json_db: add some type hints and asserts

diff --git a/electrum/json_db.py b/electrum/json_db.py
index b493983..144a958 100644
--- a/electrum/json_db.py
+++ b/electrum/json_db.py
@@ -25,7 +25,7 @@
 import threading
 import copy
 import json
-from typing import TYPE_CHECKING, Optional, Sequence, List, Union
+from typing import TYPE_CHECKING, Optional, Sequence, List, Union, Any
 
 import jsonpatch
 import jsonpointer
@@ -97,13 +97,16 @@ def stored_in(name, _type=dict):
         return func
     return decorator
 
+_FLEX_KEY = str | int | None
 
-def key_path(path: Sequence[Union[str, int]], key: Optional[str]) -> str:
-    def to_str(x):
+def key_path(path: Sequence[_FLEX_KEY], key: _FLEX_KEY) -> str:
+    def to_str(x: _FLEX_KEY) -> str:
+        assert isinstance(x, _FLEX_KEY), repr(x)
+        assert x is not None
         if isinstance(x, int):
             return str(int(x))
         else:
-            assert isinstance(x, str)
+            assert isinstance(x, str), f"unexpected key type for: {x!r}"
             return x
     items = [to_str(x) for x in path]
     if key is not None:
@@ -113,15 +116,17 @@ def key_path(path: Sequence[Union[str, int]], key: Optional[str]) -> str:
 class BaseStoredObject:
 
     _db: 'JsonDB' = None
-    _key = None
-    _parent = None
-    _lock = None
+    _key: _FLEX_KEY = None
+    _parent: Optional['BaseStoredObject'] = None
+    _lock: threading.RLock = None
 
     def set_db(self, db):
         self._db = db
         self._lock = self._db.lock if self._db else threading.RLock()
 
-    def set_parent(self, key, parent):
+    def set_parent(self, *, key: _FLEX_KEY, parent: Optional['BaseStoredObject']) -> None:
+        assert (key == "") == (parent is None), f"{key=!r}, {parent=!r}"
+        assert isinstance(key, _FLEX_KEY), repr(key)
         self._key = key
         self._parent = parent
 
@@ -130,7 +135,7 @@ class BaseStoredObject:
         return self._lock
 
     @property
-    def path(self) -> Sequence[str]:
+    def path(self) -> Sequence[_FLEX_KEY] | None:
         # return None iff we are pruned from root
         x = self
         s = [x._key]
@@ -142,15 +147,18 @@ class BaseStoredObject:
         assert self._db is not None
         return s
 
-    def db_add(self, key, value):
+    def db_add(self, key: _FLEX_KEY, value) -> None:
+        assert isinstance(key, _FLEX_KEY), repr(key)
         if self.path:
             self._db.add(self.path, key, value)
 
-    def db_replace(self, key, value):
+    def db_replace(self, key: _FLEX_KEY, value) -> None:
+        assert isinstance(key, _FLEX_KEY), repr(key)
         if self.path:
             self._db.replace(self.path, key, value)
 
-    def db_remove(self, key):
+    def db_remove(self, key: _FLEX_KEY) -> None:
+        assert isinstance(key, _FLEX_KEY), repr(key)
         if self.path:
             self._db.remove(self.path, key)
 
@@ -158,7 +166,8 @@ class BaseStoredObject:
 class StoredObject(BaseStoredObject):
     """for attr.s objects """
 
-    def __setattr__(self, key, value):
+    def __setattr__(self, key: str, value):
+        assert isinstance(key, str), repr(key)
         if self.path and not key.startswith('_'):
             if value != getattr(self, key):
                 self.db_replace(key, value)
@@ -185,7 +194,8 @@ class StoredDict(dict, BaseStoredObject):
             self.__setitem__(k, v)
 
     @locked
-    def __setitem__(self, key, v):
+    def __setitem__(self, key: _FLEX_KEY, v) -> None:
+        assert isinstance(key, _FLEX_KEY), repr(key)
         is_new = key not in self
         # early return to prevent unnecessary disk writes
         if not is_new and self._db and json.dumps(v, cls=self._db.encoder) == json.dumps(self[key], cls=self._db.encoder):
@@ -204,18 +214,20 @@ class StoredDict(dict, BaseStoredObject):
             v.set_db(self._db)
         # set parent
         if isinstance(v, BaseStoredObject):
-            v.set_parent(key, self)
+            v.set_parent(key=key, parent=self)
         # set item
         dict.__setitem__(self, key, v)
         self.db_add(key, v) if is_new else self.db_replace(key, v)
 
