old_contents: import pytest
  from mock import patch, MagicMock
  
  from open_folder import *
  
  def test_folder_exists():
      with patch('subprocess.check_call', MagicMock(return_value="NOOP")):
          result = open_folder(".")
          assert result == None
  
  def test_folder_does_not_exists():
      with patch('subprocess.check_call', MagicMock(return_value="NOOP")):
          with pytest.raises(Exception):
              open_folder("it_is_very_unlikely_that_this_file_exists_20150718")
  
  def test_unsupported_os():
      with patch('subprocess.check_call', MagicMock(return_value="NOOP")):
          with patch('platform.system', MagicMock(return_value="NotDarwinWindowsLinux")):
              with pytest.raises(Exception):
                  result = open_folder("/")
  
  def test_supported_os():
      with patch('subprocess.check_call', MagicMock(return_value="NOOP")):
  
          with patch('platform.system', MagicMock(return_value="Linux")):
              result = open_folder("/")
              assert result == None
          with patch('platform.system', MagicMock(return_value="Darwin")):
              result = open_folder("/")
              assert result == None
          with patch('platform.system', MagicMock(return_value="Windows")):
              result = open_folder("/")
              assert result == None
  
new_contents: import pytest
  from mock import patch, MagicMock
  
  from open_folder import *
  
  def test_folder_exists():
      with patch('subprocess.check_call', MagicMock(return_value="NOOP")):
          result = open_folder(".")
          assert result == None
  
  def test_folder_does_not_exists():
      with patch('subprocess.check_call', MagicMock(return_value="NOOP")):
          with pytest.raises(Exception) as excinfo:
              open_folder("it_is_very_unlikely_that_this_file_exists_20150718")
          assert str(excinfo.value) == ('Folder does not exist.')
  
  def test_unsupported_os():
      with patch('subprocess.check_call', MagicMock(return_value="NOOP")):
  
          with patch('platform.system', MagicMock(return_value="NotDarwinWindowsLinux")):
              with pytest.raises(Exception) as excinfo:
                  open_folder("/")
              assert str(excinfo.value).startswith('Your operating system was not recognized.')
  
  def test_supported_os():
      with patch('subprocess.check_call', MagicMock(return_value="NOOP")):
  
          with patch('platform.system', MagicMock(return_value="Linux")):
              result = open_folder("/")
              assert result == None
          with patch('platform.system', MagicMock(return_value="Darwin")):
              result = open_folder("/")
              assert result == None
          with patch('platform.system', MagicMock(return_value="Windows")):
              result = open_folder("/")
              assert result == None
  
subject: Check to ensure the excpetions return the text we expect.