图书管理系统借书还书Python程序

以下是一个简单的图书管理系统借书还书Python程序，它可以实现借书和还书的功能：

import sqlite3

# 连接到数据库
conn = sqlite3.connect('library.db')

# 借书函数
def borrow_book():
    book_id = input("请输入要借阅的书籍ID：")
    user_id = input("请输入您的用户ID：")
    cursor = conn.cursor()
    # 查询书籍是否存在
    cursor.execute("SELECT * FROM books WHERE id=?", (book_id,))
    book = cursor.fetchone()
    if book is None:
        print("该书籍不存在！")
        return
    # 查询用户是否存在
    cursor.execute("SELECT * FROM users WHERE id=?", (user_id,))
    user = cursor.fetchone()
    if user is None:
        print("该用户不存在！")
        return
    # 查询用户是否已经借阅该书籍
    cursor.execute("SELECT * FROM user_book WHERE user_id=? AND book_id=?", (user_id, book_id))
    user_book = cursor.fetchone()
    if user_book is not None:
        print("您已经借阅了该书籍！")
        return
    # 更新用户借阅信息
    cursor.execute("INSERT INTO user_book (user_id, book_id) VALUES (?, ?)", (user_id, book_id))
    # 更新书籍库存信息
    cursor.execute("UPDATE books SET stock=stock-1 WHERE id=?", (book_id,))
    conn.commit()
    print("借阅成功！")

# 还书函数
def return_book():
    book_id = input("请输入要归还的书籍ID：")
    user_id = input("请输入您的用户ID：")
    cursor = conn.cursor()
    # 查询书籍是否存在
    cursor.execute("SELECT * FROM books WHERE id=?", (book_id,))
    book = cursor.fetchone()
    if book is None:
        print("该书籍不存在！")
        return
    # 查询用户是否存在
    cursor.execute("SELECT * FROM users WHERE id=?", (user_id,))
    user = cursor.fetchone()
    if user is None:
        print("该用户不存在！")
        return
    # 查询用户是否已经借阅该书籍
    cursor.execute("SELECT * FROM user_book WHERE user_id=? AND book_id=?", (user_id, book_id))
    user_book = cursor.fetchone()
    if user_book is None:
        print("您没有借阅该书籍！")
        return
    # 更新用户借阅信息
    cursor.execute("DELETE FROM user_book WHERE user_id=? AND book_id=?", (user_id, book_id))
    # 更新书籍库存信息
    cursor.execute("UPDATE books SET stock=stock+1 WHERE id=?", (book_id,))
    conn.commit()
    print("归还成功！")

# 主函数
def main():
    while True:
        print("欢迎使用图书管理系统！")
        print("1. 借书")
        print("2. 还书")
        print("3. 退出")
        choice = input("请选择操作：")
        if choice == '1':
            borrow_book()
        elif choice == '2':
            return_book()
        elif choice == '3':
            break
        else:
            print("无效的操作！")

    # 关闭数据库连接
    conn.close()

if __name__ == '__main__':
    main()