python编程 从入门到实践6--文件和异常

从文件中读取数据

读取圆周率文件pi_digits.txt

with open('pi_digits.txt') as file_object:
    contents = file_object.read()#读取整个文件
    print(contents.rstrip())

with open('pi_digits.txt') as file_object:
    for line in file_object:#逐行读取
        print(line.rstrip())

with open('pi_digits.txt') as file_object:
    lines = file_object.readlines()
#创建一个包含文件各行内容的列表
for line in lines:
    print(line.rstrip())

结果相同：

3.1415926535
  8979323846
  2643383279

使用文件内容

with open('pi_digits.txt') as file_object:
    lines = file_object.readlines()
    
pi_string = ''
for line in lines:
    pi_string += line.strip()
print(pi_string)
print(len(pi_string))

line.strip()删除左右两边的空格

3.141592653589793238462643383279
32

写入文件

with open('programming.txt','w') as file_object1:
    file_object1.write('I love you\n')
    file_object1.write('mylove\n')

打开文件时open(),三个模式：读取模式’r’ 写入模式’w’ 附加模式’a’

附加模式处理文件不会删除覆盖i原有内容，而是再文件末尾添加你写入文件的行。

异常

try-except-else代码块工作原理如下:
Python尝试执行try代码块中的代码（只有可能引发异常的代码才需要放try语句中）
except代码块告诉python，如果它尝试运行try代码块中的代码时引发指定异常如何操作。
else代码块中存放仅在try代码块成功执行时才需要运行的代码。

print("Give me two numbers, and I'll divide them.")
print("Enter 'q' to quit.")

while True:
    first_number = input("\nFirst number: ")
    if first_number == 'q':
        break
    second_number = input("Second number: ")
    try:
        answer = int(first_number) / int(second_number)
    except ZeroDivisionError:
        print("You can't divide by 0!")
    else:
        print(answer)

Give me two numbers, and I'll divide them.
Enter 'q' to quit.

First number: 5
Second number: 0
You can't divide by 0!

First number: q

如果不加else：

print("Give me two numbers, and I'll divide them.")
print("Enter 'q' to quit.")

while True:
    first_number = input("\nFirst number: ")
    if first_number == 'q':
        break
    second_number = input("Second number: ")
    try:
        answer = int(first_number) / int(second_number)
    except ZeroDivisionError:
        print("You can't divide by 0!")
        print(answer)

同样输入5 和0
会报错：

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "D:/code/python code/PythonCrashCourse/资源/chapter_10/division.py", line 14, in <module>
    print(answer)
NameError: name 'answer' is not defined

因为不合法，所以answer变量不存在，所以无法打出来。

pass

Python中pass的用法：
1、空语句 do nothing
2、保证格式完整
3、保证语义完整
4、在失败时一声不吭（由你决定什么时候向用户报什么错误）

存储数据

json.dump() 2个参数 ：要存储的数据 及 可用于存储数据的文件对象
json.load() 1个参数： 文件对象
例：第一次使用。询问用户名，存入文件。再次登陆，直接呈现欢迎语句。

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:
        print("Welcome back, " + username + "!")
    else:
        username = get_new_username()
        print("We'll remember you when you come back, " + username + "!")

greet_user()

利用了重构的思路：将代码划分为一系列完成具体工作的函数。

扩展例子：在打印并欢迎用户回来的消息前，先询问她用户名是否是对的，如果不对，输入正确用户名。

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(username):
    """Prompt for a new username."""
    print("There're nobody in json,you're the one.")
    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 = input("What is your name? ")
    storedname = get_stored_username()
    if storedname:
        while(username != storedname):
            username = input("no,What is your name? ")
        print("Welcome back, " + username+ "!")
    else:
        storedname = get_new_username(username)
        print("We'll remember you when you come back," + storedname+"!")

greet_user()

第一次输入：

What is your name? l
There're nobody in json,you're the one.
We'll remember you when you come back,l!

后面输入：

What is your name? pp
no,What is your name? oo
no,What is your name? kk
no,What is your name? l
Welcome back, l!