ref: use a more typing-friendly singleton for backup ScrubbedData (#90297)

avoids this crash in mypy: python/mypy#18964

<!-- Describe your PR here. -->

Author: asottile-sentry

src/sentry/backup/comparators.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import enum
 import re
 from abc import ABC, abstractmethod
 from collections import defaultdict
@@ -22,17 +23,9 @@
 
 UNIX_EPOCH = unix_zero_date = datetime.fromtimestamp(0, timezone.utc).isoformat()
 
-
-class ScrubbedData:
-    """A singleton class used to indicate data has been scrubbed, without indicating what that data
-    is. A unit type indicating "scrubbing was successful" only."""
-
-    instance: ScrubbedData
-
-    def __new__(cls):
-        if getattr(cls, "instance", None) is None:
-            cls.instance = super().__new__(cls)
-        return cls.instance
+# A singleton class used to indicate data has been scrubbed, without indicating what that data
+# is. A unit type indicating "scrubbing was successful" only.
+ScrubbedData = enum.Enum("ScrubbedData", "SCRUBBED_DATA")
 
 
 class JSONScrubbingComparator(ABC):
@@ -106,7 +99,7 @@ def __scrub__(
         right: Any,
         f: (
             Callable[[list[str]], list[str]] | Callable[[list[str]], ScrubbedData]
-        ) = lambda _: ScrubbedData(),
+        ) = lambda _: ScrubbedData.SCRUBBED_DATA,
     ) -> None:
         """Removes all of the fields compared by this comparator from the `fields` dict, so that the
         remaining fields may be compared for equality. Public callers should use the inheritance-safe wrapper, `scrub`, rather than using this internal method directly.

tests/sentry/backup/test_comparators.py

@@ -291,12 +291,12 @@ def test_good_auto_suffix_comparator_scrubbed():
     }
     cmp.scrub(left, right)
     assert left["scrubbed"]
-    assert left["scrubbed"]["AutoSuffixComparator::same"] is ScrubbedData()
-    assert left["scrubbed"]["AutoSuffixComparator::suffixed"] is ScrubbedData()
+    assert left["scrubbed"]["AutoSuffixComparator::same"] is ScrubbedData.SCRUBBED_DATA
+    assert left["scrubbed"]["AutoSuffixComparator::suffixed"] is ScrubbedData.SCRUBBED_DATA
 
     assert right["scrubbed"]
-    assert right["scrubbed"]["AutoSuffixComparator::same"] is ScrubbedData()
-    assert right["scrubbed"]["AutoSuffixComparator::suffixed"] is ScrubbedData()
+    assert right["scrubbed"]["AutoSuffixComparator::same"] is ScrubbedData.SCRUBBED_DATA
+    assert right["scrubbed"]["AutoSuffixComparator::suffixed"] is ScrubbedData.SCRUBBED_DATA
 
 
 def test_good_datetime_equality_comparator():
@@ -970,10 +970,10 @@ def test_good_foreign_key_comparator_scrubbed():
     right = deepcopy(left)
     cmp.scrub(left, right)
     assert left["scrubbed"]
-    assert left["scrubbed"]["ForeignKeyComparator::user"] is ScrubbedData()
+    assert left["scrubbed"]["ForeignKeyComparator::user"] is ScrubbedData.SCRUBBED_DATA
 
     assert right["scrubbed"]
-    assert right["scrubbed"]["ForeignKeyComparator::user"] is ScrubbedData()
+    assert right["scrubbed"]["ForeignKeyComparator::user"] is ScrubbedData.SCRUBBED_DATA
 
 
 def test_bad_foreign_key_comparator_set_primary_key_maps_not_called():
@@ -1171,11 +1171,11 @@ def test_good_ignored_comparator_scrubbed():
     right = deepcopy(left)
     cmp.scrub(left, right)
     assert left["scrubbed"]
-    assert left["scrubbed"]["IgnoredComparator::ignored_field"] is ScrubbedData()
+    assert left["scrubbed"]["IgnoredComparator::ignored_field"] is ScrubbedData.SCRUBBED_DATA
     assert left["scrubbed"].get("IgnoredComparator::other_field") is None
 
     assert right["scrubbed"]
-    assert right["scrubbed"]["IgnoredComparator::ignored_field"] is ScrubbedData()
+    assert right["scrubbed"]["IgnoredComparator::ignored_field"] is ScrubbedData.SCRUBBED_DATA
     assert right["scrubbed"].get("IgnoredComparator::other_field") is None
 
 
@@ -1271,10 +1271,10 @@ def test_good_secret_hex_comparator_scrubbed():
     }
     cmp.scrub(left, right)
     assert left["scrubbed"]
-    assert left["scrubbed"]["SecretHexComparator::secret_hex_field"] is ScrubbedData()
+    assert left["scrubbed"]["SecretHexComparator::secret_hex_field"] is ScrubbedData.SCRUBBED_DATA
 
     assert right["scrubbed"]
-    assert right["scrubbed"]["SecretHexComparator::secret_hex_field"] is ScrubbedData()
+    assert right["scrubbed"]["SecretHexComparator::secret_hex_field"] is ScrubbedData.SCRUBBED_DATA
 
 
 def test_good_subscription_id_comparator():
@@ -1406,10 +1406,16 @@ def test_good_subscription_id_comparator_scrubbed():
     }
     cmp.scrub(left, right)
     assert left["scrubbed"]
-    assert left["scrubbed"]["SubscriptionIDComparator::subscription_id_field"] is ScrubbedData()
+    assert (
+        left["scrubbed"]["SubscriptionIDComparator::subscription_id_field"]
+        is ScrubbedData.SCRUBBED_DATA
+    )
 
     assert right["scrubbed"]
-    assert right["scrubbed"]["SubscriptionIDComparator::subscription_id_field"] is ScrubbedData()
+    assert (
+        right["scrubbed"]["SubscriptionIDComparator::subscription_id_field"]
+        is ScrubbedData.SCRUBBED_DATA
+    )
 
 
 def test_good_unordered_list_comparator():
@@ -1520,10 +1526,16 @@ def test_good_unordered_list_comparator_scrubbed():
     }
     cmp.scrub(left, right)
     assert left["scrubbed"]
-    assert left["scrubbed"]["UnorderedListComparator::unordered_list_field"] is ScrubbedData()
+    assert (
+        left["scrubbed"]["UnorderedListComparator::unordered_list_field"]
+        is ScrubbedData.SCRUBBED_DATA
+    )
 
     assert right["scrubbed"]
-    assert right["scrubbed"]["UnorderedListComparator::unordered_list_field"] is ScrubbedData()
+    assert (
+        right["scrubbed"]["UnorderedListComparator::unordered_list_field"]
+        is ScrubbedData.SCRUBBED_DATA
+    )
 
 
 def test_good_uuid4_comparator():
@@ -1625,10 +1637,10 @@ def test_good_uuid4_comparator_scrubbed():
     }
     cmp.scrub(left, right)
     assert left["scrubbed"]
-    assert left["scrubbed"]["UUID4Comparator::guid_field"] is ScrubbedData()
+    assert left["scrubbed"]["UUID4Comparator::guid_field"] is ScrubbedData.SCRUBBED_DATA
 
     assert right["scrubbed"]
-    assert right["scrubbed"]["UUID4Comparator::guid_field"] is ScrubbedData()
+    assert right["scrubbed"]["UUID4Comparator::guid_field"] is ScrubbedData.SCRUBBED_DATA
 
 
 def test_good_user_password_obfuscating_comparator_claimed_user():