TotT Better Stubbing in Python

TotT Better Stubbing in Python

So you've learned all about method stubs, mock objects, and fakes. You might be tempted to stub out slow or I/O-dependent built-ins. For example:

 def Foo(path):
   if os.path.exists(path):
     return DoSomething()
   else:
     return DoSomethingElse()

 def testFoo(self):         # Somewhere in your unit test class
   old_exists = os.path.exists
   try:
     os.path.exists = lambda x: True
     self.assertEqual(Foo('bar'), something)
     os.path.exists = lambda x: False
     self.assertEqual(Foo('bar'), something_else)
   finally:
     # Remember to clean-up after yourself!
     os.path.exists = old_exists

Congratulations, you just achieved 100% coverage! Unfortunately, you might find that this test fails in strange ways. For example, given the following DoSomethingElse which checks the existence of a different file:

 def DoSomethingElse():
   assert os.path.exists(some_other_file)
   return some_other_file

Foo will now throw an exception in its second invocation because os.path.exists returns False so the assertion fails.

You could avoid this problem by stubbing or mocking out DoSomethingElse, but the task might be daunting in a real-life situation. Instead, it is safer and faster to parameterize the built-in:

 def Foo(path, path_checker=os.path.exists):
   if path_checker(path):
     return DoSomething()
   else:
     return DoSomethingElse()

 def testFoo(self):
   self.assertEqual(Foo('bar', lambda x: True), something)
   self.assertEqual(Foo('bar', lambda x: False), something_else)

Remember to download this episode of Testing on the Toilet, print it, and flyer your office.