Moving from java to python.

[Larry Bates]

Paul Hankin wrote:
> On Nov 12, 3:25 pm, "PeterBrad... at googlemail.com"
> <PeterBrad... at googlemail.com> wrote:
>> Hi,
>>
>> I have recently been learning python, and coming from a java
>> background there are many new ways of doing things that I am only just
>> getting to grips with.
>>
>> I wondered if anyone could take a look at a few pieces of code I have
>> written to see if there are any places where I am still using java-
>> esque techniques, and letting me know the appropriate python way of
>> doing things.
>>
>> Here is a node class I wrote for use in graph traversal algorithms:
>>
>> #====
>>
>> class Node:
>>         """
>>         Node models a single piece of connected data.
>>
>>         Author: Peter Braden
>>         Last Modified : Nov. '07
>>         """
>>
>>         def __init__(self, connections = None, uid = None):
>>                 """
>>                 Args:
>>                         [connections - a list of (connected node, weight) tuples.       ]
>>                         [uid - an identifier for comparisons.]
>>                 """
>>                 self._connected = []
>>                 self._weights = []
>>
>>                 if connections:
>>                         for i in connections:
>>                                 self.connected.append(i[0])
>>                                 self.weights.append(i[1])
>>
>>                 if not uid:
>>                         self.id = id(self)
>>                 else:
>>                         self.id = uid
>>
>>         def __eq__(self, other):
>>                 return self.id == other.id
>>
>>         def getConnected(self):
>>                 return self._connected
>>
>>         def getWeights(self):
>>                 return self._weights
>>
>>         def getConnections(self):
>>                 connections = []
>>                 for i in range(len(connected)):
>>                         connections.append((self._connected[i],self._weight[i]))
>>                 return connections
>>
>>         connected = property(getConnected, None)
>>         weights = property (getWeights, None)
>>         connections = property(getConnections, None)
>>
>>         def addConnection(self, node, weight = 0):
>>                 self.connected.append(node)
>>                 self.weights.append(weight)
>>
>>         def removeConnection(self, node):
>>                 i = self._connected.index(node)
>>                 del self._connected[i]
>>                 del self._weights[i]
> 
> There's no reason to make 'connected' and 'weights' properties: just
> use them directly.
> 
> Make 'connections' default to [] rather than None, and replace the
> slightly clumsy initialisation with more direct code:
> 
> self.connected = [i[0] for i in connections]
> self.weights = [i[1] for i in connections]
> 
> getConnections can be much simpler...
> 
> def getConnections(self):
>     return zip(self.connected, self.weights)
> 
> The 'uid' business makes me suspicious: what's it for? If you need it,
> you probably need to initialise with an explicit test for None rather
> than just 'if uid' which will be wrong if you use a uid of 0...
> 
> self.id = uid if uid is not None else id(self)
> 
> HTH
> --
> Paul Hankin
> 
Be VERY careful with using [] as default arguments.  Mutables are only evaluated 
once (not at every call).  This question comes up about once a week on this 
forum where people don't understand this.

I would recommend using (if you can):

    def __init__(self, connected = None, weights=None, uid = None):
        self.connected = connected or []
        self.weights = weights or []


If you want to stay with single connections list do this:

    def __init__(self, connections = None, uid = None):
         if connections is not None:
             self.connected=[c[0] for c in connections]
             self.weights=[c(1) for c in connections]

	else:
             self.connected=[]
             self.weights=[]

Others have addressed the lack of need for accessors.

-Larry