How to Fix Identity Map Errors

user1=find_user(1)
user2=find_user(1)
user1.name='Alice'
user2.name='Bob'
save_user(user1)  # Alice saved
save_user(user2)  # Bob overwrites! Same row, different objects.

class IdentityMap:
    def __init__(self): self._map={}
    def get(self,cls,uid): return self._map.get((cls,uid))
    def add(self,obj): self._map[(type(obj),obj.id)]=obj
    def clear(self): self._map.clear()
class UserMapper:
    def __init__(self,conn,identity_map):
        self.conn=conn; self.identity_map=identity_map
    def find(self,uid):
        cached=self.identity_map.get(User,uid)
        if cached: return cached
        row=self.conn.execute('SELECT id,name FROM users WHERE id=?',(uid,)).fetchone()
        if not row: return None
        u=User(row[1]); u.id=row[0]
        self.identity_map.add(u)
        return u
im=IdentityMap(); mapper=UserMapper(conn,im)
u1=mapper.find(1); u2=mapper.find(1); u1 is u2  # True