Merge with latest, and fix tests related to bug fix

Author: Dr-Irv

doc/source/whatsnew/v0.24.0.txt

@@ -1315,6 +1315,7 @@ Reshaping
 - Bug in :func:`pandas.concat` when concatenating a multicolumn DataFrame with tz-aware data against a DataFrame with a different number of columns (:issue`22796`)
 - Bug in :func:`merge_asof` where confusing error message raised when attempting to merge with missing values (:issue:`23189`)
 - Bug in :meth:`DataFrame.nsmallest` and :meth:`DataFrame.nlargest` for dataframes that have :class:`MultiIndex`ed columns (:issue:`23033`).
+- Bug in :meth:`DataFrame.join` when joining on partial MultiIndex would drop names (:issue:`20452`).
 
 .. _whatsnew_0240.bug_fixes.sparse:
 

pandas/tests/reshape/merge/test_merge.py

@@ -1913,16 +1913,16 @@ def test_merge_index_types(index):
     assert_frame_equal(result, expected)
 
 
-@pytest.mark.parametrize("on,left_on,right_on,left_index,right_index,nms,nm", [
-    (['outer', 'inner'], None, None, False, False, ['outer', 'inner'], 'B'),
-    (None, None, None, True, True, ['outer', 'inner'], 'B'),
-    (None, ['outer', 'inner'], None, False, True, None, 'B'),
-    (None, None, ['outer', 'inner'], True, False, None, 'B'),
-    (['outer', 'inner'], None, None, False, False, ['outer', 'inner'], None),
-    (None, None, None, True, True, ['outer', 'inner'], None),
-    (None, ['outer', 'inner'], None, False, True, None, None),
-    (None, None, ['outer', 'inner'], True, False, None, None)])
-def test_merge_series(on, left_on, right_on, left_index, right_index, nms, nm):
+@pytest.mark.parametrize("on,left_on,right_on,left_index,right_index,nm", [
+    (['outer', 'inner'], None, None, False, False, 'B'),
+    (None, None, None, True, True, 'B'),
+    (None, ['outer', 'inner'], None, False, True, 'B'),
+    (None, None, ['outer', 'inner'], True, False, 'B'),
+    (['outer', 'inner'], None, None, False, False, None),
+    (None, None, None, True, True, None),
+    (None, ['outer', 'inner'], None, False, True, None),
+    (None, None, ['outer', 'inner'], True, False, None)])
+def test_merge_series(on, left_on, right_on, left_index, right_index, nm):
     # GH 21220
     a = pd.DataFrame({"A": [1, 2, 3, 4]},
                      index=pd.MultiIndex.from_product([['a', 'b'], [0, 1]],
@@ -1932,7 +1932,7 @@ def test_merge_series(on, left_on, right_on, left_index, right_index, nms, nm):
                   names=['outer', 'inner']), name=nm)
     expected = pd.DataFrame({"A": [2, 4], "B": [1, 3]},
                             index=pd.MultiIndex.from_product([['a', 'b'], [1]],
-                            names=nms))
+                            names=['outer', 'inner']))
     if nm is not None:
         result = pd.merge(a, b, on=on, left_on=left_on, right_on=right_on,
                           left_index=left_index, right_index=right_index)