Initial commit to fix csv loading bug pandas-dev#56929

Author: CYHSM

pandas/_libs/parsers.pyi

@@ -656,52 +656,58 @@ cdef class TextReader:
 
             # Header is in the file
             for level, hr in enumerate(prelim_header):
+                print(f"Processing level {level}, header row {hr}")
 
                 this_header = []
 
                 if self.parser.lines < hr + 1:
+                    print("Tokenizing rows...")
                     self._tokenize_rows(hr + 2)
 
                 if self.parser.lines == 0:
+                    print("No lines in parser.")
                     field_count = 0
                     start = self.parser.line_start[0]
 
                 # e.g., if header=3 and file only has 2 lines
-                elif (self.parser.lines < hr + 1
-                      and not isinstance(self.orig_header, list)) or (
-                          self.parser.lines < hr):
+                elif (self.parser.lines < hr + 1 and not isinstance(self.orig_header, list)) or (self.parser.lines < hr):
                     msg = self.orig_header
                     if isinstance(msg, list):
                         joined = ",".join(str(m) for m in msg)
                         msg = f"[{joined}], len of {len(msg)},"
-                    raise ParserError(
-                        f"Passed header={msg} but only "
-                        f"{self.parser.lines} lines in file")
+                    raise ParserError(f"Passed header={msg} but only {self.parser.lines} lines in file")
 
                 else:
                     field_count = self.parser.line_fields[hr]
                     start = self.parser.line_start[hr]
+                    print(f"Field count: {field_count}, Start: {start}")
 
                 unnamed_count = 0
                 unnamed_col_indices = []
 
                 for i in range(field_count):
                     word = self.parser.words[start + i]
 
-                    name = PyUnicode_DecodeUTF8(word, strlen(word),
-                                                self.encoding_errors)
+                    name = PyUnicode_DecodeUTF8(word, strlen(word), self.encoding_errors)
+                    print(f"Word {i}: '{word}', Decoded name: '{name}'")
 
                     if name == "":
+                        print("Empty name found.")
                         if self.has_mi_columns:
                             name = f"Unnamed: {i}_level_{level}"
+                            print(f"Setting multi-index column name: {name}")
                         else:
                             name = f"Unnamed: {i}"
+                            print(f"Setting unnamed column name: {name}")
 
                         unnamed_count += 1
                         unnamed_col_indices.append(i)
 
                     this_header.append(name)
 
+                print(f"This header: {this_header}")
+
+
                 if not self.has_mi_columns:
                     # Ensure that regular columns are used before unnamed ones
                     # to keep given names and mangle unnamed columns
@@ -744,9 +750,10 @@ cdef class TextReader:
                         lc = len(this_header)
                         ic = (len(self.index_col) if self.index_col
                               is not None else 0)
+                        print(f"LC {lc}, IC {ic} Unnamed Count {unnamed_count}")                        
 
                         # if wrong number of blanks or no index, not our format
-                        if (lc != unnamed_count and lc - ic > unnamed_count) or ic == 0:
+                        if (lc != unnamed_count and lc - ic >= unnamed_count) or ic == 0:
                             hr -= 1
                             self.parser_start -= 1
                             this_header = [None] * lc
@@ -2156,4 +2163,4 @@ def sanitize_objects(ndarray[object] values, set na_values) -> int:
         else:
             memo[val] = val
 
-    return na_count
+    return na_count

pandas/_libs/parsers.pyx

@@ -14,7 +14,7 @@
     compat,
 )
 import pandas._testing as tm
-
+from pandas.testing import assert_frame_equal
 
 class TestToCSV:
     def test_to_csv_with_single_column(self):
@@ -368,6 +368,22 @@ def test_to_csv_multi_index(self):
         exp = tm.convert_rows_list_to_csv_str(exp_rows)
         assert df.to_csv(index=False) == exp
 
+    def test_to_csv_multi_index_nan(self):
+        # Create a MultiIndex DataFrame
+        columns = pd.MultiIndex.from_tuples([('Level 1', 'Level 2')], names=['level1', 'level2'])
+        data = [[np.nan], [0.1], [0.4]]
+        df_complex = pd.DataFrame(data, columns=columns)
+
+        # Expected DataFrame
+        expected_df = pd.DataFrame(data, columns=columns, index=range(3))
+
+        # Save and load the DataFrame as a CSV
+        with tm.ensure_clean("complex_data.csv") as path:
+            df_complex.to_csv(path)
+            loaded_df_complex = pd.read_csv(path, header=[0, 1], index_col=0)
+
+        assert_frame_equal(loaded_df_complex, expected_df)
+
     @pytest.mark.parametrize(
         "ind,expected",
         [