : f538741
python : 3.11.7.final.0
python-bits : 64
OS : Darwin
OS-release : 23.2.0
Version : Darwin Kernel Version 23.2.0: Wed Nov 15 21:53:18 PST 2023; root:xnu-10002.61.3~2/RELEASE_ARM64_T6000
machine : arm64
processor : arm
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 2.2.0
numpy : 1.26.3
pytz : 2023.3.post1
dateutil : 2.8.2
setuptools : 69.0.3
pip : 23.3.2
Cython : None
pytest : None
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : None
IPython : 8.20.0
pandas_datareader : None
adbc-driver-postgresql: None
adbc-driver-sqlite : None
bs4 : None
bottleneck : None
dataframe-api-compat : None
fastparquet : None
fsspec : None
gcsfs : None
matplotlib : None
numba : 0.58.1
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pyreadstat : None
python-calamine : None
pyxlsb : None
s3fs : None
scipy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
zstandard : None
tzdata : 2023.4
qtpy : None
pyqt5 : None

[rhshadrach]

Thanks for the report, confirmed on main. Further investigations and an PRs to fix are welcome!

cc @lithomas1

[andremcorreia]

take

[andremcorreia]

Hello, after finding a solution to this issue, I encountered a failing local test: "pytest pandas/tests/groupby/transform/test_numba.py::test_index_data_correctly_passed," which, if I undestood correctly, currently expects the results to be in the wrong order due to this bug.
(test in question for reference)

def f(values, index):
        return index - 1
df = DataFrame({"group": ["A", "A", "B"], "v": [4, 5, 6]}, index=[-1, -2, -3])
result = df.groupby("group").transform(f, engine="numba")
expected = DataFrame([-4.0, -3.0, -2.0], columns=["v"], index=[-1, -2, -3])

It expects a dataframe in which index -1 corresponds to the value -4, index -2 to the value -3, and index -3 to the value -2. I believe the intended behavior would be index -1, value -2, index -2, value -3, and index -3, value -4.
I'd just like to make sure I'm not misunderstanding the test before modifying it and moving to make a pull request for the issue.

[rhshadrach]

Agreed - I think this is saying the same, but I would think the expected here should be:

expected = DataFrame([-2.0, -3.0, -4.0], columns=["v"], index=[-1, -2, -3])