


An article to help you understand the introduction and case analysis of Python exceptions
1. Introduction to exceptions
When Python detects an error, the interpreter cannot continue to execute. Instead, some error prompts appear. This is what is called "abnormal".
2. Case Analysis
Open a non-existent file 123 .txt, when the 123.txt file cannot be found, an IOError type error will be thrown to us, No such file or directory: 123.txt (there is no such file or directory as 123.txt).
print'-----test--1---' open('123.txt','r') print '-----test--2---'
Run result:
## Catching exception try...except...
Example:
try: print('-----test--1---') open('123.txt','r') print('-----test--2---') except IOError: pass
Running result:
Description:
This program cannot be viewed Any error is detected because the IOError exception is captured using except and a method for handling it is added.
pass 表示实现了相应的实现,但什么也不做;如果把pass改为print语句,那么就会输出其他信息。
小总结 :
把可能出现问题的代码,放在try中。
把处理异常的代码,放在except中。
except捕获多个异常
例 :
try: print(num) except IOError: print('产生错误了')
运行结果 :
想一想:
上例程序,已经使用except来捕获异常了,为什么还会看到错误的信息提示?
答:
except捕获的错误类型是IOError,而此时程序产生的异常为 NameError ,所以except没有生效。
修改后的代码为 :
try: print(num) except NameError: print('产生错误了')
运行结果
实际开发中,捕获多个异常的方式,如下:
#coding=utf-8 try: print('-----test--1---') open('123.txt','r') # 如果123.txt文件不存在,那么会产生 IOError 异常 print('-----test--2---') print(num)# 如果num变量没有定义,那么会产生 NameError 异常 except (IOError,NameError): #如果想通过一次except捕获到多个异常可以用一个元组的方式
注意:
当捕获多个异常时,可以把要捕获的异常的名字,放到except 后,并使用元组的方式仅进行存储。
获取异常的信息描述
try: open("a.txt") except(NameError,IOError) as result: print("捕抓到异常") print("信息展示:",result)
运行结果:
捕获所有异常
try: open("a.txt") except Exception as result: print("捕抓到异常") print("信息展示:",result)
运行结果:
else
咱们应该对else
并不陌生,在if中,它的作用是当条件不满足时执行的实行;同样在try...except...中也是如此,即如果没有捕获到异常,那么就执行else中的事情。
try: num = 100 print(num) except NameError as errorMsg: print('产生错误了:%s' % errorMsg) else: print('没有捕获到异常,真高兴')
运行结果如下:
try...finally...
try...finally...语句用来表达这样的情况:
在程序中,如果一个段代码必须要执行,即无论异常是否产生都要执行,那么此时就需要使用finally。比如文件关闭,释放锁,把数据库连接返还给连接池等。
demo:
import time try: f = open('test.txt') try: while True: content = f.readline() if len(content) == 0: break time.sleep(2) print(content) except: #如果在读取文件的过程中,产生了异常,那么就会捕获到 #比如 按下了 ctrl+c pass finally: f.close() print('关闭文件') except: print("没有这个文件")
运行结果:
说明:
test.txt文件中每一行数据打印,但是我有意在每打印一行之前用time.sleep方法暂停2秒钟。这样做的原因是让程序运行得慢一些。在程序运行的时候,按Ctrl+c中断(取消)程序。
我们可以观察到KeyboardInterrupt异常被触发,程序退出。但是在程序退出之前,finally从句仍然被执行,把文件关闭。
三、总结
本文讲解了Python基础之异常知识 ,介绍了常见的异常操作,用丰富的案例,帮助大家更好的去了解异常常见的的基础操作。
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