diff --git a/BaseClasses.py b/BaseClasses.py
index a60951b2..e5d92b95 100644
--- a/BaseClasses.py
+++ b/BaseClasses.py
@@ -7,6 +7,7 @@ import json
 import functools
 from collections import OrderedDict, Counter, deque
 from typing import List, Dict, Optional, Set, Iterable, Union, Any, Tuple, TypedDict, Callable
+import typing  # this can go away when Python 3.8 support is dropped
 import secrets
 import random
 
@@ -563,9 +564,20 @@ class MultiWorld():
         return False
 
 
+PathValue = Tuple[str, Optional["PathValue"]]
+
+
 class CollectionState():
-    additional_init_functions: List[Callable] = []
-    additional_copy_functions: List[Callable] = []
+    prog_items: typing.Counter[Tuple[str, int]]
+    world: MultiWorld
+    reachable_regions: Dict[int, Set[Region]]
+    blocked_connections: Dict[int, Set[Entrance]]
+    events: Set[Location]
+    path: Dict[Union[Region, Entrance], PathValue]
+    locations_checked: Set[Location]
+    stale: Dict[int, bool]
+    additional_init_functions: List[Callable[[CollectionState, MultiWorld], None]] = []
+    additional_copy_functions: List[Callable[[CollectionState, CollectionState], CollectionState]] = []
 
     def __init__(self, parent: MultiWorld):
         self.prog_items = Counter()
@@ -603,6 +615,7 @@ class CollectionState():
             if new_region in rrp:
                 bc.remove(connection)
             elif connection.can_reach(self):
+                assert new_region, "tried to search through an Entrance with no Region"
                 rrp.add(new_region)
                 bc.remove(connection)
                 bc.update(new_region.exits)
@@ -633,7 +646,8 @@ class CollectionState():
                   spot: Union[Location, Entrance, Region, str],
                   resolution_hint: Optional[str] = None,
                   player: Optional[int] = None) -> bool:
-        if not hasattr(spot, "can_reach"):
+        if isinstance(spot, str):
+            assert isinstance(player, int), "can_reach: player is required if spot is str"
             # try to resolve a name
             if resolution_hint == 'Location':
                 spot = self.world.get_location(spot, player)
@@ -644,7 +658,7 @@ class CollectionState():
                 spot = self.world.get_region(spot, player)
         return spot.can_reach(self)
 
-    def sweep_for_events(self, key_only: bool = False, locations: Set[Location] = None):
+    def sweep_for_events(self, key_only: bool = False, locations: Optional[Iterable[Location]] = None) -> None:
         if locations is None:
             locations = self.world.get_filled_locations()
         new_locations = True
@@ -656,6 +670,7 @@ class CollectionState():
             new_locations = reachable_events - self.events
             for event in new_locations:
                 self.events.add(event)
+                assert isinstance(event.item, Item), "tried to collect Event with no Item"
                 self.collect(event.item, True, event)
 
     def has(self, item: str, player: int, count: int = 1) -> bool:
@@ -670,7 +685,7 @@ class CollectionState():
     def count(self, item: str, player: int) -> int:
         return self.prog_items[item, player]
 
-    def has_group(self, item_name_group: str, player: int, count: int = 1):
+    def has_group(self, item_name_group: str, player: int, count: int = 1) -> bool:
         found: int = 0
         for item_name in self.world.worlds[player].item_name_groups[item_name_group]:
             found += self.prog_items[item_name, player]
@@ -678,7 +693,7 @@ class CollectionState():
                 return True
         return False
 
-    def count_group(self, item_name_group: str, player: int):
+    def count_group(self, item_name_group: str, player: int) -> int:
         found: int = 0
         for item_name in self.world.worlds[player].item_name_groups[item_name_group]:
             found += self.prog_items[item_name, player]