Sunday, March 8, 2009

Automatic __repr__ and __eq__ for Data Structure Classes

For the compilers class project that I'm working on, I recently wrote a couple of classes that do simple structural equality and automatic __repr__ generation. We have a lot of simple data structure IR classes in our project, and they all need __eq__ and __repr__ for testing and debugging. Structural equality is easy; all you have to do is introspect on __dict__ and see that the attributes match recursively. Automatic __repr__ is more difficult, though, because to produce valid Python source, you have to know the order of the arguments to the instance's __init__ method. Fortunately, with the inspect module, you can call inspect.getargspec(self.__init__) and get that information. We use the simple convention for our IR nodes that the arguments to __init__ all become attributes of the same name, so you can then use the argument names and getattr to generate the reprs of the subnodes. Good times!

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