Python creating static objects which are shared amongst all class objects

Question

I have a class node something like this. It's a typical node object for a graph.

class Node(object):
    def __init__(self, data, edges = []):
        super(Node, self).__init__()
        self.data = data
        self.edges = edges
        self.visited = False

    def addEdge(self, *args):
        print(self)
        self.edges.extend(args)
        print(self.edges)

I create two objects like this -

one = Node(1)
two = Node(2)

Next I add a pointer of two to one using the addEdge method defined above -

one.addEdge(two)

Now comes the surprising bit. When I check the values of one.edges and two.edges I get this -

one.edges [<main.Node object at 0x109ed3e50>]

two.edges [<main.Node object at 0x109ed3e50>].

If you see both the objects have gotten the value. I'm quite puzzled at this and have no idea why this is happening. Is this how python behaves? If so can you explain this behaviour?

Possible duplicate of "Least Astonishment" and the Mutable Default Argument — jasonharper
– jasonharper, Commented Dec 29, 2018 at 5:02

Mark · Accepted Answer · 2018-12-29 04:59:29Z

3

You need to be careful when using an array literal as a default value because you don't get a new array with each instance — you get a reference to the same one. In your example you will see:

>> one.edges is two.edges
True

You need to do something to make sure you get a new array each time. One things you can do is:

self.edges = list(edges)

Another option is:

def __init__(self, data, edges = None):

   if edges is None:
        self.edges = []
   else:
        self.edges = edges

But edges is still mutable so it may also lead to subtle bugs if the caller is not expecting it to be changed.

edited Dec 29, 2018 at 4:59

answered Dec 29, 2018 at 4:54

Mark

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4 Comments

JacobIRR Over a year ago

So this edges=[] in the init method is the culprit?

Mark Over a year ago

Yes, many will say it's a bad idea to use mutable objects as default parameters.

Raj Kumar Over a year ago

Ok. That was unexpected. Does the same happen when we pass {} literals as default parameters?

Mark Over a year ago

Yes, @RajKumar the same thing will happen.

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