     @locked
-    def __delitem__(self, key):
+    def __delitem__(self, key: _FLEX_KEY) -> None:
+        assert isinstance(key, _FLEX_KEY), repr(key)
         dict.__delitem__(self, key)
         self.db_remove(key)
 
     @locked
-    def pop(self, key, v=_RaiseKeyError):
+    def pop(self, key: _FLEX_KEY, v=_RaiseKeyError) -> Any:
+        assert isinstance(key, _FLEX_KEY), repr(key)
         if key not in self:
             if v is _RaiseKeyError:
                 raise KeyError(key)
@@ -227,7 +239,8 @@ class StoredDict(dict, BaseStoredObject):
             r._parent = None
         return r
 
-    def setdefault(self, key, default = None, /):
+    def setdefault(self, key: _FLEX_KEY, default = None, /):
+        assert isinstance(key, _FLEX_KEY), repr(key)
         if key not in self:
             self.__setitem__(key, default)
         return self[key]
@@ -283,7 +296,7 @@ class JsonDB(Logger):
         data = self._convert_dict([], data)
         # convert dict to StoredDict
         self.data = StoredDict(data, self)
-        self.data.set_parent('', None)
+        self.data.set_parent(key='', parent=None)
         # write file in case there was a db upgrade
         if self.storage and self.storage.file_exists():
             self.write_and_force_consolidation()
@@ -357,13 +370,16 @@ class JsonDB(Logger):
         self.pending_changes.append(json.dumps(patch, cls=self.encoder))
         self.set_modified(True)
 
-    def add(self, path, key, value):
+    def add(self, path, key: _FLEX_KEY, value) -> None:
+        assert isinstance(key, _FLEX_KEY), repr(key)
         self.add_patch({'op': 'add', 'path': key_path(path, key), 'value': value})
 
-    def replace(self, path, key, value):
+    def replace(self, path, key: _FLEX_KEY, value) -> None:
+        assert isinstance(key, _FLEX_KEY), repr(key)
         self.add_patch({'op': 'replace', 'path': key_path(path, key), 'value': value})
 
-    def remove(self, path, key):
+    def remove(self, path, key: _FLEX_KEY) -> None:
+        assert isinstance(key, _FLEX_KEY), repr(key)
         self.add_patch({'op': 'remove', 'path': key_path(path, key)})
 
     @locked
@@ -419,7 +435,9 @@ class JsonDB(Logger):
     def _should_convert_to_stored_dict(self, key) -> bool:
         return True
 
-    def _convert_dict_key(self, path):
+    def _convert_dict_key(self, path: List[str]) -> _FLEX_KEY:
+        """Maybe convert key from str to python type (typically int or IntEnum)"""
+        assert all(isinstance(x, str) for x in path), repr(path)
         key = path[-1]
         parent_key = path[-2] if len(path) > 1 else None
         gp_key = path[-3] if len(path) > 2 else None
@@ -431,9 +449,11 @@ class JsonDB(Logger):
             convert_key = None
         if convert_key:
             key = convert_key(key)
+        assert isinstance(key, _FLEX_KEY), f"unexpected type for {key=!r} at {path=}"
         return key
 
-    def _convert_dict_value(self, path, v):
+    def _convert_dict_value(self, path: List[str], v) -> Any:
+        assert all(isinstance(x, str) for x in path), repr(path)
         key = path[-1]
         if key in registered_dicts:
             constructor, _type = registered_dicts[key]
@@ -453,8 +473,9 @@ class JsonDB(Logger):
             v = self._convert_dict(path, v)
         return v
 
-    def _convert_dict(self, path, data: dict):
-        # recursively convert dict to StoredDict
+    def _convert_dict(self, path: List[str], data: dict):
+        # recursively convert json dict to StoredDict
+        assert all(isinstance(x, str) for x in path), repr(path)
         d = {}
         for k, v in list(data.items()):
             child_path = path + [k]