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Isn't overriding __init__ a risky thing to do in python?

+1 vote
31 views

This code is from The Python Cookbook, 2nd edition, 12.2 Counting Tags in a Document:

from xml.sax.handler import ContentHandler
import xml.sax
class countHandler(ContentHandler):
 def __init__(self):
 self.tags={}
 def startElement(self, name, attr):
 self.tags[name] = 1 + self.tags.get(name, 0)

Isn't overriding __init__ a risky thing to do? The docs don't mention it as a method I should override, and also don't define what's in there or if I'd need to call the base class __init__. Moreover, startDocument is provided for parser setup.

As it happens, ContentHandler.__init__ isn't empty, so the above code could fail if the parser isn't prepared for _locator to be undefined. Is the above code is an acceptable idiom?

posted Aug 30, 2013 by Meenal Mishra

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1 Answer

+1 vote

I think this is a bad idea unless you want to avoid the parent class __init__ specifically (in which case a comment stating why is mandatory). I do not like that this recipe shows behavior that might be fine in this instance, but is not a good general practice.

 def __init__(self):
 super(ContentHandler, self).__init__()
 #OR ContentHandler.__init__(self)
 self.tags={}

I personally think the super() line is better of the two options.

answer Aug 30, 2013 by Majula Joshi
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+1 vote

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.

+1 vote

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(never huts to improve ones coding skills).

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