BUG: read_csv raising error for python engine when names longer than header but equal to data (#44654)

Author: phofl

doc/source/whatsnew/v1.4.0.rst

@@ -668,6 +668,7 @@ I/O
 - Bug in :func:`read_csv` with :code:`float_precision="round_trip"` which did not skip initial/trailing whitespace (:issue:`43713`)
 - Bug in :func:`read_csv` not applying dtype for ``index_col`` (:issue:`9435`)
 - Bug in dumping/loading a :class:`DataFrame` with ``yaml.dump(frame)`` (:issue:`42748`)
+- Bug in :func:`read_csv` raising ``ValueError`` when names was longer than header but equal to data rows for ``engine="python"`` (:issue:`38453`)
 - Bug in :class:`ExcelWriter`, where ``engine_kwargs`` were not passed through to all engines (:issue:`43442`)
 - Bug in :func:`read_csv` raising ``ValueError`` when ``parse_dates`` was used with ``MultiIndex`` columns (:issue:`8991`)
 - Bug in :func:`read_csv` converting columns to numeric after date parsing failed (:issue:`11019`)

pandas/io/parsers/python_parser.py

@@ -448,7 +448,15 @@ def _infer_columns(self):
                 self._clear_buffer()
 
             if names is not None:
-                if len(names) > len(columns[0]):
+                # Read first row after header to check if data are longer
+                try:
+                    first_line = self._next_line()
+                except StopIteration:
+                    first_line = None
+
+                len_first_data_row = 0 if first_line is None else len(first_line)
+
+                if len(names) > len(columns[0]) and len(names) > len_first_data_row:
                     raise ValueError(
                         "Number of passed names did not match "
                         "number of header fields in the file"

pandas/tests/io/parser/test_header.py

@@ -574,6 +574,19 @@ def test_multi_index_unnamed(all_parsers, index_col, columns):
         tm.assert_frame_equal(result, expected)
 
 
+@skip_pyarrow
+def test_names_longer_than_header_but_equal_with_data_rows(all_parsers):
+    # GH#38453
+    parser = all_parsers
+    data = """a, b
+1,2,3
+5,6,4
+"""
+    result = parser.read_csv(StringIO(data), header=0, names=["A", "B", "C"])
+    expected = DataFrame({"A": [1, 5], "B": [2, 6], "C": [3, 4]})
+    tm.assert_frame_equal(result, expected)
+
+
 @skip_pyarrow
 def test_read_csv_multiindex_columns(all_parsers):
     # GH#6051