Multiple inheritance is hard, we all know it, but even after warning people about it myself, I found myself tripped up by it yesterday. All I wanted was a mixin for my unittest.TestCase class.
Unittest’s TestCases use setUp and tearDown to affect the test state, and I wanted a way to share setUp and tearDown implementations between two different test classes. A mixin seemed like a good solution. I already have a BaseTestCase of my own that inherits from unittest.TestCase, so my mixin looked like this:
#.. set up that everyone needs ..
#.. tear down that everyone needs ..
#.. do something here ..
#.. do something here ..
class RealTestCase(BaseTestCase, MyMixin):
# I didn't need setUp and tearDown here..
#.. etc ..
The theory is that mixins avoid some complications of multiple inheritance: they provide methods, but no attributes, and don’t inherit in a way that produces diamonds in the inheritance hierarchy. And I did the right things here, using super to keep the MRO (method resolution order) intact.
But this code doesn’t work, MyMixin.setUp is never invoked. Why? It’s because TestCase in unittest doesn’t invoke super():
# unittest.py (some details omitted :)
The method resolution order of RealTestCase is:
Since TestCase.setUp doesn’t use super, the sequence stops there, and MyMixin is never consulted. At first I thought, “TestCase.setup should use super!” But, if it did, it would fail in simpler hierarchies that don’t use a mixin, because it would try to invoke object.setUp(), which doesn’t exist.
I suppose TestCase could be re-written like this:
# unittest.py (hypothetical)
parent = super(Base, self)
if hasattr(parent, "setUp"):
This works, and now MyMixin.setUp() is invoked, but then it fails for the same reason: it tries to invoke object.setUp, which doesn’t exist, so MyMixin also needs to have the defensive check of its parent. Yuk.
The simple solution is to swap the order of the mixin and the base class:
class RealTestCase(MyMixin, BaseTestCase):
With this class declaration, the MRO is: RealTestCase, MyMixin, BaseTestCase, TestCase, object. All the setUp’s are invoked, and it ends cleanly with no complicated parentage check.
But I can’t help feeling this isn’t great. For one thing, conceptually, BaseTestCase is the real parent, and the mixin is just tossed in, so I would rather be able to write my base classes in the other order. But more worrying, this solution also means that I have to be very careful to know the lineage of all my classes all the way up to object.
Maybe this is the fundamental truth about Multiple Inheritance, and why it is so difficult: details about base classes that you thought were abstracted away from you can suddenly be critical to understand. Like all action at a distance, this is mysterious and confusing.
My take-aways from this:
- Mixins come first even though it looks odd.
- Multiple inheritance is hard.
- There are yet more Python details to master.
The question remaining in my mind: would class hierarchies be better if the top-most classes (derived from object) used the defensive super style? Or is that overkill that defers rather than removes the pain? Would something else bite me later?