dippim wrote:
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 =one,b = None)
self.start =
self.end =
from datetime import datetime
c =atetime(2009,8,13,6,15,0)
d =atetime(2009,8,14,12,0,0)
afoo =oo(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 =atetime(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 =one,b = None):
self._start =
self._end =
def get_start(self):
return self._start
def set_start(self, value):
if self._end is None or value < self._end:
self._start =alue
else:
self._end =alue
start =roperty(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 =alue
else:
self._start =alue
end =roperty(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!
DANGER- WILL ROBINSON!
Don't use this code as-is. There is a nasty surprise waiting for the
caller when he sets start and end, and discovers that one of them gets
thrown out, and an old value still remains.
obj= foo(3, 5)
obj.start = 8
obj.end = 12
print obj.start, obj.end
will print out 3, 12. Not what the caller expected.
Four fixes, in order of preference:
0) Trust your user to read and obey your docstrings. This was what JM
was implying, by changing the names of the formal parameters.
1) make a new method that sets both values, making these two properties
readonly. That new method would make sure the two parameters are
self-consistent. Making the actual values readonly can be done with a
descriptor as well, or even a decorator.
2) Raise an exception in the getter methods if they're out of order
3) do the min/max logic on the getter methods, but I don't like that one
at all.
DaveA
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