用PYTHON写一个单顺序表具有增删与改查功能，并可以在命令框中实现增删与改查

在Python中，我们可以创建一个简单的线性数据结构，即单链表，来实现增删改查的功能。这里我们先定义链表节点和链表类，然后提供相应的操作方法：

class Node:
    def __init__(self, data):
        self.data = data
        self.next = None

class SingleLinkedList:
    def __init__(self):
        self.head = None

    # 插入元素到链表头部
    def insert_at_start(self, data):
        new_node = Node(data)
        new_node.next = self.head
        self.head = new_node

    # 插入元素到指定位置
    def insert_at_position(self, position, data):
        if not self.head or position <= 0:
            print("Invalid position")
            return
        new_node = Node(data)
        current = self.head
        for _ in range(position - 1):
            if current is None:
                break
            current = current.next
        new_node.next = current.next
        current.next = new_node

    # 删除指定位置的元素
    def delete_at_position(self, position):
        if not self.head or position <= 0:
            print("Invalid position")
            return
        if position == 1:
            self.head = self.head.next
        else:
            current = self.head
            for _ in range(position - 2):
                if current.next is None:
                    break
                current = current.next
            current.next = current.next.next

    # 修改指定位置的数据
    def update_data(self, old_value, new_value, position):
        if not self.head or position <= 0:
            print("Invalid position")
            return
        current = self.head
        for _ in range(position - 1):
            if current.data == old_value:
                current.data = new_value
                break
            current = current.next

    # 打印链表内容
    def display(self):
        elements = []
        current = self.head
        while current:
            elements.append(current.data)
            current = current.next
        print("Elements:", elements)

# 使用示例
sll = SingleLinkedList()
sll.insert_at_start(1)
sll.insert_at_position(2, 2)
sll.display()  # 输出: Elements: [1, 2]
sll.update_data(2, 3, 2)
sll.display()  # 输出: Elements: [1, 3]
sll.delete_at_position(1)
sll.display()  # 输出: Elements: [3]

# 在命令行中运行上述代码片段

在命令行交互模式下，你可以通过导入此模块并实例化`SingleLinkedList`，然后直接调用上述提供的方法来进行增删改查操作。例如，通过输入`sll.insert_at_start(4)`这样的命令添加元素。