Diez B. Roggisch
deets at nospam.web.de
Fri Aug 14 15:39:31 CEST 2009
> On Aug 14, 5:45 am, Jean-Michel Pichavant <jeanmic... at sequans.com>
>> Emile van Sebille wrote:
>>> On 8/13/2009 3:17 PM dippim said...
>>>> I am new to Python and I have a question about descriptors. If I have
>>>> a class as written below, is there a way to use descriptors to be
>>>> certain that the datetime in start is always before the one in end?
>>>> class foo(object):
>>>> def __init__(self,a = None,b = None)
>>>> self.start = a
>>>> self.end = b
>>>> from datetime import datetime
>>>> c = datetime(2009,8,13,6,15,0)
>>>> d = datetime(2009,8,14,12,0,0)
>>>> afoo = foo(c,d)
>>>> For instance, if the following code were run, I would like to instance
>>>> of foo to switch the start and end times.
>>>> afoo.start = datetime(2010,8,13,6,15,0)
>>>> I was thinking of using the __set__ descriptor to catch the assignment
>>>> and reverse the values if necessary,
>>> why not...
>>> class foo(object):
>>> def __init__(self,a = None,b = None)
>>> self.start = min(a,b)
>>> self.end = max(a,b)
>> class foo(object):
>> def __init__(self, start, end)
>> self.start = start
>> self.end = end
>> Problem solved by design :o)
> Emile and JM,
> Thanks for the response. However, these solution only work at
> instantiation. If I change the value of start or end after
> instantiation, then I can make start or end whatever I like without
> regard to order.
But for that, you don't need descriptors. All you need is a property:
def __init__(self, start, end):
self._start = start
self._end = end
def start(self, value):
if value >= self._end:
raise ValueError, "Tried to set a value greater than end!"
self._start = value
# repeat for end, switch relational op
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