Object indexing in python

Question

I have created the following objects in python

It creates an object of class arc and then creates another object network using the object arc

class Arc:
    # Creates an object of class arc
    def __init__(self, tailNode = 0, headNode = 0, lowerBound = 0, upperBound = 0, cost = 0):
        self.tailNode = tailNode
        self.headNode = headNode
        self.lowerBound = lowerBound
        self.upperBound = upperBound
        self.cost = cost
    def displayArc(self):
        print("Tail Node : ", self.tailNode,  "\nHead Node : ", self.headNode, "\nLower Bound : ", self.lowerBound, "\nUpper Bound ; ", self.upperBound, "\nCost : ", self.cost, "\n")

class Node:
    # Create an object of class node
    def __init__(self, nodeName = 0, distLabel = 0, preNode = 0):
        self.nodeName = nodeName
        self.distLabel = distLabel
        self.preNode = preNode

class Network:
     # Creates a network from given arcs
     def __init__(self, fileName):
        global arcNo 
        arcNo = 0

        self.fileName = fileName
        f = open(self.fileName)
        x = f.readlines()

        arcList = [ Arc() for i in range(len(x))]

    for i in range(len(x)):
        temp = x[i]
        temp = temp.split("\n")
        temp = ",".join(map(str, temp))
        temp = temp.split(",")
        arcList[i] = Arc(temp[0], temp[1], temp[2], temp[3], temp[4])
        arcNo += 1

    print(arcNo)

net = Network("arcList.txt")
print(type(net))
print(net[1])

When the print statement comes says

4
<class '__main__.Network'>
Traceback (most recent call last):
      File "Dijkstra.py", line 54, in <module>
    print(net[1])
TypeError: 'Network' object does not support indexing

How do I support indexing so that I can call the network object by its index ?

I was hoping to see a list of all the attributes given to it like 1,2,0,100,6 — Surya Pavan Pynda
– Surya Pavan Pynda, Commented Aug 2, 2016 at 7:38
On a dumber note, in the initialisation is the arcList created assigned to net ? — Surya Pavan Pynda
– Surya Pavan Pynda, Commented Aug 2, 2016 at 7:50
no, right now, arcList can only be accessed from your __init__() method. If you need to be part of your class instance (net), you need to write self.arcList = .... You can then call net.arcList — Julien Spronck
– Julien Spronck, Commented Aug 2, 2016 at 7:56

Community · Accepted Answer · 2017-05-23 10:28:46Z

3

Assuming net[index] returns the arcList variable you can simply override your [] operator as such

class Network:
    def __getitem__(self,index):
         return arcList[index]

Printing a class needs a method as well. This might help you How to print a class or objects of class using print()?

edited May 23, 2017 at 10:28

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answered Aug 2, 2016 at 5:45

Prasanna

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1 Comment

Surya Pavan Pynda Over a year ago

I want to access the class. Print is only for checking. For example if I give a=net[1], I want a string or array assigned to "a".

Julien Spronck · Accepted Answer · 2016-08-02 05:03:27Z

1

In order to support indexing, your Network class should have a __getitem__() method (http://docs.python.org/reference/datamodel.html#object.getitem).

answered Aug 2, 2016 at 5:03

Julien Spronck

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