mend

Author: tisjayy

doc/source/whatsnew/v3.0.0.rst

@@ -814,10 +814,7 @@ I/O
 - Bug in :meth:`set_option` where setting the pandas option ``display.html.use_mathjax`` to ``False`` has no effect (:issue:`59884`)
 - Bug in :meth:`to_csv` where ``quotechar``` is not escaped when ``escapechar`` is not None (:issue:`61407`)
 - Bug in :meth:`to_excel` where :class:`MultiIndex` columns would be merged to a single row when ``merge_cells=False`` is passed (:issue:`60274`)
-- Bug in :func:`read_csv` with ``engine="python"`` and callable ``on_bad_lines`` 
-  where a ``ParserWarning`` for extra fields returned by the callable was only 
-  raised when ``index_col`` was ``None``. Now the warning is consistently raised 
-  regardless of ``index_col`` (:issue:`#61837`)
+- Bug in :func:`read_csv` with ``engine="python"`` and callable ``on_bad_lines`` where a ``ParserWarning`` for extra fields returned by the callable was only raised when ``index_col`` was ``None``. Now the warning is consistently raised regardless of ``index_col`` (:issue:`#61837`)
 
 Period
 ^^^^^^

pandas/io/parsers/base_parser.py

@@ -614,11 +614,7 @@ def _check_data_length(
         columns: list of column names
         data: list of array-likes containing the data column-wise.
         """
-        if not self.index_col and len(columns) != len(data) and columns:
-            # error: No overload variant of "__ror__" of "ndarray" matches
-            # argument type "ExtensionArray"
-            if len(data) > len(columns) :
-                return
+        if columns and len(data)!=len(columns):
             warnings.warn(
                 f"Length of header or names ({len(columns)}) does not match number of "
                 f"fields in line ({len(data)}). Extra field will be dropped.",

pandas/tests/io/parser/test_python_parser_only.py

@@ -332,11 +332,12 @@ def test_on_bad_lines_extra_fields_warns(python_parser_only):
 
     def line_fixer(_line):
         return ["1", "2", "3", "4", "5"]
+
     for index_col in [None, 0]:
         with tm.assert_produces_warning(ParserWarning):
             parser.read_csv(
-                    StringIO(data), on_bad_lines=line_fixer, index_col=index_col
-                    )
+                StringIO(data), on_bad_lines=line_fixer, index_col=index_col
+            )
 
 
 def test_python_engine_file_no_next(python_parser_only):