On Mon, 08 Aug 2011 03:07:30 +1000
Steven D'Aprano <steve+comp.lang.pyt...@pearwood.info> 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.extend(s.order[1:3])
s.order = [[99, 99],[100, 100]]
import random
random.shuffle(s.order)
etc
[...]
> 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.__length=length
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
else:
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.
Regards,
John
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