# Buggy code
def __setitem__(self, key, v):
    key = self.convert_key(key)
    is_new = key not in self
    # early return to prevent unnecessary disk writes
    if not is_new and self[key] == v:
        return
    # recursively set db and path
    if isinstance(v, StoredDict):
        v.db = self.db
        v.path = self.path + [key]
        for k, vv in v.items():
            v[k] = vv
    # recursively convert dict to StoredDict.
    # _convert_dict is called breadth-first
    elif isinstance(v, dict):
        if self.db:
            v = self.db._convert_dict(self.path, key, v)
        v = StoredDict(v, self.db, self.path + [key])
    # convert_value is called depth-first
    if isinstance(v, dict) or isinstance(v, str):
        if self.db:
            v = self.db._convert_value(self.path, key, v)
    # set parent of StoredObject
    if isinstance(v, StoredObject):
        v.set_db(self.db)
    # set item
    dict.__setitem__(self, key, v)
    if self.db:
        self.db.set_modified(True)


# Traceback (TypeError)
E | gui.qt.exception_window.Exception_Hook | exception caught by crash reporter
Traceback (most recent call last):
File "...\electrum\electrum\gui\qt\main_window.py", line 2838, in closeEvent
self.clean_up()
File "...\electrum\electrum\gui\qt\main_window.py", line 2856, in clean_up
self.gui_object.close_window(self)
File "...\electrum\electrum\gui\qt\__init__.py", line 339, in close_window
self.daemon.stop_wallet(window.wallet.storage.path)
File "...\electrum\electrum\daemon.py", line 453, in stop_wallet
wallet.stop_threads()
File "...\electrum\electrum\wallet.py", line 314, in stop_threads
self.save_keystore()
File "...\electrum\electrum\wallet.py", line 1908, in save_keystore
self.db.put('keystore', self.keystore.dump())
File "...\electrum\electrum\json_db.py", line 38, in wrapper
return func(self, *args, **kwargs)
File "...\electrum\electrum\json_db.py", line 184, in put
self.data[key] = copy.deepcopy(value)
File "...\Python38\lib\copy.py", line 146, in deepcopy
y = copier(x, memo)
File "...\Python38\lib\copy.py", line 230, in _deepcopy_dict
y[deepcopy(key, memo)] = deepcopy(value, memo)
File "...\Python38\lib\copy.py", line 172, in deepcopy
y = _reconstruct(x, memo, *rv)
File "...\Python38\lib\copy.py", line 270, in _reconstruct
state = deepcopy(state, memo)
File "...\Python38\lib\copy.py", line 146, in deepcopy
y = copier(x, memo)
File "...\Python38\lib\copy.py", line 230, in _deepcopy_dict
y[deepcopy(key, memo)] = deepcopy(value, memo)
File "...\Python38\lib\copy.py", line 172, in deepcopy
y = _reconstruct(x, memo, *rv)
File "...\Python38\lib\copy.py", line 270, in _reconstruct
state = deepcopy(state, memo)
File "...\Python38\lib\copy.py", line 146, in deepcopy
y = copier(x, memo)
File "...\Python38\lib\copy.py", line 230, in _deepcopy_dict
y[deepcopy(key, memo)] = deepcopy(value, memo)
File "...\Python38\lib\copy.py", line 161, in deepcopy
rv = reductor(4)
TypeError: cannot pickle '_thread.RLock' object

# Fixed code
def __setitem__(self, key, v):
    key = self.convert_key(key)
    is_new = key not in self
    # early return to prevent unnecessary disk writes
    if not is_new and self[key] == v:
        return
    # recursively set db and path
    if isinstance(v, StoredDict):
        v.db = self.db
        v.path = self.path + [key]
        for k, vv in v.items():
            v[k] = vv
    # recursively convert dict to StoredDict.
    # _convert_dict is called breadth-first
    elif isinstance(v, dict):
        if self.db:
            v = self.db._convert_dict(self.path, key, v)
        if not self.db or self.db._should_convert_to_stored_dict(key):
            v = StoredDict(v, self.db, self.path + [key])
    # convert_value is called depth-first
    if isinstance(v, dict) or isinstance(v, str):
        if self.db:
            v = self.db._convert_value(self.path, key, v)
    # set parent of StoredObject
    if isinstance(v, StoredObject):
        v.set_db(self.db)
    # set item
    dict.__setitem__(self, key, v)
    if self.db:
        self.db.set_modified(True)