How do I access class variables without changing them in python?

Question

I'm new to programming so sorry for the basic question. I am trying to write a search algorithm for a class, and I thought creating a class for each search node would be helpful.

class Node(object):

    def __init__(self, path_to_node, search_depth, current_state):
        self.path_to_node = path_to_node
        self.search_depth = search_depth
        self.current_state = current_state

    ...

With some functions too. I am now trying to define a function outside of the class to create children nodes of a node and add them to a queue. node.current_state is a list

def bfs_expand(node, queuey, test_states):
    # Node Queue List -> Queue List
    # If legal move and not already in test states create and put children nodes
    # into the queue and their state into test_states. Return queue and test states

    # Copy original path, depth, and state to separate variables
    original_path = node.path_to_node
    original_depth = node.search_depth
    original_state = node.current_state

    # Check if up is legal, if so add new node to queue and state to test state
    if node.is_legal_move('Up'):
        up_state = original_state
        a = up_state.index(0)
        b = a - 3
        up_state[a], up_state[b] = up_state[b], up_state[a]
        if up_state not in test_states:
            test_states.append(up_state)
            up_node = Node(original_path + ['Up'], original_depth + 1, up_state)
            queuey.put(up_node)

    print(test_states)
    print(original_state)

I then try to proceed through down, left and right with similar if statements, but they are messed up because the original_state has changed. When I print the original state after that up statement, it returns the up_state created in the if statement. I realize (well, I think) that this is because original_state, and therefore up_state, are actually calling node.current_state and do not store the list in a separate variable. How should I get the variable from a node to manipulate independently? Should I not even be using a class for something like this, maybe a dictionary? I don't need code written for me but a conceptual nudge would be greatly appreciated!

Must read: https://nedbatchelder.com/text/names.html – chepner May 10 '17 at 16:06 — chepner, May 10 '17 at 16:06
@chepner Thanks very helpful – user7953599 May 10 '17 at 17:13 — user7953599, May 10 '17 at 17:13

score 1 · Accepted Answer · answered May 10 '17 at 15:47

You should use copy.deepcopy if you want to avoid modifying the original

original_path = copy.deepcopy(node.path_to_node)
original_depth = copy.deepcopy(node.search_depth)
original_state = copy.deepcopy(node.current_state)

Or essentially whichever object you want to use as a "working copy" should be a deep copy of the original if you don't want to modify the original version of it.

score 1 · Answer 2 · edited May 23 '17 at 10:31

Expanding a bit on @CoryKramer's answer: In Python, objects have reference semantics, which means that saying

a = b

where a and b are objects, makes both a and b references to the same object, meaning that changing a property on a will change that same property on b as well. In order to actually get a new object with the same properties as the old one, you should use copy.deepcopy as already stated. However, be careful when using that function. If your object contains a reference cycle (i.e.: It contains a reference to an object which contains a reference to itself), copy.deepcopy will lead to an infinite loop.

For this reason, there is also copy.copy, which does not follow object references contained in the object to copy.

How do I access class variables without changing them in python?

2 Answers2