unittest help

[Duncan Booth]

Qiangning Hong wrote:

> As you see, it is an OO wrapper on _extmod, which is a pyrex extension
> module.  The question is: how to unittest this class?  As the _extmod
> is hardware-dependent, I want to use a mock class to replace it in unit
> test.  But how can I let myclass in unittest to import the mock class?
> Like the following:
> 
> class MyClassTest(unittest.TestCase):
>     def setUp(self):
>         import myclass
>         import mocklib
>         myclass.change_extmod(mocklib.MockExtMod())
>         self.testobj = myclass.MyClass()  # here MyClass.__init__ will
> call the open
>                                           # method of MockExtMod class
> instead of
>                                           # _extmod.open()
>     ...
> 
> How to implement the change_extmod?  (Or maybe my idea is totally
> wrong?)
> 

One way is simply to do:

    def setUp(self):
        import myclass
        self.real_extmod = myclass._extmod
        myclass._extmod = mocklib.MockExtMod()
        self.testobj = myclass.MyClass()

    def tearDown(self):
        import myclass
        if hasattr(self, testobj):
            del self.testobj
        myclass._extmod = self.real_extmod

This can be less intrusive than passing the mock object to a constructor, 
but it depends very much on the way the objects are used: changing global 
state for a unit test is a risky business, for example if an exception is 
thrown then tearDown would be called *before* your __del__ method is 
invoked. You can work round this by ensuring that the _extmod value is 
saved in your instance but that takes you pretty much back to André Malo's 
suggestion.

BTW, accessing a global variable from a __del__ method is a bad idea 
generally: there is no guarantee that the global variable will still be set 
if __del__ is called during program exit.