高级编程技术（Python）作业10

10-2 C语言学习笔记：可使用方法replace()将字符串中的特定单词都替换为另一个单词。下面是一个简单的示例，演示了如何将句子中的’dog’ 替换为’cat’：

>>> message = "I really like dogs."
>>> message.replace('dog', 'cat')
'I really like cats.'

读取你刚创建的文件learning_python.txt中的每一行，将其中的Python都替换为另一门语言的名称，如C。将修改后的各行都打印到屏幕上。

Solution：

filename = 'learning_python.txt'

with open(filename) as file_object:
    contents = file_object.read()
    message = contents.replace('Python', 'C')
    print(message)

Output:

In C you can open a file.
In C you can close a file.

10-7 加法计算器：将你为完成练习10-6而编写的代码放在一个while循环中，让用户犯错（输入的是文本而不是数字）后能够继续输入数字。

Solution：

while(1):
    print("Please enter 2 numbers, and I will add them.")
    print("Enter q to quit.")

    first_number = input("\n First number: ")
    if first_number == 'q':
        break
    second_number = input("\n Second number: ")
    if second_number == 'q':
        break

    try:
        answer = int(first_number) + int(second_number)
    except ValueError:
        print("You should input numbers!")
    else:
        print(answer)

Output:

Please enter 2 numbers, and I will add them.
Enter q to quit.

First number: dsfsa

Second number: sadfsda
You should input numbers!

Please enter 2 numbers, and I will add them.
Enter q to quit.

First number: 1

Second number: 1
2
Please enter 2 numbers, and I will add them.
Enter q to quit.

First number: quit

Second number: q

10-13 验证用户：最后一个remember_me.py版本假设用户要么已输入其用户名，要么是首次运行该程序。我们应修改这个程序，以应对这样的情形：当前和最后一次运行该程序的用户并非同一个人。
为此，在greet_user() 中打印欢迎用户回来的消息前，先询问他用户名是否是对的。如果不对，就调用get_new_username()让用户输入正确的用户名。

Solution：

import json

def get_stored_username():
    """Get stored username if available."""
    filename = 'username.json'
    try:
        with open(filename) as f_obj:
            username = json.load(f_obj)
    except FileNotFoundError:
        return None
    else:
        return username

def get_new_username():
    """Prompt for a new username."""
    username = input("What is your name? ")
    filename = 'username.json'
    with open(filename, 'w') as f_obj:
        json.dump(username, f_obj)
    return username

def greet_user():
    """Greet the user by name."""
    username = get_stored_username()
    if username:
        response = input("Is " + username + " your name? Y/N ")
        if response == 'Y':
            print("Welcome back, " + username + "!")
        elif response == 'N':
            username = get_new_username()
            print("We'll remember you when you come back, " + username + "!")
        else:
            print("Wrong input!")
    else:
        username = get_new_username()
        print("We'll remember you when you come back, " + username + "!")

greet_user()

Output of the first time：

What is your name? ashero
We'll remember you when you come back, ashero!

Output of the second time：

Is ashero your name? Y/N N
What is your name? faker
We'll remember you when you come back, faker!

Output of the third time：

Is faker your name? Y/N Y
Welcome back, faker!