[TIP] mocking out mixins and subclasses

Michael Foord fuzzyman at voidspace.org.uk
Mon Mar 5 12:34:20 PST 2007

Nate Lowrie wrote:
> I have several mixin classes that are subclassed by other classes.  I
> am unit testing the mixin classes on there own. Some of the classes
> that subclass these mixins call the mixin functions in some of the
> methods.  I know that I can rebind the methods to mocked function
> after I instantiate an object but there are some calls to the mixin
> methods in the init section as well.  Ideally, I would like to mock
> the mixin before creating any objects.
> How are other people handling unit testing for mixins and objects
> using the mixin?  Is there an easy way to do this that I am
> overlooking?
Hmm... mocking out base classes before instantiation is something I 
haven't had to do.

As the code snippet below shows, you can do it by assigning to 
'__bases__', to swap out the base class.

 >>> class x(object):
...  z = 3
 >>> class y(object):
...  z = 6
 >>> class z(x):
...  pass
 >>> z.z
 >>> z.__bases__
(<class '__main__.x'>,)
 >>> z.__bases__ = (y,)
 >>> z.z

So you should be able to swap out the base class before instantiating 
your object - this will work so long as the mixin methods are called 
using 'self'.

If the calls in init reference the class by name (rather than self), 
then you can just mock on the base class of course.



> Cheers,
> Nate L.
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