Add to_flat_index method to MultiIndex

doc/source/api.rst

@@ -1725,6 +1725,7 @@ MultiIndex Components
    MultiIndex.set_levels
    MultiIndex.set_labels
    MultiIndex.to_hierarchical
+   MultiIndex.to_index
    MultiIndex.to_frame
    MultiIndex.is_lexsorted
    MultiIndex.sortlevel

doc/source/whatsnew/v0.24.0.txt

@@ -214,6 +214,7 @@ Other Enhancements
 - Compatibility with Matplotlib 3.0 (:issue:`22790`).
 - Added :meth:`Interval.overlaps`, :meth:`IntervalArray.overlaps`, and :meth:`IntervalIndex.overlaps` for determining overlaps between interval-like objects (:issue:`21998`)
 - :meth:`Timestamp.tz_localize`, :meth:`DatetimeIndex.tz_localize`, and :meth:`Series.tz_localize` have gained the ``nonexistent`` argument for alternative handling of nonexistent times. See :ref:`timeseries.timezone_nonexsistent` (:issue:`8917`)
+- :meth:`MultiIndex.to_index` has been added to flatten multiple levels into a single-level :class:`Index` object.
 
 .. _whatsnew_0240.api_breaking:
 

pandas/core/indexes/base.py

@@ -1105,6 +1105,52 @@ def _format_attrs(self):
         """
         return format_object_attrs(self)
 
+    def to_index(self, sep=None):
+        """
+        Convert a MultiIndex to an Index of Tuples containing the level values.
+
+        .. versionadded:: 0.24.0
+
+        Parameters
+        ----------
+        sep : str, optional
+            Not yet implemented.
+
+        Returns
+        -------
+        pd.Index : an Index with the MultiIndex data represented in Tuples.
+
+        Notes
+        -----
+        This method will simply return the caller if called by anything other
+        than a MultiIndex.
+
+        See Also
+        --------
+        to_series : Similar method to construct a Series.
+        to_frame : Similar method to construct a DataFrame.
+
+        Examples
+        --------
+        >>> index = pd.MultiIndex.from_product(
+        ...     [['foo', 'bar'], ['baz', 'qux']],
+        ...     names=['a', 'b'])
+        >>> index.to_index()
+        Index([('foo', 'baz'), ('foo', 'qux'),
+               ('bar', 'baz'), ('bar', 'qux')],
+              dtype='object')
+        """
+        if sep is not None:
+            # TODO: Add support for separator to return strs instad of tuples
+            raise NotImplementedError
+
+        if not isinstance(self, ABCMultiIndex):
+            return self
+
+        idx = Index(self.values, tupleize_cols=False)
+
+        return idx
+
     def to_series(self, index=None, name=None):
         """
         Create a Series with both index and values equal to the index keys

pandas/core/indexes/multi.py

@@ -194,6 +194,7 @@ class MultiIndex(Index):
     set_levels
     set_labels
     to_frame
+    to_index
     is_lexsorted
     sortlevel
     droplevel

pandas/tests/indexes/multi/test_conversion.py

@@ -169,3 +169,11 @@ 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_index(idx):
+    expected = pd.Index((('foo', 'one'), ('foo', 'two'), ('bar', 'one'),
+                         ('baz', 'two'), ('qux', 'one'), ('qux', 'two')),
+                        tupleize_cols=False)
+    result = idx.to_index()
+    tm.assert_index_equal(result, expected)

pandas/tests/indexes/test_base.py

@@ -2201,6 +2201,18 @@ def test_tab_complete_warning(self, ip):
             with provisionalcompleter('ignore'):
                 list(ip.Completer.completions('idx.', 4))
 
+    def test_to_index(self, indices):
+        # 22866
+        if isinstance(indices, MultiIndex):
+            pytest.skip("Separate expectation for MultiIndex")
+
+        result = indices.to_index()
+        tm.assert_index_equal(result, indices)
+
+    def test_to_index_sep_raises(self, indices):
+        with pytest.raises(NotImplementedError):
+            indices.to_index(sep="")
+
 
 class TestMixedIntIndex(Base):
     # Mostly the tests from common.py for which the results differ