python基础（八）

异常
1.1 使用try-except代码块

try:  #如果try代码块中没有问题，python将跳过except代码块
    print(5/0)
except ZeroDivisionError:  #try代码块中引发异常，将执行except代码块
    print("You can't divide by zero!")

结果：

You can't divide by zero!

1.2 使用异常避免崩溃

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) #只有可能引发异常的代码块才需要放在try语句中
    except ZeroDivisionError:
        print("You can't divide by zero!")
    else:
        print(answer)   #else中执行成功的代码块

结果：

image.png

1.3 处理FileNotFoundError异常

file_name='alice.txt'
try:
    with open(file_name) as file_object:
        contends=file_object.read()
except FileNotFoundError:
    msg="Sorry,the file "+file_name+" does not exist."
    print(msg)

结果:

Sorry,the file alice.txt does not exist.

1.4 分析文本

title="Alice in Wonderland"
print(title.split())    #split()根据一个字符串创建一个单词列表

file_name='alice.txt'
try:
    with open(file_name) as file_object:
        contents=file_object.read()
except FileNotFoundError:
    msg="Sorry, the file "+file_name+" does not exist."
    print(msg)
else:
    words=contents.split()  #split()根据一个字符串创建一个单词列表,该方法以空格为分隔符将字符串分拆成多个部分，并将这些部分都存储到一个列表中
    num_words=len(words)
    print("The file "+file_name+" has about "+str(num_words)+" words.")

结果：

The file alice.txt has about 23005 words.

注意：使用文本来自项目：http://www.gutenberg.org/
1.5 使用多个文件

def count_words(filename):
    '''计算一个文件大致包含多少个单词'''
    try:
        with open(file_name) as file_object:
            contents=file_object.read()
    except FileNotFoundError:
        msg="Sorry, the file "+file_name+" does not exist."
        print(msg)
    else:
        words=contents.split()  #split()根据一个字符串创建一个单词列表
        num_words=len(words)
        print("The file "+file_name+" has about "+str(num_words)+" words.")
file_names=['alice.txt','guest.txt','add.txt','reason.txt']
for file_name in file_names:
    count_words(file_name)

结果：

The file alice.txt has about 23005 words.
The file guest.txt has about 7 words.
Sorry, the file add.txt does not exist.
The file reason.txt has about 9 words

1.6 失败时一声不吭

def count_words(filename):
    '''计算一个文件大致包含多少个单词'''
    try:
        with open(file_name) as file_object:
            contents=file_object.read()
    except FileNotFoundError:
        # msg="Sorry, the file "+file_name+" does not exist."
        # print(msg)
        pass    #pass让python什么都不要做，同时充当占位符，可能以后会在这里做些什么
    else:
        words=contents.split()  #split()根据一个字符串创建一个单词列表
        num_words=len(words)
        print("The file "+file_name+" has about "+str(num_words)+" words.")
file_names=['alice.txt','guest.txt','add.txt','reason.txt']
for file_name in file_names:
    count_words(file_name)

结果:

The file alice.txt has about 23005 words.  #找不到文件时直接跳过
The file guest.txt has about 7 words.
The file reason.txt has about 9 words.

2、存储数据
2.1 使用json.dump()和json.load()
2.1.1 使用json.dump()

import json #导入json模块

numbers=[2,3,5,7,11,19]

filename='numbers.json'
with open(filename,'w') as file_object:
    json.dump(numbers,file_object)  #json.dump()函数来存储数字列表，两个实参为要存储的数据及存储数据的文件对象

结果:

numbers.json
[2, 3, 5, 7, 11, 19]

2.1.2 使用json.load()

import json #导入json模块

file_name='numbers.json'
with open(file_name) as f_obj:
    numbers=json.load(f_obj)    #json.load()函数读取内存数据，接受参数为存储数据的文件对象
print(numbers)

结果:

[2, 3, 5, 7, 11, 19, 23]

2.2 保存和读取用户生成的数据

import json
filename='username.json'
try:
    with open(filename) as f_obj:
        username=json.load(f_obj)
except FileNotFoundError:
    username=input("What is your name?")
    with open(filename,'w') as f_obj:
        json.dump(username,f_obj)
        print("We'll remember you when you come back, "+username+"!")
else:
    print("Welcome back, "+username+"!")

结果：

Welcome back, wangwu!

2.3 重构

import json
def get_stored_username():
    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():
    '''提示用户输入用户名'''
    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():    #直接调用其他的功能函数
    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()

根据执行功能分别定义函数，使代码清晰而易于维护。