#coding=gbk

#Python编程从入门到实践笔记——异常和存储数据

#10.3异常

#Python使用被称为异常的特殊对象来管理程序执行期间发生的错误。每当发生让Python不知所措的错误时，它都会创建一个异常对象。

#如果编写了处理该异常的代码，程序将继续运行；如果你未对异常进行处理，程序将停止，并显示一个traceback，其中包含有关异常的报告。

#异常是使用try-except代码块处理的。try-except 代码块让Python执行指定的操作，同时告诉Python发生异常时怎么办。

#使用了try-except代码块时，即便出现异常，程序也将继续运行：显示你编写的友好的错误消息，而不是令用户迷惑的traceback。

#1.处理ZeroDivisionError异常

#print(5/0)

#2.使用try-except 代码块

try:

    print(5/0)

except ZeroDivisionError:

    print("You can't devide by zero.")

#3.使用异常避免崩溃

#4.else 代码块

#依赖于try 代码块成功执行的代码都应放到else 代码块中：

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)

#5.处理FileNotFoundError异常

filename = 'alice.txt'

try:

    with open(filename) as f_obj:

        contents = f_obj.read()

except FileNotFoundError:

    msg = "Sorry, the file " + filename + " does not exist."

    print(msg)

#6.分析文本

filename = 'alice.txt'

try:

    with open(filename) as f_obj:

        contents = f_obj.read()

except FileNotFoundError:

    msg = "Sorry, the file " + filename + " does not exist."

    print(msg)

else:

    # 计算文件大致包含多少个单词

    words = contents.split()

    num_words = len(words)

    print("The file " + filename + " has about " + str(num_words) + " words.")

#7.使用多个文件

def count_words(filename):

    """--snip--"""

filenames = ['alice.txt', 'siddhartha.txt', 'moby_dick.txt', 'little_women.txt']

for filename in filenames:

    count_words(filename)

#8.失败时一声不吭 pass语句

def count_words(filename):

    """计算一个文件大致包含多少个单词"""

    try:

        """--snip--"""

    except FileNotFoundError:

        pass

    else:

        """--snip--"""

#10.4存储数据

#1.使用json.dump() 和json.load()

import json

numbers = [2, 3, 5, 7, 11, 13]

filename = 'numbers.json'

with open(filename, 'w') as f_obj:

    json.dump(numbers, f_obj)

#2.保存和读取用户生成的数据

import json

username = input("What is your name? ")

filename = 'username.json'

with open(filename, 'w') as f_obj:

    json.dump(username, f_obj)

    print("We'll remember you when you come back, " + username + "!")

with open(filename) as f_obj:

    username = json.load(f_obj)

    print("Welcome back, " + username + "!")