Description
Pandas version checks
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I have checked that this issue has not already been reported.
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I have confirmed this bug exists on the latest version of pandas.
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I have confirmed this bug exists on the main branch of pandas.
Reproducible Example
import pandas as pd
import datetime as dt
print(pd.__version__) # 2.2.3
left = pd.DataFrame({
'left_date': pd.date_range(dt.date(2025, 1, 1), dt.date(2025, 1, 5)),
})
left = left.drop([0, 1])
right = pd.DataFrame({
'right_date': pd.date_range(dt.date(2025, 1, 1), dt.date(2025, 1, 5)),
'value': range(5)
})
merged = pd.merge_asof(left, right, left_on='left_date', right_on='right_date')
index_before = left.index.tolist()
index_after = merged.index.tolist()
print(index_before) # [2, 3, 4] since first two dropped
print(index_after) # should be [2, 3, 4] but is [0, 1, 2]Issue Description
merge_asof is has the unexpected side effect of resetting a RangeIndex. If there are gaps in the RangeIndex in the left dataframe ahead of the call, there will be no gaps in the dataframe resulting from the call.
Expected Behavior
By default, I would expect the index to be untouched, or at least expect an ignore_index keyword argument to be able to control this behavior.
Installed Versions
INSTALLED VERSIONS
commit : 0691c5c
python : 3.11.9
python-bits : 64
OS : Windows
OS-release : 10
Version : 10.0.19045
machine : AMD64
processor : AMD64 Family 25 Model 97 Stepping 2, AuthenticAMD
byteorder : little
LC_ALL : None
LANG : None
LOCALE : English_United States.1252
pandas : 2.2.3
numpy : 2.2.2
pytz : 2025.1
dateutil : 2.9.0.post0
pip : 25.0.1
Cython : None
sphinx : None
IPython : None
adbc-driver-postgresql: None
adbc-driver-sqlite : None
bs4 : None
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 : 2025.1
qtpy : None
pyqt5 : None