Python - append list to other list?

Question

Leetcode 113 - Path Sum II

Given the root of a binary tree and an integer targetSum, return all root-to-leaf paths where each path's sum equals targetSum.

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right

class Solution:
    def pathSum(self, root: TreeNode, targetSum: int) -> List[List[int]]:
        if not root: return []
        
        res = []
        
        def dfs(node, total = 0, path = []):
            if not node: return 
            
            path.append(node.val)
            total += node.val
            if not node.left and not node.right and total == targetSum:
                res.append(list(path))
            else:
                dfs(node.left, total, path)
                dfs(node.right, total, path)
            total -= path.pop()
        
        dfs(root)
        return res

Why we have to use res.append(list(path)) rather than use res.append(path) ?

If we only append path into res, it would only append empty list into res .

Answer 1

This is a subtle Python issue. When you use a default parameter like that (in the def dfs line), it captures that default object at definition time. So, every time that function runs, path is set to the SAME empty list. If you change that list, everyone with a reference to it sees the change. I believe they use list(path) to make a copy of that list, rather than store a link to the common list.