I am implementing a basic node object in python. Basically, I implemented a node class with the attribute f_pointers and set it to the default value []. When ever I try to change f_pointers of (lets say) node_a, I will end up changing f_pointers of node_b, which are programmed to be completely unrelated.
I have already solved the problem by instead changing the default value to None and setting up the forward_pointers in __init__. However, I would still like to know how to avoid this problem in the future and possibly learn something new about Python.
For the sake of simplicity, I removed some unnecessary parts of the code.
class Node:
def __init__(self, f_pointers = []):
self.f_pointers = f_pointers
def get_pointers(self):
return self.f_pointers
def add_pointers(self, new_pointer):
self.f_pointers.append(new_pointer)
a = Node()
b = Node()
print(a.get_pointers, b.get_pointers)
>>> [] []
a.add_pointers("a")
print(a.get_pointers, b.get_pointers)
>> ["a"] ["a"]
a.add_pointers("b")
print(a.get_pointers, b.get_pointers)
>> ["a","b"] ["a","b"]
As can be seen, a and b are completely unrelated objects (other than the fact that they are of the same type Node) but will affect each other. Why does this happen?
It's because you are referencing to the same list (the one instantiated in the __init__ default params list definition like __init__(self, f_pointers=[]). What happens is that when you say in the __init__ method code block that self.f_points = f_pointers you are basically referencing the same list every time you instantiate a new Node object.
The reasons are explained further here
What you do want to do instead is instantiate a new list for every init like:
def __init__(self, f_pointers=None):
self.f_pointers = []