I think you might use a tuple instead of a list for OrderElement, that would
make much easier your code:
class
OrderElement(tuple):
def __new__(cls, x, y):
if not isinstance(x, int) or not isinstance(y, int):
raise TypeError("Order element must receives two
integers")
return tuple.__new__(cls, (x, y))
class Order(list):
def __setitem__(self, item):
assert isinstance(item, OrderElement)
super(Order, self).__setitem__(item)
I didn't check your module condition since it isn't quite clear to me, but
you could add a second condition two Order class.
2011/8/7 Steven D'Aprano <steve+comp.lang.pyt...@pearwood.info>
> Roy Smith wrote:
>
> > In article <mailman.2010.1312731312.1164.python-l...@python.org>,
> > John O'Hagan <resea...@johnohagan.com> wrote:
> >
> >> I'm looking for good ways to ensure that attributes are only writable
> >> such that they retain the characteristics the class requires.
> >
> > Sounds like you're trying to do
> > http://en.wikipedia.org/wiki/Design_by_contract. Which is not a bad
> > thing. But, I think a more pythonic way to implement this would be to
> > verify behaviors, not types.
> >
> > I would start by writing a assert_invarient() method which validates the
> > object. I'm guessing all you really need is that you can index [0] and
> > [1] and get ints, so test for that. Something like:
> >
> > def assert_invarient(self):
> > try:
> > assert isinstance(data[0], int)
> > assert isinstance(data[1], int)
> > except:
> > raise ValueError
>
> Don't do that. assert is for testing program logic, not verifying data. The
> problem with assert is that the user can turn all assertions off, simply by
> launching Python with the -O switch. Your verification code then becomes:
>
> def assert_invarient(self):
> try:
> pass
> except:
> raise ValueError
>
> which is useless.
>
> When should you use an assertion? If you've ever written code like this:
>
> if condition:
> do_something()
> else:
> # This should never happen. But you know what they say: code that
> # can't happen, does!
> raise RuntimeError('condition unexpectedly false')
>
>
> that's a prime candidate for turning into an assertion:
>
>
> assert condition, 'condition unexpectedly false'
> do_something()
>
>
>
> --
> Steven
>
> --
> http://mail.python.org/mailman/listinfo/python-list
>
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