BUG: preserve _id in MultiIndex.copy(deep=False) (#47900)

Author: jbrockmendel

pandas/core/indexes/multi.py

@@ -1190,20 +1190,23 @@ def copy(
  This could be potentially expensive on large MultiIndex objects.
  """
  names = self._validate_names(name=name, names=names, deep=deep)
+ keep_id = not deep
  if levels is not None:
  warnings.warn(
  "parameter levels is deprecated and will be removed in a future "
  "version. Use the set_levels method instead.",
  FutureWarning,
  stacklevel=find_stack_level(),
  )
+ keep_id = False
  if codes is not None:
  warnings.warn(
  "parameter codes is deprecated and will be removed in a future "
  "version. Use the set_codes method instead.",
  FutureWarning,
  stacklevel=find_stack_level(),
  )
+ keep_id = False
 
  if deep:
  from copy import deepcopy
@@ -1225,6 +1228,8 @@ def copy(
  )
  new_index._cache = self._cache.copy()
  new_index._cache.pop("levels", None) # GH32669
+ if keep_id:
+ new_index._id = self._id
 
  if dtype:
  warnings.warn(

pandas/tests/indexes/multi/test_copy.py

@@ -104,3 +104,15 @@ def test_copy_deprecated_parameters(deep, param_name, param_value):
  idx_copy = idx.copy(deep=deep, **{param_name: param_value})
 
  assert [list(i) for i in getattr(idx_copy, param_name)] == param_value
+
+
+def test_copy_deep_false_retains_id():
+ # GH#47878
+ idx = MultiIndex(
+ levels=[["foo", "bar"], ["fizz", "buzz"]],
+ codes=[[0, 0, 0, 1], [0, 0, 1, 1]],
+ names=["first", "second"],
+ )
+
+ res = idx.copy(deep=False)
+ assert res._id is idx._id