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Author: mroeschke

pandas/_libs/index.pyi

@@ -69,12 +69,12 @@ class MaskedUInt8Engine(MaskedIndexEngine): ...
 class MaskedBoolEngine(MaskedUInt8Engine): ...
 
 class BaseMultiIndexCodesEngine:
-    levels: list[np.ndarray]
+    levels: tuple[np.ndarray]
     offsets: np.ndarray  # ndarray[uint64_t, ndim=1]
 
     def __init__(
         self,
-        levels: list[np.ndarray],  # all entries hashable
+        levels: tuple[np.ndarray],  # all entries hashable
         labels: list[np.ndarray],  # all entries integer-dtyped
         offsets: np.ndarray,  # np.ndarray[np.uint64, ndim=1]
     ) -> None: ...

pandas/core/indexes/multi.py

@@ -840,7 +840,7 @@ def size(self) -> int:
     # Levels Methods
 
     @cache_readonly
-    def levels(self) -> tuple:
+    def levels(self) -> tuple[np.ndarray, ...]:
         # Use cache_readonly to ensure that self.get_locs doesn't repeatedly
         # create new IndexEngine
         # https://github.com/pandas-dev/pandas/issues/31648
@@ -877,7 +877,7 @@ def _set_levels(
             level_numbers = list(range(len(new_levels)))
         else:
             level_numbers = [self._get_level_number(lev) for lev in level]
-            new_levels_list = list(self._levels)
+            new_levels_list: tuple[Index, ...] = list(self._levels)
             for lev_num, lev in zip(level_numbers, levels):
                 new_levels_list[lev_num] = ensure_index(lev, copy=copy)._view()
             new_levels = tuple(new_levels_list)
@@ -889,7 +889,7 @@ def _set_levels(
             self._codes = new_codes
 
         names = self.names
-        self._levels = new_levels
+        self._levels: tuple[Index, ...] = new_levels
         if any(names):
             self._set_names(names)
 
@@ -1056,7 +1056,7 @@ def _set_codes(
             level_numbers = range(len(new_codes))
         else:
             level_numbers = [self._get_level_number(lev) for lev in level]
-            new_codes_list = list(self._codes)
+            new_codes_list: list[np.ndarray[np.int8]] = list(self._codes)
             for lev_num, level_codes in zip(level_numbers, codes):
                 lev = self.levels[lev_num]
                 new_codes_list[lev_num] = _coerce_indexer_frozen(

pandas/core/reshape/merge.py

@@ -1767,9 +1767,9 @@ def _convert_to_multiindex(index: Index) -> MultiIndex:
     # so that dropped levels can be appended
     join_index = _convert_to_multiindex(join_index)
 
-    join_levels = join_index.levels
-    join_codes = join_index.codes
-    join_names = join_index.names
+    join_levels = list(join_index.levels)
+    join_codes = list(join_index.codes)
+    join_names = list(join_index.names)
 
     # Iterate through the levels that must be restored
     for dropped_level_name in dropped_level_names:
@@ -1792,9 +1792,9 @@ def _convert_to_multiindex(index: Index) -> MultiIndex:
         else:
             restore_codes = algos.take_nd(codes, indexer, fill_value=-1)
 
-        join_levels = list(join_levels) + [restore_levels]
-        join_codes = list(join_codes) + [restore_codes]
-        join_names = list(join_names) + [dropped_level_name]
+        join_levels = join_levels + [restore_levels]
+        join_codes = join_codes + [restore_codes]
+        join_names = join_names + [dropped_level_name]
 
     return join_levels, join_codes, join_names