[song] rebirth of me in py introductory training (9): exception handling

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The first 1 Turn off ：Python Exception class and custom exception

Task description

Skilled use of exception handling mechanism plays an important role in improving the robustness and fault tolerance of programs , At the same time, you can put Python Obscure error prompts are converted into friendly prompts and displayed to end users . The task of this chapter is to let readers deeply understand the meaning of exceptions , And master the method of customizing exceptions .
#### Related knowledge
##### The basic concept of exceptions
What on earth is abnormal ？ The following example can enlighten you ：

1. x, y = 12, 5
2. a = x / y
3. print(A) # Spelling mistakes ,Python Case sensitive , Variables are not defined A

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Programming requirements

Customize as required MyError Exception class , Properly MyError When an exception occurs , The execution evaluation code can output the following information .
Test input ：
4
Expected output ：
This is my definition of the second 4 Exceptions

# -*- coding: utf-8 -*-

class MyError(Exception):
    #********begin*********#
    def __init__(self,value):  
        self.value = value  
    def __str__(self):  
        return ' This is my definition of the second %d Exceptions '%(self.value)  
    
    
    
    #******** end*********#

The first 2 Turn off ：Python Exception handling structure in

Task description

The task of this level is to let students learn how to apply exception handling methods in the program , master Python Exception handling structure in .
#### Related knowledge
#####try...except... structure
The most common and basic exception handling structure is try...except... structure .
try The code block in the clause contains statements that may have exceptions , and except Clause is used to catch the corresponding exception ,except The code block in the clause is used to handle exceptions .

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Programming requirements

Complete the code on the right as required . To achieve the following functions ：

• When the length of the test input string is greater than 3 Throw when MyError abnormal , And the output length is too long , Greater than 3 Otherwise, the output length is appropriate .
• And whether or not an exception is thrown , After the execution is completed, output the execution completion .

       #*********begin*********#
      try:  
          if(x>3):  
              raise MyError()  
      except MyError as e:  
          print(e)  
      else:  
          print(' The length is right ')  
      finally:  
          print(' completion of enforcement ') 
      #********* end*********#

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