Properties and Objects...

[Gabriel Genellina]

En Sat, 22 Sep 2007 15:06:38 -0300, George V. Neville-Neil  
<gnn at neville-neil.com> 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