API: harmonize Index.to_flat_index to MultiIndex.to_flat_index

closes #23670

Author: toobaz

doc/source/whatsnew/v0.24.1.rst

@@ -69,6 +69,9 @@ Fixed Regressions
 
 - Fixed the warning for implicitly registered matplotlib converters not showing. See :ref:`whatsnew_0211.converters` for more (:issue:`24963`).
 
+**MultiIndex**
+
+- :meth:`Index.to_flat_index` now always returns an :class:`Index` containing tuples, even in the case the original object was already a flat :class:`Index` (:issue:`23670`).
 
 **Other**
 

pandas/core/indexes/base.py

@@ -1105,23 +1105,31 @@ def summary(self, name=None):
 
     def to_flat_index(self):
         """
-        Identity method.
+        Convert a MultiIndex to an Index of Tuples containing the level values.
 
         .. versionadded:: 0.24.0
 
-        This is implemented for compatability with subclass implementations
-        when chaining.
-
         Returns
         -------
         pd.Index
-            Caller.
+            Index with the MultiIndex data represented in Tuples.
 
-        See Also
+        Examples
         --------
-        MultiIndex.to_flat_index : Subclass implementation.
+        >>> index = pd.MultiIndex.from_product(
+        ...     [['foo', 'bar'], ['baz', 'qux']],
+        ...     names=['a', 'b'])
+        >>> index.to_flat_index()
+        Index([('foo', 'baz'), ('foo', 'qux'),
+               ('bar', 'baz'), ('bar', 'qux')],
+              dtype='object')
         """
-        return self
+
+        from .multi import MultiIndex
+        if not isinstance(self, MultiIndex):
+            self = MultiIndex.from_product([self])
+
+        return Index(self.values, tupleize_cols=False)
 
     def to_series(self, index=None, name=None):
         """

pandas/core/indexes/multi.py

@@ -164,7 +164,6 @@ class MultiIndex(Index):
     set_levels
     set_codes
     to_frame
-    to_flat_index
     is_lexsorted
     sortlevel
     droplevel
@@ -1523,34 +1522,6 @@ def to_hierarchical(self, n_repeat, n_shuffle=1):
                       FutureWarning, stacklevel=2)
         return MultiIndex(levels=levels, codes=codes, names=names)
 
-    def to_flat_index(self):
-        """
-        Convert a MultiIndex to an Index of Tuples containing the level values.
-
-        .. versionadded:: 0.24.0
-
-        Returns
-        -------
-        pd.Index
-            Index with the MultiIndex data represented in Tuples.
-
-        Notes
-        -----
-        This method will simply return the caller if called by anything other
-        than a MultiIndex.
-
-        Examples
-        --------
-        >>> index = pd.MultiIndex.from_product(
-        ...     [['foo', 'bar'], ['baz', 'qux']],
-        ...     names=['a', 'b'])
-        >>> index.to_flat_index()
-        Index([('foo', 'baz'), ('foo', 'qux'),
-               ('bar', 'baz'), ('bar', 'qux')],
-              dtype='object')
-        """
-        return Index(self.values, tupleize_cols=False)
-
     @property
     def is_all_dates(self):
         return False

pandas/tests/indexes/multi/test_conversion.py

@@ -214,11 +214,3 @@ def test_to_series_with_arguments(idx):
     assert s.values is not idx.values
     assert s.index is not idx
     assert s.name != idx.name
-
-
-def test_to_flat_index(idx):
-    expected = pd.Index((('foo', 'one'), ('foo', 'two'), ('bar', 'one'),
-                         ('baz', 'two'), ('qux', 'one'), ('qux', 'two')),
-                        tupleize_cols=False)
-    result = idx.to_flat_index()
-    tm.assert_index_equal(result, expected)

pandas/tests/indexes/test_common.py

@@ -113,12 +113,17 @@ def test_corner_union(self, indices, fname, sname, expected_name):
         tm.assert_index_equal(union, expected)
 
     def test_to_flat_index(self, indices):
-        # 22866
-        if isinstance(indices, MultiIndex):
-            pytest.skip("Separate expectation for MultiIndex")
+        # 22866, 23670
 
         result = indices.to_flat_index()
-        tm.assert_index_equal(result, indices)
+
+        if isinstance(indices, MultiIndex):
+            values = [tuple(v) for v in indices]
+        else:
+            values = [(v,) for v in indices]
+        expected = pd.Index(values, tupleize_cols=False)
+
+        tm.assert_index_equal(result, expected)
 
     def test_wrong_number_names(self, indices):
         with pytest.raises(ValueError, match="^Length"):