Mocking complex data structures in Python

Question

I have set of scripts designed to administer a database cluster. I have converted these scripts to an object-oriented design and am having a hell of a time trying to write unit tests. Here's a simplified example of what I'm trying to achieve:

class DBServer:
    def __init__(self,hostname):
        self.hostname=hostname

    def get_instance_status(self):
        if (<instance_is_online>):
            return True
        else:
            return false

    def shutdown_instance(self):
        #<insert code here to stop the DB instance on that node>


class DBCluster:
    def __init__(self,clustername):
        self.servers=[DBServer('hostname1'), DBServer('hostname2'), DBServer('hostname3'), DBServer('hostname4')]
    
    def get_servers_online(self):
        #returns an array of DBServer objects where the DB instance is online
        serversonline=[]
        for server in self.servers:
            if server.get_instance_status():
                serversonline.append(server)
        return serversonline
            
    def shutdown_cluster(self):
        for server in self.servers:
            if server.get_instance_status():
                server.shutdown_instance()

As you can see, a DBServer is an object with a hostname and some functions that can be run against each server. A DBCluster is an object with an array of DBServers, and some functions that manage the entire cluster.

Imagine a workflow where the user wants to get a list of servers in the cluster where the DB instance is online. Then the user should pick one and shut it down:

def shutdown_user_instance(node_number):
    #get list of online instances in cluster, display to user and shut down the chosen instance:
    cluster=DBCluster()
    servers=cluster.get_servers_online()
    print ('Choose server to shut down:')
    i=0
    for server in servers:
        print (str(i) +': ' +server.hostname)
        i++
    reply=str(input('Option: '))
    instance=int(reply)
    servers[instance].shutdown_instance()

How do we unit test this function? I'm thinking it should mock the cluster.get_instances_online() function to return an array of mock DBServer objects. I can then create a return value for server.hostname for each object in that array, and then mock server.shutdown_instance(). The trouble is I don't know where to start. Something like this?

class TestCluster(unittest.TestCase):

    @patch(stack.DBCluster)
    @patch(stack.DBServer)
    @patch(builtins.input)
    @patch.object(stack.DBserver,'shutdown_instance')
    def test_shutdown(self, mock_cluster, mock_server, mock_input, mock_shutdown):
        mock_server.get_instance_status.side_effect[True,False,True,False] #for the purpose of this test, let's say nodes 0 and 2 are online, 1 and 3 are offline
        mock_cluster.get_servers_online.return_value=[mock_server,mock_server] #How should I achieve this array of mocked objects to test??
        mock_input.return_value=0
        shutdown_user_instance() #theoretically this should mock a shutdown of node 0