Monday, October 6, 2014

Mocking out python OS specific imports

Testing python code that needs to run on a different OS is painful. A major part of the difficulty is that even though you can (somewhat) easily mock out the API calls used, you can't import the code successfully because the modules only exist on the target OS. Lets take an example of code that imports and calls functions from the win32api module. How do you mock out the import so you can test it on linux? I know of two main approaches.

One is the proxy module. Basically you define a module to hide all of the OS-specific imports behind, and do a conditional import in that module. So instead of having code like this:
import win32api
you do
import windows_imports
and then in windows_imports/
import platform

if platform.system() == "Windows":
  import win32api
  import winerror
  import wmi
Then inside your tests you need to create stubs to replace your API calls, e.g. for windows_imports.win32api.GetLogicalDriveStrings. Theoretically this should be fairly straightforward, but when I started down this path it got fairly complicated and I struggled to make it work. In the end I gave up and settled on the second approach, as below.

The second approach, described here, is to delay the import of the OS specific code in your tests until after you modify sys.modules to stub out all the OS-specific modules. This has the distinct advantage of leaving your production code untouched, and having all the complexity in your test code. Using the mock library makes this much easier. Below is an example of mocking out a WMI call made from python.
import mock
import test_fixture
import unittest

class WindowsTests(unittest.TestCase):

  def setUp(self):
    self.wmimock = mock.MagicMock()
    self.win32com = mock.MagicMock()
    self.win32com.client = mock.MagicMock()
    modules = {
        "_winreg": mock.MagicMock(),
        "pythoncom": mock.MagicMock(),
        "pywintypes": mock.MagicMock(),
        "win32api": mock.MagicMock(),
        "win32com": self.win32com,
        "win32com.client": self.win32com.client,
        "win32file": mock.MagicMock(),
        "win32service": mock.MagicMock(),
        "win32serviceutil": mock.MagicMock(),
        "winerror": mock.MagicMock(),
        "wmi": self.wmimock

    self.module_patcher = mock.patch.dict("sys.modules", modules)

    # Now we're ready to do the import
    from myrepo.actions import windows = windows

  def tearDown(self):

  def testEnumerateInterfaces(self):

    # Stub out wmi.WMI().Win32_NetworkAdapterConfiguration(IPEnabled=1)
    wmi_object = self.wmimock.WMI.return_value
    wmi_object.Win32_NetworkAdapterConfiguration.return_value = [

    enumif =
    interface_dict_list = list(enumif.RunWMIQuery())

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