Merge branch 'negative-header' of https://github.com/MarcoGorelli/pandas

Author: Marco Gorelli

pandas/io/common.py

@@ -125,6 +125,13 @@ def _validate_header_arg(header) -> None:
             "the row(s) making up the column names"
         )
 
+    if header == -1:  # In versions prior to 0.25, this
+        # would have worked.
+        raise ValueError(
+            "Passing -1 to header is invalid. "
+            "For no header, use header=None instead"
+        )
+
 
 def _stringify_path(
     filepath_or_buffer: FilePathOrBuffer[AnyStr]

pandas/tests/io/parser/test_header.py

@@ -39,6 +39,20 @@ def test_bool_header_arg(all_parsers, header):
         parser.read_csv(StringIO(data), header=header)
 
 
+def test_negative_header(all_parsers):
+    parser = all_parsers
+    data = """1,2,3,4,5
+6,7,8,9,10
+11,12,13,14,15
+"""
+    with pytest.raises(
+        ValueError,
+        match="Passing -1 to header is invalid. "
+        "For no header, use header=None instead",
+    ):
+        parser.read_csv(StringIO(data), header=-1)
+
+
 def test_no_header_prefix(all_parsers):
     parser = all_parsers
     data = """1,2,3,4,5