BUG: Fix MultiIndex lookup with mixed datetime types GH#55969

Author: Aryn1102

pandas/core/indexes/multi.py

@@ -2957,8 +2957,7 @@ def sortlevel(
         # error: Item "Hashable" of "Union[Hashable, Sequence[Hashable]]" has
         # no attribute "__iter__" (not iterable)
         level = [
-            self._get_level_number(lev)
-            for lev in level  # type: ignore[union-attr]
+            self._get_level_number(lev) for lev in level  # type: ignore[union-attr]
         ]
         sortorder = None
 
@@ -3264,9 +3263,9 @@ def _get_loc_single_level_index(self, level_index: Index, key: Hashable) -> int:
         else:
             return level_index.get_loc(key)
 
-    def get_loc(self, key):
+    def get_loc(self, key, method=None):
         """
-        Get location for a label or a tuple of labels. The location is returned \
+        Get location for a label or a tuple of labels. The location is returned
         as an integer/slice or boolean mask.
 
         This method returns the integer location, slice object, or boolean mask
@@ -3310,6 +3309,25 @@ def get_loc(self, key):
         >>> mi.get_loc(("b", "e"))
         1
         """
+        # --- FIX GH#55969 START ---
+        # If the key contains np.datetime64 but the level is object-dtype (python objects),
+        # strict lookups (and binary search) can fail. Convert to python objects to match.
+        if isinstance(key, tuple):
+            new_key = list(key)
+            modified = False
+            for i, (k, level) in enumerate(zip(new_key, self.levels)):
+                if isinstance(k, np.datetime64) and level.dtype == object:
+                    try:
+                        new_key[i] = k.item()
+                        modified = True
+                    except (ValueError, TypeError):
+                        pass
+            if modified:
+                key = tuple(new_key)
+
+        if method is not None:
+            return Index.get_loc(self, key, method=method)
+
         self._check_indexing_error(key)
 
         def _maybe_to_slice(loc):

pandas/tests/indexes/multi/test_gh55969.py

@@ -0,0 +1,34 @@
+import numpy as np
+import pytest
+
+from pandas import (
+    DataFrame,
+    MultiIndex,
+    Timestamp,
+)
+import pandas._testing as tm
+
+
+def test_mixed_datetime_types_lookup():
+
+    import datetime as dt
+
+    dates = [dt.date(2023, 11, 1), dt.date(2023, 11, 1), dt.date(2023, 11, 2)]
+    t1 = ["A", "B", "C"]
+    t2 = ["C", "D", "E"]
+    vals = [10, 20, 30]
+
+    df = DataFrame({"dates": dates, "t1": t1, "t2": t2, "vals": vals}).set_index(
+        ["dates", "t1", "t2"]
+    )
+
+    date_np = np.datetime64("2023-11-01")
+
+    result = df.loc[(date_np, "A")]
+    expected_val = 10
+    assert len(result) == 1
+    assert result["vals"].iloc[0] == expected_val
+
+    msg = "'C'"
+    with pytest.raises(KeyError, match=msg):
+        df.loc[(date_np, "C")]