BUG: Throw a ParserError when header rows have unequal column counts …

pandas/io/parsers/base_parser.py

@@ -341,6 +341,14 @@ def _extract_multi_indexer_columns(
         # extract the columns
         field_count = len(header[0])
 
+        # check if header lengths are equal
+        for header_iter in range(len(header)):
+            if len(header[header_iter]) != field_count:
+                raise ParserError(
+                    "Header rows must have equal number of columns. "
+                    f"Mismatch found at header {header_iter}."
+                )
+
         def extract(r):
             return tuple(r[i] for i in range(field_count) if i not in sic)
 

pandas/tests/io/parser/test_header.py

@@ -585,3 +585,20 @@ def test_read_csv_multiindex_columns(all_parsers):
     tm.assert_frame_equal(df1, expected.iloc[:1])
     df2 = parser.read_csv(StringIO(s2), header=[0, 1])
     tm.assert_frame_equal(df2, expected)
+
+
+def test_read_csv_multi_header_length_check(all_parsers):
+    # GH#43102
+    parser = all_parsers
+
+    case = """row11,row12,row13
+row21,row22, row23
+row31,row32
+"""
+
+    with pytest.raises(
+        ParserError,
+        match="Header rows must have equal number of columns. "
+        "Mismatch found at header 1.",
+    ):
+        parser.read_csv(StringIO(case), sep=",", header=[0, 2])