Pandas version checks
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[x] I have checked that this issue has not already been reported.
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[x] I have confirmed this bug exists on the latest version of pandas.
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[x] I have confirmed this bug exists on the main branch of pandas.
Reproducible Example
import pandas as pd
# Create a datetime index
t = pd.date_range("2025-07-06", periods=3, freq="h")
# Left dataframe: one row per timestamp
df1 = pd.DataFrame({"time": t, "val1": [1, 2, 3]})
# Right dataframe: two rows per timestamp (duplicates)
df2 = pd.DataFrame({"time": t.repeat(2), "val2": [10, 20, 30, 40, 50, 60]})
# This works
print(pd.merge(df1, df2, on="time", how="left"))
# This fails
print(
pd.merge(
df1.convert_dtypes(dtype_backend="pyarrow"),
df2.convert_dtypes(dtype_backend="pyarrow"),
on="time", # pyarrow datetime column causes error
how="left",
)
)
Issue Description
Error message:
ValueError: Length mismatch: Expected axis has 6 elements, new values have 3 elements
Expected Behavior
The merge should succeed and return 6 rows, like it does when not using dtype_backend="pyarrow".
Installed Versions
INSTALLED VERSIONS
------------------
commit : c888af6d0bb674932007623c0867e1fbd4bdc2c6
python : 3.12.11
python-bits : 64
OS : Darwin
OS-release : 24.5.0
Version : Darwin Kernel Version 24.5.0: Tue Apr 22 19:54:29 PDT 2025; root:xnu-11417.121.6~2/RELEASE_ARM64_T6030
machine : arm64
processor : arm
byteorder : little
LC_ALL : None
LANG : None
LOCALE : C.UTF-8
pandas : 2.3.1
numpy : 2.3.1
pytz : 2025.2
dateutil : 2.9.0.post0
pip : 25.1.1
Cython : None
sphinx : None
IPython : 9.4.0
adbc-driver-postgresql: None
adbc-driver-sqlite : None
bs4 : None
blosc : None
bottleneck : None
dataframe-api-compat : None
fastparquet : None
fsspec : 2025.5.1
html5lib : None
hypothesis : None
gcsfs : None
jinja2 : 3.1.6
lxml.etree : None
matplotlib : 3.10.3
numba : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
psycopg2 : None
pymysql : None
pyarrow : 20.0.0
pyreadstat : None
pytest : None
python-calamine : None
pyxlsb : None
s3fs : None
scipy : 1.16.0
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
xlsxwriter : None
zstandard : None
tzdata : 2025.2
qtpy : None
pyqt5 : None