implementing descriptors

[Diez B. Roggisch]

dippim schrieb:
> On Aug 14, 5:45 am, Jean-Michel Pichavant <jeanmic... at sequans.com>
> wrote:
>> 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)
>>> Emile
>> or
>>
>> class foo(object):
>>     def __init__(self, start, end)
>>        self.start = start
>>        self.end = end
>>
>> Problem solved by design :o)
>>
>> JM
> 
> 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:

class Foo(object):

    def __init__(self, start, end):
        self._start = start
        self._end = end


    @property
    def start(self):
        return start


    @start.setter
    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


Diez