How to mock python's read()

Question

I'm trying to test the read() method in the following class:

class Channel(sam.Sam):
  def __open(self):
    try:
      self.__channel = open('%s/channel.ini' % os.path.dirname(os.path.realpath(__file__)), 'r+')
    except Exception as e:
      traceback.print_exc(file = sys.stdout)
      raise e

  def read(self):
    try:
      self.__open()
      return JSONEncoder().encode({
        "status": True,
        "channel": self.__channel.read().strip()
      })
    except Exception as e:
      traceback.print_exc(file = sys.stdout)
      return JSONEncoder().encode({
        "status": False
      })
    finally:
      self.__close()

As I understand it, I should be mocking the file.read() method (in self.__channel.read(), or maybe the os.open() method, but none of the examples I've found have the call to os.open() or file.read() deep inside a class.

I already tried __builtin__.read = MagicMock(return_value="something"), and many variations thereof, but not one of them even make sense. I'm kind of lost as to how to even start this.

Is this even the right way?

Answer 1

Mock the open function; you can use the mock_open() utility function to provide a suitable mock:

from unittest.mock import mock_open

with patch('your_module.open', mock_open(read_data=JSON_TEST_DATA), create=True) as m:
    result = Channel().read()
    assert m.assert_called_once_with(expected_file_name)

The patch() call creates a new global open object in your your_module namespace, so when the Channel.__open() method runs it'll find that object rather than the open() built-in function.

By passing in a read_data argument to mock_open(), you can dictate what is returned by the self.__channel.read() call.