I've dome some reading on the difference between __new__ and __init__, and
never really groked it. I just followed the advice that you should almost
always use __init__.
I recently came across a task that required using __new__ and not __init__. I
was a bit intimidated at first, but it was quick and easy. This simple
programming exercise really cleared a lot of things up for me.
Not to be immodest, but I think something like this ought to be the canonical
example for explaining when/how to override __new__.
The task? I want to make a class that behaves exactly like a tuple, except
changing the constructor argument signature and adding some extra methods. An
example should clarify what I needed.
> x = ParetoTuple(1, 2, 0)
> x[1]
>> 2
> len(x)
>> 3
> 2 in x
>> True
> -1 in x
>> False
> x.dominates(ParetoTuple(1, 3, 0))
>> True
> x.equivalent(ParetoTuple(1, 2 + 1e-5, 0))
>> True
etc.
Since I want the constructor to take an (almost) arbitrary number of arguments,
each of which will be elements of the resulting ParetoTuple, I need to override
__new__. I don't need to overwrite __init__, because the tuple.__new__ will
populate it's data when the arguments are properly formatted.
Also, since the world of Pareto comparisons makes sense only with 2 or more
goals, I want my specialized constructor to take at least 2 arguments in a
natural way.
Here is the code
class ParetoTuple(tuple) :
def __new__ (cls, obj1, obj2, *rest):
return super(ParetoTuple, cls).__new__(cls, (obj1, obj2) + rest)
# nothing special about the dominates, equivalents methods...
# no __init__ needed
I understand some people argue in favor of using a factory pattern for this
sort of situation, but I disagree. I think the cognitive overhead of factories
requires a more complicated task than re-signaturing the constructor method.
At any rate, hope it helps others like it helped me.
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