test_my_class_a_open.py

import unittest
import mock
from module.my_class_a import MyClassA

class TestMyClassA(unittest.TestCase):
  
  def setUp(self):
    self._fake_path = '/path/to/the/fake/yaml/file'
    
  @mock.patch('module.my_class_a.yaml')
  def test_to_test(self, mock_yaml):
    
    a = MyClassA()
    
    mock_yaml.load = mock.MagicMock()
    mock_yaml.load.return_value = dict(test='testing')
    
    mock_open = mock.mock_open()
    with patch('module.my_class_a.open', mock_open, create=True):
      output = a.to_test(self._fake_path)
      self.assertEqual('testing', output['test'])  
    
    mock_yaml.load.assert_called_with()
    mock_open.assert_called_with(self._fake_path, 'r')