How do you do a level-order (BFS) traversal of a tree?

from collections import deque

class TreeNode:
    def __init__(self, val, left=None, right=None):
        self.val, self.left, self.right = val, left, right

def level_order(root):
    if not root:
        return []
    levels, q = [], deque([root])
    while q:
        size = len(q)
        level = []
        for _ in range(size):          # one full level
            node = q.popleft()
            level.append(node.val)
            if node.left:  q.append(node.left)
            if node.right: q.append(node.right)
        levels.append(level)
    return levels


# --- demo ---  3 / (9) , (20 / 15,7)
root = TreeNode(3, TreeNode(9), TreeNode(20, TreeNode(15), TreeNode(7)))
print(level_order(root))   # [[3], [9, 20], [15, 7]]