How to Fix Strangler Fig Pattern Errors

def rewrite_all():
    # Stop the world: rewrite entire monolith at once
    old_system.stop()
    new_system.deploy()
    # If new system fails, everything is down

class LegacyService:
    def get_user(self,uid): return {'id':uid,'name':'Legacy'}
class NewService:
    def get_user(self,uid): return {'id':uid,'name':'New'}
class Router:
    def __init__(self):
        self.legacy=LegacyService(); self.new=NewService()
        self.migrated_users=set()
    def get_user(self,uid):
        if uid in self.migrated_users:
            return self.new.get_user(uid)
        return self.legacy.get_user(uid)
router=Router()
# Migrate users gradually:
router.migrated_users.add(1)  # user 1 now on new system
print(router.get_user(1))  # New service
print(router.get_user(2))  # Legacy service