George V. Neville-Neil wrote:
> I have been trying to switch this over to using properties, which seem
> at first glance to be cleaner, but which I am having a lot of problems
> with. In particular I want to add a property to an object, not a
> class.
You can't. The underlying mechanism (descriptors) works on the class level.
> The field list in a class is actually relatively static but I
> wind up with ugly code like:
>
> class ethernet(pcs.Packet):
> """Ethernet"""
> __layout__ = pcs.Layout()
> _map = ethernet_map.map
>
> src = pcs.StringField("src", 48)
> dst = pcs.StringField("dst", 48)
> type = pcs.Field("type", 16)
>From the piece of code you present I would guess that the above class
attributes are superfluous. Or doesn't pcs.Packet.__init__() do the
self.src = ...
thingy?
> def __init__(self, bytes = None, timestamp = None):
> """initialize an ethernet packet"""
>
> src = pcs.StringField("src", 48)
> dst = pcs.StringField("dst", 48)
> type = pcs.Field("type", 16)
>
> pcs.Packet.__init__(self, [dst, src, type], bytes = bytes)
> self.description = inspect.getdoc(self)
>
> and assigning the properties at class time means that it's hard to have
> variations, packets with the same name but with slightly varying field
> lists.
What's so hard about subclassing?
> So, is there a way to assign a property to an object, like this:
>
> def __init__(....
> self.src = property()
>
> or something like that?
You can make a property that delegates its behaviour, e. g:
import new
def get_src_one(self):
return 42
class A(object):
src = property(lambda self: self.get_src())
def __init__(self, get):
self.get_src = new.instancemethod(get, self)
a = A(get_src_one)
print a.src
But -- ceterum censeo -- subclassing is probably better, cleaner and
faster. Then instead of calling the class directly make a factory function
that picks the appropriate subclass.
Peter
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