I'd like to write this test case:

  with patch('builtins.open') as pyopen:
    mock_open(pyopen, read_data="foo")
    run()
    pyopen.assert_has_calls([call("filename", "wt"),
                             call().write("gazonk"),
                             call().close()])

and I shouldn't have to care if the code is written like this:

  def run():
    f = open("filename", "wt")
    try:
      write("gazonk")
    finally:
      f.close()

or like this:

  def run():
    with open("filename", "wt") as f:
      write("gazonk")

Also see Issue44185 for __exit__.