BUG: Fix of handle missing CSV MI column names

Before, only the first index name got replaced
with `None` so long as it had the string "Unnamed"
in it.

Now we replace all index names with `None` if they
were deliberately set with placeholders.

Author: gfyoung

doc/source/whatsnew/v0.24.0.txt

@@ -1275,6 +1275,7 @@ Notice how we now instead output ``np.nan`` itself instead of a stringified form
 - Bug in :func:`to_string()` that broke column alignment when ``index=False`` and width of first column's values is greater than the width of first column's header (:issue:`16839`, :issue:`13032`)
 - Bug in :func:`DataFrame.to_csv` where a single level MultiIndex incorrectly wrote a tuple. Now just the value of the index is written (:issue:`19589`).
 - Bug in :meth:`HDFStore.append` when appending a :class:`DataFrame` with an empty string column and ``min_itemsize`` < 8 (:issue:`12242`)
+- Bug in :meth:`read_csv()` in which :class:`MultiIndex` index names were being improperly handled in the cases when they were not provided (:issue:`23484`)
 
 Plotting
 ^^^^^^^^

pandas/_libs/parsers.pyx

@@ -302,6 +302,7 @@ cdef class TextReader:
         object tupleize_cols
         object usecols
         list dtype_cast_order
+        set unnamed_cols
         set noconvert
 
     def __cinit__(self, source,
@@ -536,7 +537,7 @@ cdef class TextReader:
                 self.header = [ header ]
 
         self.names = names
-        self.header, self.table_width = self._get_header()
+        self.header, self.table_width, self.unnamed_cols = self._get_header()
 
         if not self.table_width:
             raise EmptyDataError("No columns to parse from file")
@@ -727,6 +728,8 @@ cdef class TextReader:
             cdef StringPath path = _string_path(self.c_encoding)
 
         header = []
+        unnamed_cols = set()
+
         if self.parser.header_start >= 0:
 
             # Header is in the file
@@ -758,7 +761,6 @@ cdef class TextReader:
                     start = self.parser.line_start[hr]
 
                 counts = {}
-                unnamed_count = 0
                 for i in range(field_count):
                     word = self.parser.words[start + i]
 
@@ -776,8 +778,7 @@ cdef class TextReader:
                                     .format(i=i, lvl=level))
                         else:
                             name = 'Unnamed: {i}'.format(i=i)
-                        unnamed_count += 1
-
+                        unnamed_cols.add(name)
                     count = counts.get(name, 0)
 
                     if not self.has_mi_columns and self.mangle_dupe_cols:
@@ -798,6 +799,8 @@ cdef class TextReader:
                         lc = len(this_header)
                         ic = (len(self.index_col) if self.index_col
                               is not None else 0)
+                        unnamed_count = len(unnamed_cols)
+
                         if lc != unnamed_count and lc - ic > unnamed_count:
                             hr -= 1
                             self.parser_start -= 1
@@ -830,7 +833,7 @@ cdef class TextReader:
             if self.parser.lines < 1:
                 self._tokenize_rows(1)
 
-            return None, self.parser.line_fields[0]
+            return None, self.parser.line_fields[0], unnamed_cols
 
         # Corner case, not enough lines in the file
         if self.parser.lines < data_line + 1:
@@ -864,7 +867,7 @@ cdef class TextReader:
             elif self.allow_leading_cols and passed_count < field_count:
                 self.leading_cols = field_count - passed_count
 
-        return header, field_count
+        return header, field_count, unnamed_cols
 
     def read(self, rows=None):
         """

pandas/io/parsers.py

@@ -1265,6 +1265,7 @@ def __init__(self, kwds):
         self.prefix = kwds.pop('prefix', None)
 
         self.index_col = kwds.get('index_col', None)
+        self.unnamed_cols = set()
         self.index_names = None
         self.col_names = None
 
@@ -1374,7 +1375,8 @@ def _extract_multi_indexer_columns(self, header, index_names, col_names,
         # clean the index_names
         index_names = header.pop(-1)
         index_names, names, index_col = _clean_index_names(index_names,
-                                                           self.index_col)
+                                                           self.index_col,
+                                                           self.unnamed_cols)
 
         # extract the columns
         field_count = len(header[0])
@@ -1454,7 +1456,8 @@ def _make_index(self, data, alldata, columns, indexnamerow=False):
             if not self._name_processed:
                 (self.index_names, _,
                  self.index_col) = _clean_index_names(list(columns),
-                                                      self.index_col)
+                                                      self.index_col,
+                                                      self.unnamed_cols)
                 self._name_processed = True
             index = self._get_complex_date_index(data, columns)
             index = self._agg_index(index, try_parse_dates=False)
@@ -1732,6 +1735,7 @@ def __init__(self, src, **kwds):
         kwds['usecols'] = self.usecols
 
         self._reader = parsers.TextReader(src, **kwds)
+        self.unnamed_cols = self._reader.unnamed_cols
 
         passed_names = self.names is None
 
@@ -1792,7 +1796,8 @@ def __init__(self, src, **kwds):
                 self._name_processed = True
                 (index_names, self.names,
                  self.index_col) = _clean_index_names(self.names,
-                                                      self.index_col)
+                                                      self.index_col,
+                                                      self.unnamed_cols)
 
