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
>>> def line_fixer(line):
... return [1, 2, 3, 4, 5]
...
>>> df = pd.read_csv('test.csv', engine='python', on_bad_lines=line_fixer)
<stdin>:1: ParserWarning: Length of header or names does not match length of data. This leads to a loss of data with index_col=False.
>>> df = pd.read_csv('test.csv', engine='python', on_bad_lines=line_fixer, index_col=0)
>>>Issue Description
test.csv, with extra column ("E") in row 3
id,field_1,field_2
101,A,B
102,C,D,E
103,F,G
Callable line_fixer returns a list with 5 elements, which is more elements than expected.
Documentation for the read_csv() on_bad_lines callable states:
If the function returns a new list of strings with more elements than expected, a ParserWarning will be emitted while dropping extra elements.
This behavior is correctly seen when index_col=None (the default), but not when index_col is set.
Expected Behavior
A ParserWarning should be raised regardless of the index_col parameter. In either case, data (elements 4 and 5, in this example) are being lost, but this is done silently when index_col is set.
Installed Versions
INSTALLED VERSIONS
commit : c888af6
python : 3.10.4
python-bits : 64
OS : Windows
OS-release : 10
Version : 10.0.26100
machine : AMD64
processor : Intel64 Family 6 Model 140 Stepping 1, GenuineIntel
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : English_United States.1252
pandas : 2.3.1
numpy : 2.2.5
pytz : 2025.2
dateutil : 2.9.0.post0
pip : 25.1.1
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 : 20.0.0
pyreadstat : None
pytest : None
python-calamine : None
pyxlsb : None
s3fs : None
scipy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
xlsxwriter : 3.2.3
zstandard : None
tzdata : 2025.2
qtpy : None
pyqt5 : None