使用python语言完成如下操作：我将会输入依次删除一个二叉树各叶节点的删除顺序，请你输出所有叶节点被删除前该二叉树的先序遍历

以下是使用 Python 语言实现的代码：

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

def preorder_traversal(root):
    if root is None:
        return []
    return [root.val] + preorder_traversal(root.left) + preorder_traversal(root.right)

def delete_leaves(root, sequence):
    if root is None:
        return None
    if root.left is None and root.right is None:
        sequence.append(root.val)
        return None
    root.left = delete_leaves(root.left, sequence)
    root.right = delete_leaves(root.right, sequence)
    return root

if __name__ == '__main__':
    # 构造二叉树
    root = TreeNode(1)
    root.left = TreeNode(2)
    root.right = TreeNode(3)
    root.left.left = TreeNode(4)
    root.left.right = TreeNode(5)
    root.right.left = TreeNode(6)
    root.right.right = TreeNode(7)

    # 依次删除叶节点并输出先序遍历
    for i in range(7):
        sequence = []
        root = delete_leaves(root, sequence)
        print(preorder_traversal(root))

使用上述代码，我们可以构造一个二叉树并依次删除所有叶节点，同时输出每次删除前的先序遍历结果。