On Aug 14, 2:34 am, Raymond Hettinger <pyt...@rcn.com> wrote: > [David] > > > > > 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, but I can't figure out how to > > determine which values is being set. > > You're on the right track, but it is easier to use property() than to > write your own custom descriptor with __get__ and __set__. > > class foo(object): > def __init__(self,a = None,b = None): > self._start = a > self._end = b > def get_start(self): > return self._start > def set_start(self, value): > if self._end is None or value < self._end: > self._start = value > else: > self._end = value > start = property(get_start, set_start) > def get_end(self): > return self._end > def set_end(self, value): > if self._start is None or value > self._start: > self._end = value > else: > self._start = value > end = property(get_end, set_end) > > Raymond
Raymond, This functionality is exactly what I was looking for. Thanks! I'll be using this to solve my problem. Now that I'm on the right track, I'm still a bit confused about how __get__ and __set__ are useful. Admittedly, I don't need to understand them to solve this problem, but perhaps they may be useful in the future. If I wanted to solve this problem using __get__ and __set__ could it be done? Thanks Again! -- http://mail.python.org/mailman/listinfo/python-list
