Why does another module get added?

import unittest
import client
from tkinter import Tk
from unittest.mock import patch


class TestClient(unittest.TestCase):
    @patch('client.Client.socket')
    def test_send(self, mock_socket):
        test_client = client.Client(Tk())
        test_client.send('TESTING')
        mock_socket.send.assert_called_with(unittest.mock.ANY)

I asked a similar question about similar code. Then I refactored to the shown code. When I execute this with python -m unittest I get:

AttributeError: does not have the attribute 'socket'

I have no idea where the first client. is coming from.