Suggesting for overloading the assign operator

Ian Bicking ianb at colorstudy.com
Wed Jul 2 00:13:01 CEST 2003


On Tue, 2003-07-01 at 13:01, Alan Kennedy wrote:
> Rim wrote:
> 
> > What do you think about providing a statement to assign values
> > without assigning type?
> > 
> > '=' statement assigns value and type
> > ':=' statement assigns value ony
> 
> I think the concept has some merit. I think that sometimes it useful to ensure
> that the target of a rebind operation have the same type as the object which was
> bound before the rebind.
> 
> I know that the same effect can be achieved by descriptors or overriding
> "setattr" et al. Or something like this

Or maybe:

class stricttype(object):

    counter = 1

    def __init__(self, required_type, doc=None, real_attr=None):
        self.required_type = required_type
        if not real_attr:
            real_attr = '_stricttype_attr_%i' % stricttype.counter
            stricttype.counter += 1
        self.real_attr = real_attr
        self.doc = doc

    def __set__(self, obj, value):
        if not isinstance(value, self.required_type):
            raise TypeError, 'must be of type %r' % self.required_type
        setattr(obj, self.real_attr, value)

    def __get__(self, obj, cls):
        return getattr(obj, self.real_attr)

    def __del__(self, obj):
        delattr(obj, self.real_attr)

    def __doc__(self, obj):
        return self.doc


class IntPoint(object):

    def __init__(self, x, y):
        self.x = x
        self.y = y

    def __str__(self):
        return 'Point: (%i, %i)' % (self.x, self.y)

    x = stricttype(int)
    y = stricttype(int)

>>> p = IntPoint(1, 1)
>>> p
Point: (1, 1)
>>> p.x = 2
>>> p
Point: (2, 1)
>>> p.x = 2.2
Traceback (most recent call last):
  File "stricttype.py", line 44, in ?
    p.x = 2.2
  File "stricttype.py", line 15, in __set__
    raise TypeError, 'must be of type %r' % self.required_type
TypeError: must be of type <type 'int'>


If we're talking about optimizations, there's no reason something like
Psyco couldn't specifically look for specific descriptors like
stricttype, and probably replace that implementation with something
better optimized.

  Ian







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