En Sat, 22 Sep 2007 15:06:38 -0300, George V. Neville-Neil
<[EMAIL PROTECTED]> escribi�:
> 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. 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)
>
> 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)
You don't have to repeat the fields:
pcs.Packet.__init__(self, [self.dst, self.src, self.type], bytes
= bytes)
does the same thing.
And you can remove self.description=... and use (at the class level):
description = __doc__
or:
description = property(lambda self: self.__doc__)
to automatically enable subclases to share the definition
One could say that the field order is important so I'd finally write it as:
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)
_fields = [dst, src, type]
description = property(lambda self: self.__doc__)
def __init__(self, bytes = None, timestamp = None):
"""initialize an ethernet packet"""
pcs.Packet.__init__(self, self._fields, bytes = bytes)
> 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.
This is the part I don't understand. Can't you define a new class, in case
you want a different field list? Using the above class:
class ethernet_modified(ethernet):
"""Modified ethernet class"""
added_field = pcs.StringField("whatever", 16)
_fields = [dst, src, type, added_field]
And it doesn't even requiere an __init__ method
> So, is there a way to assign a property to an object, like this:
>
> def __init__(....
> self.src = property()
>
> or something like that?
I'd use different classes for different kind of objects.
> And, failing that, is there a clean way to modify the class in it's
> __init__() method so I don't have to call the Field objects twice,
> once for the property effect and once to put into the list in the base
> Packet class?
I think my example above does what you want.
--
Gabriel Genellina
--
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