Restricted attribute writing

John O'Hagan research at
Mon Aug 8 06:59:23 CEST 2011

On Mon, 08 Aug 2011 03:07:30 +1000
Steven D'Aprano <steve+comp.lang.python at> wrote:

> John O'Hagan wrote:
> > I'm looking for good ways to ensure that attributes are only writable such
> > that they retain the characteristics the class requires.
> That's what properties are for.
> > My particular case is a class attribute which is initialised as a list of
> > lists of two integers, the first of which is a modulo remainder. I need to
> > be able to write to it like a normal list, but want to ensure it is only
> > possible to do so without changing that format.
> Then you have two problems to solve.
> First, you need a special type of list that only holds exactly two integers.
> Your main class can't control what happens inside the list, so you need the
> list to validate itself.
> Secondly, you should use a property in your main class to ensure that the
> attribute you want to be a special list-of-two-ints can't (easily) be
> changed to something else.

Although experience shows you're usually right :) , I thought I had three problems, the third being what I perhaps wasn't clear enough about: that the two-integer containers live in a list which should only contain the two-integer things, but aside from that should be able to do all the other list operations on it. AFAIK making this attribute a property only protects it from incorrect assignment, but not from unwanted appends etc.  

That's what the other helper class Order is meant for, it subclasses list, and overrides __setitem__ to ensure every item is an OrderElement, and __getitem__ to ensure slices are the same class. I've also since realised it must override append, insert and extend. I think I need all this to ensure the required behaviour, including:

s = SeqSim([[15, 2]], 12)
s.order[0][1] = 100
s.order[0][1:] = [100]
s.order += [[22, 11]]
s.order *= 2
s.order[2] = [[15, 8]]
s.order[1:5:2]) = [[1, 1],[2, 2]]
s.order.extend([[1, 1],[2, 2]])
s.order.insert(2, [2, 29])
s.order.append([26, 24])
s.order = [[99, 99],[100, 100]]
import random
> I'd take this approach instead:
> # Untested.
> class ThingWithTwoIntegers(object):
>     def __init__(self, a, b):
>         self.a = a
>         self.b = b
>     def __getitem__(self, index):
>         # Slicing not supported, because I'm lazy.
>         if index < 0: index += 2
>         if index == 0: return self.a
>         elif index == 1: return self.b
>         else: raise IndexError
>     def __setitem__(self, index, value):
>         # Slicing not supported, because I'm lazy.
>         if index < 0: index += 2
>         if index == 0: self.a = value
>         elif index == 1: self.b = value
>         else: raise IndexError
>     def _geta(self):
>         return self._a
>     def _seta(self, value):
>         if isinstance(value, (int, long)):  # drop long if using Python 3
>             self._a = value
>         else:
>             raise TypeError('expected an int but got %s' % type(value))
>     a = property(_geta, _seta)
>     # and the same for b: _getb, _setb, making the obvious changes
> Obviously this isn't a full blown list, but if you don't need all the
> list-like behaviour (sorting, inserting, deleting items, etc.) why support
> it?

Thanks for this, I can see that the __data attribute I was using was unnecessary and I've redone the OrderElement class accordingly, although I do want slicing and don't need dot-notation access:

class OrderElement():

    def __init__(self, length, a, b):
        self.__a = a
        self.__b = b
        self[:] = a, b
    def __setitem__(self, index, item):
        if isinstance(index, slice):
            for k, i in zip(range(*index.indices(2)), item):
                self[k] = i
        elif isinstance(item, int) and index in (0, 1):
            if index == 0:
                self.__a = item % self.__length
            elif index == 1:
                self.__b = item
            raise TypeError("OrderElement takes two integers")
    def __getitem__(self, index):
        if isinstance(index, slice):
           return [self[i] for i in range(*index.indices(2))]
        if index == 0:
            return self.__a
        if index == 1:
            return self.__b
        raise IndexError

As for the rest, I take your point that a simple idea need not be simple to implement, and I'm starting to think my solution may be about as complicated as it needs to be. 



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