Add to_flat_index method to MultiIndex

doc/source/api.rst

@@ -1708,6 +1708,7 @@ 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

@@ -203,6 +203,7 @@ Other Enhancements
 - :meth:`Index.to_frame` now supports overriding column name(s) (:issue:`22580`).
 - New attribute :attr:`__git_version__` will return git commit sha of current build (:issue:`21295`).
 - Compatibility with Matplotlib 3.0 (:issue:`22790`).
+- :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/multi.py

@@ -194,6 +194,7 @@ class MultiIndex(Index):
     set_levels
     set_labels
     to_frame
+    to_index
     is_lexsorted
     sortlevel
     droplevel
@@ -1198,6 +1199,28 @@ def to_frame(self, index=True, name=None):
             result.index = self
         return result
 
+    def to_index(self, sep=None):
+        """
+        Convert a MultiIndex to an Index of Tuples containing the level values.
+
+        .. versionadded:: 0.24.0
+
+        Returns
+        -------
+        pd.Index : an Index with the MultiIndex data represented in Tuples.
+
+        See also
+        --------
+        Index
+        """
+        if sep is not None:
+            # TODO: Add support for separator to return strs instad of tuples
+            raise NotImplementedError
+        else:
+            idx = Index(self.values, tupleize_cols=False)
+
+        return idx
+
     def to_hierarchical(self, n_repeat, n_shuffle=1):
         """
         .. deprecated:: 0.24.0

pandas/tests/indexes/multi/test_conversion.py

@@ -4,6 +4,7 @@
 import numpy as np
 import pandas as pd
 import pandas.util.testing as tm
+import pytest
 from pandas import DataFrame, MultiIndex, date_range
 from pandas.compat import range
 
@@ -169,3 +170,16 @@ 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)
+
+
+def test_to_index_sep_raises(idx):
+    with pytest.raises(NotImplementedError):
+        idx.to_index(sep="")