How to mock python's read()

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?