BUG: break reference cycle in Index._engine

Fixes pandas-dev#27585

Author: crepererum

doc/source/whatsnew/v0.25.1.rst

@@ -25,6 +25,8 @@ Other enhancements
 Bug fixes
 ~~~~~~~~~
 
+- Break reference cycle in :meth:`Index._engine` (:issue:`27585`)
+
 
 Categorical
 ^^^^^^^^^^^

pandas/core/indexes/base.py

@@ -687,7 +687,11 @@ def _cleanup(self):
     @cache_readonly
     def _engine(self):
         # property, for now, slow to look up
-        return self._engine_type(lambda: self._ndarray_values, len(self))
+
+        # to avoid a refernce cycle, bind `_ndarray_values` to a local variable, so
+        # `self` is not passed into the lambda.
+        _ndarray_values = self._ndarray_values
+        return self._engine_type(lambda: _ndarray_values, len(self))
 
     # --------------------------------------------------------------------
     # Array-Like Methods

pandas/tests/indexes/test_base.py

@@ -1,5 +1,6 @@
 from collections import defaultdict
 from datetime import datetime, timedelta
+import gc
 from io import StringIO
 import math
 import operator
@@ -2424,6 +2425,13 @@ def test_deprecated_contains(self):
             with tm.assert_produces_warning(FutureWarning):
                 index.contains(1)
 
+    def test_engine_reference_cycle(self):
+        # https://github.com/pandas-dev/pandas/issues/27585
+        index = pd.Index([1, 2, 3])
+        nrefs_pre = len(gc.get_referrers(index))
+        index._engine
+        assert len(gc.get_referrers(index)) == nrefs_pre
+
 
 class TestMixedIntIndex(Base):
     # Mostly the tests from common.py for which the results differ