python Exception Handling try Example Summary

Exception handling

Brief introduction

When we write program code, we often hang up the whole program because of a small problem.

What are the benefits of exception handling for our tests, I believe that students who have done automation know that our use cases are executed one by one. For example, there are hundreds of use cases in our programs. If one use case causes a program exception due to data, then all the rest will stop working. In this case, we should throw out the problem caused by this data and handle this exception.

We can use tryexcept to handle exceptions.

Structure

Put the statements that may report errors in try:, and then use except to handle exceptions. Every try must have at least 1 except.

If an error statement may occur, we can know which exceptions to report. After except, we need to keep up with the exception name

You can also use universal exception Exception, which can catch any exception

All the standard exception classes for python: See the end of the article

Example 1: Handling the specified error exception type

If we know what errors will be reported, we can directly catch and handle them, but if the name of the caught exception is incorrect, the program will also report errors.

First, we print an error of not declaring/initializing an object (no attribute), and then print another data that can be executed normally

print(aa)
print('bb')

Print results

Traceback (most recent call last):
File "C:\ Users\ Zhang Tianci\ PycharmProjects\ pythonProject\ test\ lianxi\ 111. py", line 6, in < module >
print(a)
NameError: name 'a' is not defined

Obviously print ("bb") is not executed, so we add try to the part where we may or are known to report an error: and specify NameError as to f variables with except, and print out the error

try:
    print(aa)
except NameError as f:
    print(f)
print('bb')

Print results

name 'aa' is not defined
bb

As you can see, the program prints out the error message and executes print ("bb")

What if I open a wrong file? I don't know the name of the file opening error. What should I do at this time?

try:
    open('ztc.txt','r')
except NameError as f:
    print(f)
print('bb')

Print results:

Actually, it is an error file. I can't find the exception name correctly with Nameerror

Traceback (most recent call last):
  File "C:\Users\ Zhang Tianci \PycharmProjects\pythonProject\test\lianxi\111.py", line 2, in <module>
    open('ztc.txt','r')
FileNotFoundError: [Errno 2] No such file or directory: 'ztc.txt'

See Example 2 when you encounter such a situation

Example 2: Universal exception handling

In Example 1, we said that when we can determine what type of error will be reported, we can specify this error type to handle it. Of course, in many cases, we will encounter many unknown exceptions, and it is impossible to predict all exceptions, so we can directly use the universal exception Exception

try:
    open('ztc.txt','r')
except Exception as f:
    print(f)
print('bb')

Print results:

exception can handle any error type exception in our try

[Errno 2] No such file or directory: 'ztc.txt'
bb

Example 3: try... finally...

try... finally... means that if our program has encountered an error, the code must be executed

What scenes can be used?

For example, if we link to the database, if we want to operate the database, if the program reports an error or does not report an error, we must close the database
For example, if we open an excel or txt document and want to write data, we must close this document regardless of whether an error is reported or not

First look at a normal execution situation

try:
    f = open('ztc.json','r')

finally:
    f.close()
    print(' Has been closed ')

Print results

Has been closed

If you open the file and report an error before the file is closed,

try:
    f = open('ztc.json','r')
    print(aaa)

finally:
    f.close()
    print(' Has been closed ')

Print results

Traceback (most recent call last):
File "C:\ Users\ Zhang Tianci\ PycharmProjects\ pythonProject\ test\ lianxi\ 111. py", line 3, in < module >
print(aaa)
NameError: name 'aaa' is not defined
Has been closed

Look at the above code, obviously the program printed print (aaa) when the error, but will still open the file to close

Appendix:

异常名称	描述
BaseException	所有异常的基类
SystemExit	解释器请求退出
KeyboardInterrupt	用户中断执行(通常是输入^C)
Exception	常规错误的基类
StopIteration	迭代器没有更多的值
GeneratorExit	生成器(generator)发生异常来通知退出
SystemExit	Python 解释器请求退出
StandardError	所有的内建标准异常的基类
ArithmeticError	所有数值计算错误的基类
FloatingPointError	浮点计算错误
OverflowError	数值运算超出最大限制
ZeroDivisionError	除(或取模)零 (所有数据类型)
AssertionError	断言语句失败
AttributeError	对象没有这个属性
EOFError	没有内建输入,到达EOF 标记
EnvironmentError	操作系统错误的基类
IOError	输入/输出操作失败
OSError	操作系统错误
WindowsError	系统调用失败
ImportError	导入模块/对象失败
KeyboardInterrupt	用户中断执行(通常是输入^C)
LookupError	无效数据查询的基类
IndexError	序列中没有没有此索引(index)
KeyError	映射中没有这个键
MemoryError	内存溢出错误(对于Python 解释器不是致命的)
NameError	未声明/初始化对象 (没有属性)
UnboundLocalError	访问未初始化的本地变量
ReferenceError	弱引用(Weak reference)试图访问已经垃圾回收了的对象
RuntimeError	1般的运行时错误
NotImplementedError	尚未实现的方法
SyntaxError	Python 语法错误
IndentationError	缩进错误
TabError	Tab 和空格混用
SystemError	1般的解释器系统错误
TypeError	对类型无效的操作
ValueError	传入无效的参数
UnicodeError	Unicode 相关的错误
UnicodeDecodeError	Unicode 解码时的错误
UnicodeEncodeError	Unicode 编码时错误
UnicodeTranslateError	Unicode 转换时错误
Warning	警告的基类
DeprecationWarning	关于被弃用的特征的警告
FutureWarning	关于构造将来语义会有改变的警告
OverflowWarning	旧的关于自动提升为长整型(long)的警告
PendingDeprecationWarning	关于特性将会被废弃的警告
RuntimeWarning	可疑的运行时行为(runtime behavior)的警告
SyntaxWarning	可疑的语法的警告
UserWarning	用户代码生成的警告