Weird thing in python classes ?

Question

I was trying to implement a shortest path algorithm in python. I ran into some trouble when I attempted to connect my points on the path.

I have a class called NODE I made an array of these nodes and then created new nodes in them like this:

nodes = []
for r in range ( 0, 3 ):
 for c in range ( 0, 3 ):
 nodes.append( NODE( c*WIDTH/20 + 30, r*HEIGHT/20 + 30, 10 ) )

This created nodes, which is fine, I can draw them just fine.
At first I attempted connecting them by adding a list of node objects into the the NODE class. I ran into some issues, I thought the problem is with the recursive classes, so to test I did this.
Where connections is a blank array within the NODE class.

nodes[0].connections.append( 0 )
print nodes[0].connections
nodes[1].connections.append( 1 )
print nodes[1].connections
print nodes[0].connections

This is where I found the problem, Maybe I'm just being stupid, or It's a language thing ? Dunno, but I thought I'd ask.

The output is this:
[ 0 ]
[ 0, 1 ]
[ 0, 1 ]

As you can see, I only added 1 object to the connections list in the index 1. However when I print the contents, I see two.... And this confuses me.

• Kevin: This is really hard to answer without seeing a minimal reproducible example that includes the actual NODE class.
• dhke: Let me guess, class Node has a static field connections instead of initializing connections in the constructor to a new list, so that all objects share a single connection list? Show your Node class, please.
• Aran-Fey: I'm going to go ahead and say this is a duplicate of The mutable default argument.
• suli: yep. Thats the problem. I was writing it as though I would write something in Java or cpp. Thanks for the help guys !
• user3657941: Which one is the problem? A static property or a mutable default argument?

Answer 1

Possibilty 1: your code looks like this:

class NODE:
 connections = []
 def __init__(self, value):
 self.value = value
nodes = [NODE(23), NODE(42)]
nodes[0].connections.append( 0 )
print nodes[0].connections
nodes[1].connections.append( 1 )
print nodes[1].connections
print nodes[0].connections

Solution: class instance attributes should be defined inside __init__. If you define connections at the class level, then every single instance of NODE will share the same one connections list.

class NODE:
 def __init__(self, value):
 self.value = value
 self.connections = []

---

Possibility 2: your code looks like this:

class NODE:
 def __init__(self, value, connections = []):
 self.value = value
 self.connections = connections
nodes = [NODE(23), NODE(42)]
nodes[0].connections.append( 0 )
print nodes[0].connections
nodes[1].connections.append( 1 )
print nodes[1].connections
print nodes[0].connections

Solution: the default value in an argument list is created only once, so all instances of NODE using that default value will share the same one list. use a non-mutable sentinel value to indicate that no argument was passed. None is usually suitable for this task.

class NODE:
 def __init__(self, value, connections = None):
 self.value = value
 if connections is not None:
 self.connections = connections
 else:
 self.connections = []

• suli: yea this is it. Thanks for answering ! This is gonna fix a lot of my other problems too !