                 if self.index_names is None:
                     self.index_names = index_names
@@ -1966,7 +1971,8 @@ def _get_index_names(self):
 
         if self._reader.leading_cols == 0 and self.index_col is not None:
             (idx_names, names,
-             self.index_col) = _clean_index_names(names, self.index_col)
+             self.index_col) = _clean_index_names(names, self.index_col,
+                                                  self.unnamed_cols)
 
         return names, idx_names
 
@@ -2112,7 +2118,8 @@ def __init__(self, f, **kwds):
         # Get columns in two steps: infer from data, then
         # infer column indices from self.usecols if it is specified.
         self._col_indices = None
-        self.columns, self.num_original_columns = self._infer_columns()
+        (self.columns, self.num_original_columns,
+         self.unnamed_cols) = self._infer_columns()
 
         # Now self.columns has the set of columns that we will process.
         # The original set is stored in self.original_columns.
@@ -2367,6 +2374,8 @@ def _infer_columns(self):
         names = self.names
         num_original_columns = 0
         clear_buffer = True
+        unnamed_cols = set()
+
         if self.header is not None:
             header = self.header
 
@@ -2400,24 +2409,25 @@ def _infer_columns(self):
                         if clear_buffer:
                             self._clear_buffer()
                         columns.append([None] * len(columns[-1]))
-                        return columns, num_original_columns
+                        return columns, num_original_columns, unnamed_cols
 
                     if not self.names:
                         raise EmptyDataError(
                             "No columns to parse from file")
 
                     line = self.names[:]
 
-                unnamed_count = 0
                 this_columns = []
                 for i, c in enumerate(line):
                     if c == '':
                         if have_mi_columns:
-                            this_columns.append('Unnamed: %d_level_%d'
-                                                % (i, level))
+                            col_name = ("Unnamed: {i}_level_{level}"
+                                        .format(i=i, level=level))
                         else:
-                            this_columns.append('Unnamed: %d' % i)
-                        unnamed_count += 1
+                            col_name = "Unnamed: {i}".format(i=i)
+
+                        unnamed_cols.add(col_name)
+                        this_columns.append(col_name)
                     else:
                         this_columns.append(c)
 
@@ -2443,6 +2453,8 @@ def _infer_columns(self):
                         lc = len(this_columns)
                         ic = (len(self.index_col)
                               if self.index_col is not None else 0)
+                        unnamed_count = len(unnamed_cols)
+
                         if lc != unnamed_count and lc - ic > unnamed_count:
                             clear_buffer = False
                             this_columns = [None] * lc
@@ -2513,7 +2525,7 @@ def _infer_columns(self):
                     columns = [names]
                     num_original_columns = ncols
 
-        return columns, num_original_columns
+        return columns, num_original_columns, unnamed_cols
 
     def _handle_usecols(self, columns, usecols_key):
         """
@@ -2879,7 +2891,8 @@ def _get_index_name(self, columns):
         else:
             # Case 2
             (index_name, columns_,
-             self.index_col) = _clean_index_names(columns, self.index_col)
+             self.index_col) = _clean_index_names(columns, self.index_col,
+                                                  self.unnamed_cols)
 
         return index_name, orig_names, columns
 
@@ -3178,7 +3191,7 @@ def _clean_na_values(na_values, keep_default_na=True):
     return na_values, na_fvalues
 
 
-def _clean_index_names(columns, index_col):
+def _clean_index_names(columns, index_col, unnamed_cols):
     if not _is_index_col(index_col):
         return None, columns, index_col
 
@@ -3203,10 +3216,10 @@ def _clean_index_names(columns, index_col):
             columns.remove(name)
             index_names.append(name)
 
-    # hack
-    if (isinstance(index_names[0], compat.string_types) and
-            'Unnamed' in index_names[0]):
-        index_names[0] = None
+    # Only clean index names that were placeholders.
+    for i, name in enumerate(index_names):
+        if isinstance(name, compat.string_types) and name in unnamed_cols:
+            index_names[i] = None
 
     return index_names, columns, index_col
 

pandas/tests/io/parser/index_col.py

@@ -141,3 +141,21 @@ def test_empty_with_index_col_false(self):
         result = self.read_csv(StringIO(data), index_col=False)
         expected = DataFrame([], columns=['x', 'y'])
         tm.assert_frame_equal(result, expected)
+
+    @pytest.mark.parametrize("index_names", [
+        ["", ""],
+        ["foo", ""],
+        ["", "bar"],
+        ["foo", "bar"],
+        ["NotReallyUnnamed", "Unnamed: 0"],
+    ])
+    def test_multi_index_naming(self, index_names):
+        # We don't want empty index names being replaced with "Unnamed: 0"
+        data = ",".join(index_names + ["col\na,c,1\na,d,2\nb,c,3\nb,d,4"])
+        result = self.read_csv(StringIO(data), index_col=[0, 1])
+
+        expected = DataFrame({"col": [1, 2, 3, 4]},
+                             index=MultiIndex.from_product([["a", "b"],
+                                                            ["c", "d"]]))
+        expected.index.names = [name if name else None for name in index_names]
+        tm.assert_frame_equal(result, expected)