BUG: combine_first reorders columns

[davetapley]

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({"B": [1, 2, 3], "A": [4, 5, 6]}, index=["a", "b", "c"])

print(df)  # B first, then A
print()

df_ = pd.DataFrame({"A": [7]}, index=["b"])

print(df.combine_first(df_))  # A first, then B
print()

print(df_.combine_first(df))  # A first, then B
print()


print(df_.combine_first(df)[df.columns])  # Workaround

Issue Description

I wouldn't expect combine_first to reorder the columns alphabetically, but it does.

Bug might be a stretch, but it's certainly unexpected and awkward.

Expected Behavior

Preserve the column order, as show in # Workaround.

Installed Versions

INSTALLED VERSIONS

commit : 0691c5c
python : 3.13.0+
python-bits : 64
OS : Darwin
OS-release : 21.6.0
Version : Darwin Kernel Version 21.6.0: Wed Oct 4 23:55:28 PDT 2023; root:xnu-8020.240.18.704.15~1/RELEASE_X86_64
machine : x86_64
processor : i386
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 2.2.3
numpy : 2.1.3
pytz : 2024.2
dateutil : 2.9.0.post0
pip : 24.2
Cython : None
sphinx : None
IPython : None
adbc-driver-postgresql: None
adbc-driver-sqlite : None
bs4 : 4.12.3
blosc : None
bottleneck : None
dataframe-api-compat : None
fastparquet : None
fsspec : None
html5lib : None
hypothesis : None
gcsfs : None
jinja2 : None
lxml.etree : None
matplotlib : None
numba : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
psycopg2 : None
pymysql : None
pyarrow : None
pyreadstat : None
pytest : None
python-calamine : None
pyxlsb : None
s3fs : None
scipy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
xlsxwriter : None
zstandard : None
tzdata : 2024.2
qtpy : None
pyqt5 : None

[U-S-jun]

take

[rhshadrach]

Thanks for the report, agreed this method should preserve column order. I am comfortable with calling this a bug.