Binary tree level order (Python)

level_order_traversal.py

from collections import deque
class Solution:
    def levelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        levels = []
        if not root:
            return levels
        
        level = 0
        queue = deque([root,])
        while queue:
            # start the current level
            levels.append([])
            # number of elements in the current level 
            level_length = len(queue)
            
            for i in range(level_length):
                node = queue.popleft()
                # fulfill the current level
                levels[level].append(node.val)
                
                # add child nodes of the current level
                # in the queue for the next level
                if node.left:
                    queue.append(node.left)
                if node.right:
                    queue.append(node.right)
            
            # go to next level
            level += 1
        
        return levels