Use a sentinel instead of None for cache presence

scanner/scanner/cache.py

@@ -1,9 +1,15 @@
 import asyncio
 from datetime import datetime, timedelta
 from functools import wraps
-from typing import Any, Optional, Tuple
+from typing import Any, Optional, Tuple, Final, Literal
+from enum import Enum
 
-type Cache = dict[Any, Tuple[Optional[asyncio.Event], Optional[datetime], Any]]
+# Waiting for https://github.com/python/typing/issues/689 for better sentinels
+class Sentinel(Enum):
+	NoneSentinel = 0
+
+none: Final = Sentinel.NoneSentinel
+type Cache = dict[Any, Tuple[asyncio.Event | Literal[none], datetime | Literal[none], Any]]
 
 def cache(ttl: timedelta, cache: Optional[Cache] = None, typed=False):
 	"""
@@ -25,19 +31,19 @@ def cache(ttl: timedelta, cache: Optional[Cache] = None, typed=False):
 		async def wrapper(*args, **kwargs):
 			key = make_key(args, kwargs, typed)
 
-			ret = cache.get(key, (None, None, None))
+			ret = cache.get(key, (none, none, none))
 			# First check if the same method is already running and wait for it.
-			if ret[0] is not None:
+			if ret[0] != none:
 				await ret[0].wait()
-				ret = cache.get(key, (None, None, None))
+				ret = cache.get(key, (none, none, none))
-				if ret[2] is None:
+				if ret[2] == none:
 					# ret[2] can be None if the cached method failed. if that is the case, run again.
 					return await wrapper(*args, **kwargs)
 				return ret[2]
 			# Return the cached result if it exits and is not expired
 			if (
-				ret[2] is not None
+				ret[2] != none
-				and ret[1] is not None
+				and ret[1] != none
 				and datetime.now() - ret[1] < ttl
 			):
 				return ret[2]
@@ -50,7 +56,7 @@ def cache(ttl: timedelta, cache: Optional[Cache] = None, typed=False):
 
 async def exec_as_cache(cache: Cache, key, f):
 	event = asyncio.Event()
-	cache[key] = (event, None, None)
+	cache[key] = (event, none, none)
 	try:
 		result = await f()
 	except:
@@ -58,7 +64,7 @@ async def exec_as_cache(cache: Cache, key, f):
 		event.set()
 		raise
 
-	cache[key] = (None, datetime.now(), result)
+	cache[key] = (none, datetime.now(), result)
 	event.set()
 	return result