Nested list multi index (#14467)

pandas/core/internals/construction.py

@@ -576,11 +576,16 @@ def _convert_object_array(content, columns, coerce_float=False, dtype=None):
     else:
         if len(columns) != len(content):  # pragma: no cover
             # caller's responsibility to check for this...
-            raise AssertionError(
-                f"{len(columns)} columns passed, passed data had "
-                f"{len(content)} columns"
-            )
-
+            # Its possible that the user may be trying to pass a MultiIndex here.
+            # Attempt to convert columns to MultiIndex, and check length again.
+            # This conversion should be safe to do, as the input expects an Index
+            # and the colums are left unmodified throughout the rest of this function.
+            columns = ensure_index(columns)
+            if len(columns) != len(content):
+                raise AssertionError(
+                    f"{len(columns)} columns passed, passed data had "
+                    f"{len(content)} columns"
+                )
     # provide soft conversion of object dtypes
     def convert(arr):
         if dtype != object and dtype != np.object:

pandas/tests/indexes/multi/test_nested_list_data.py

@@ -0,0 +1,42 @@
+import pandas as pd
+import pandas._testing as tm
+import numpy as np
+import pytest
+
+
+'''
+This code is meant to test the fix implemented for issue #14467.
+https://github.com/pandas-dev/pandas/issues/14467
+'''
+
+class TestNestedListDataMultiIndex:
+
+  def test_nested_list(self):
+    # Case from issue, creating data from np.array works, and should match this case
+    result = pd.DataFrame([[1, 2, 3], [4, 5, 6]],
+                      index=[['gibberish']*2, [0, 1]],
+                      columns=[['baldersash']*3, [10, 20, 30]])
+
+    expected = pd.DataFrame(np.array([[1, 2, 3], [4, 5, 6]]),
+                      index=[['gibberish']*2, [0, 1]],
+                      columns=[['baldersash']*3, [10, 20, 30]])
+    tm.assert_index_equal(result.columns, expected.columns)
+
+  def test_nest_list_with_multiIndex(self):
+    # Creating from a multiIndex should also still work
+    m = pd.MultiIndex.from_arrays([['baldersash']*3, [10, 20, 30]])
+    result = pd.DataFrame([[1, 2, 3], [4, 5, 6]],
+                      index=[['gibberish']*2, [0, 1]],
+                      columns=m)
+
+    expected = pd.DataFrame(np.array([[1, 2, 3], [4, 5, 6]]),
+                      index=[['gibberish']*2, [0, 1]],
+                      columns=[['baldersash']*3, [10, 20, 30]])
+    tm.assert_index_equal(result.columns, expected.columns)
+
+  def test_wrong_length_raises_error(self):
+    # Make sure the code raises an error if the nested lists have the wrong length
+    with pytest.raises(ValueError):
+      result = pd.DataFrame([[1, 2, 3], [4, 5, 6]],
+                        index=[['gibberish']*2, [0, 1]],
+                        columns=[['baldersash']*2, [10, 20]])