BUG: raise error when setting cached properties (#20487)

Author: jorisvandenbossche

pandas/_libs/properties.pyx

@@ -37,6 +37,9 @@ cdef class CachedProperty(object):
             PyDict_SetItem(cache, self.name, val)
         return val
 
+    def __set__(self, obj, value):
+        raise AttributeError("Can't set attribute")
+
 
 cache_readonly = CachedProperty
 

pandas/tests/indexes/test_base.py

@@ -2056,6 +2056,11 @@ def test_iadd_preserves_name(self):
         ser.index -= 1
         assert ser.index.name == "foo"
 
+    def test_cached_properties_not_settable(self):
+        idx = pd.Index([1, 2, 3])
+        with tm.assert_raises_regex(AttributeError, "Can't set attribute"):
+            idx.is_unique = False
+
 
 class TestMixedIntIndex(Base):
     # Mostly the tests from common.py for which the results differ