文件操作，应用登陆系统，注释标准，冒泡排序原理

# 写，末尾追加，指针到最后
# +表示同时可以读写，b表示已字节的方式操作
f = open("h.txt","r+")# r+最前，a+最后
# w+,先清空，在写之后，可以读，x+和w+一样，x+额外有如果文件存在，报错
f.seek(0)
# 指针回到初始
print(f.tell())#获取指针位置
d = f.read(1)
print(d)
f.write("")#写到内存，不关闭不写入
f.flush()#强行执行，写到硬盘。
f.truncate()#截断数据，仅保留指针之前
f.readline()# 只读取一行，for line in f :line也是一行一行读，读取所有行
f.close()
# with2.7之后支持同时打开两个文件
with open("h.txt","r") as obj1 ,open("new_h.txt","w") as obj2:
    for line in obj1:
        # 实现大文件（内存不足），一行一行写到新文件中，
        obj2.write(line)
# 没有关闭文件之前，指针保持不动
def x():
    # x代指函数，x()执行函数，函数可以做参数
    pass
"""
def login(username, password):
    """
    用于用户名和密码的验证
    :param username: 用户名
    :param password: 密码
    :return: True正确，False错误
    """
    with open("h.txt","r",encoding="utf-8") as f:
        for line in f:
            # strip默认无参数，移除空格，换行符。有参数则移除两侧的值
            line = line.strip()
            line_list = line.split("$")
            if username == line_list[0] and password ==line_list[1]:
                return True
        return False
def register(username,password):
    """
    注册
    :param username:用户名
    :param password:密码
    :return:True成功
    """
    with open("h.txt","a",encoding="utf-8") as f:
        f.write("
"+username+"$"+password)
    return True
def user_exist(username):
    """
    检查用户名是否存在
    :param username: 用户名
    :return: True存在，False不存在
    """
    with open("h.txt","r",encoding="utf-8") as f:
    # 无论以任何方式结束with语句，python都会自动关闭文件.(执行__exit__,close)
        for line in f:
            # 字符串操作是赋值，原字符串不改变
            line = line.strip()
            line_list = line.split("$")
            if username == line_list[0]:
                return True
    return False
def del_user(username):
    """
    删除用户
    :param username: 用户名
    :return: True
    """
    with open("h.txt","r+",encoding="utf-8") as f1,open("new_h.txt","w+",encoding="utf-8") as f2:
        for line in f1:
            line = line.strip()
            line_list = line.split("$")
            if line_list[0] != username:
                f2.write(line+"
")
    with open("h.txt","w+",encoding="utf-8") as f1,open("new_h.txt","r+",encoding="utf-8") as f2:
        for line2 in f2:
            f1.write(line2)
    return True
def revise_pwd(username,newpwd):
    """
    修改密码
    :param username: 用户名
    :param newpwd: 新密码
    :return: True
    """
    with open("h.txt","r+",encoding="utf-8") as f1,open("new_h.txt","w+",encoding="utf-8") as f2:
        for line in f1:
            line = line.strip()
            line_list = line.split("$")
            if line_list[0] == username:
                line = line.replace(line_list[1],newpwd)
                f2.write(line+"
")
            else:
                f2.write(line + "
")
    with open("h.txt","w+",encoding="utf-8") as f1,open("new_h.txt","r+",encoding="utf-8") as f2:
        for line2 in f2:
            f1.write(line2)
    return True
def main():
    print("欢迎登陆xx系统")
    inp = input("请输入（1:登陆;2:注册;3:删除用户;4:修改密码）：")
    if inp == "1":
        user = input("用户名：")
        pwd = input("密码：")
        is_login = login(user,pwd)
        if is_login :
            print("登陆成功")
        else:
            print("登陆失败")
    elif inp == "2":
        user = input("用户名：")
        is_exist = user_exist(user)
        if is_exist:
            print("用户名存在")
        else:
            pwd = input("密码：")
            result = register(user,pwd)
            if result:
                print("注册成功")
            else:
                print("注册失败")
    elif inp == "3":
        user = input("用户名：")
        pwd = input("密码：")
        is_login = login(user,pwd)
        if is_login:
            del_user(user)
            print("删除成功")
        else:
            print("账号密码错误")
    elif inp == "4":
        user = input("用户名：")
        pwd = input("密码：")
        is_login = login(user,pwd)
        if is_login:
            new_pwd = input("新密码：")
            is_revise = revise_pwd(user,new_pwd)
            if is_revise:
                print("修改成功")
        else:
            print("账号密码错误")
main()
# 冒泡排序
li = [11,22,33,44]
temp = li[1]
li[1] = li[2]
li[2] = temp
print(li)
li = [4,77,44,6,1]
for j in range(1,len(li)):
    #len(li)=5
    #j=1,2,3,4
    for i in range(len(li)-j):
        #i = 0,1,2,3
        # current = li[i]
        # next_values = li[i+1]
        if li[i] > li[i + 1]:
            temp = li[i]
            li[i] = li[i + 1]
            li[i + 1] = temp
print(li)