Python-Exception Handling

One, understand the exception###

• An exception is an event that will occur during program execution and affect the normal execution of the program.
• Under normal circumstances, an exception occurs when Python cannot handle the program normally.
• The exception is a Python object that represents an error.
• When an exception occurs in a Python script, we need to catch and handle it, otherwise the program will terminate execution.

Two, Python exception classification###

Three, exception handling

grammar

try:
  Code where the error may occur
except: 
  Code executed if an exception occurs

• Catch exception
• grammar

note:

• try: the code where the error may occur except the exception type: the code executed if the exception type is caught

1. If the exception type of the code you are trying to execute is inconsistent with the exception type to be caught, the exception cannot be caught
2. Generally, only one line of code to be executed is placed under try

• Catch multiple specified exceptions
  When catching multiple exceptions, you can put the name of the exception type to be caught after except, and write it in a tuple.

• try:. print(1/0) except (NameError, ZeroDivisionError): print('There is an error')

• Capture exception description information

• try: print(num) except (NameError, ZeroDivisionError) as result: print(result)

• Catch all exceptions
  Exception is the parent class of all program exception classes.

• try: print(num) except Exception as result: print(result)

• Abnormal else
  Else means the code to be executed if there is no exception

• try: print(1) except Exception as result: print(result) else: print('I am else, the code executed when there is no exception')

• Abnormal finally
  Finally represents the code to be executed regardless of whether it is abnormal, such as closing a file.

• try: f = open('test.txt','r') except Exception as result: f = open('test.txt','w') else: print('No exception, really happy') finally: f .close()

Fourth, abnormal transmission

Experience abnormal delivery

• demand:

1. Try to open the test.txt file in read-only mode. If the file exists, read the content of the file, and prompt the user if the file does not exist.
2. Requirements for reading content: Try to read the content in a loop. If the user accidentally terminates the program is detected during the reading process, the except captures the exception.
   Often and prompt the user

import time
try:
  f =open('test.txt')try:while True: 
     content = f.readline()iflen(content)==0:break 
     time.sleep(2)print(content) 
   except: 
     # If an exception occurs in the process of reading the file, then it will be caught
     # For example, press ctrl+c
     print('Unexpected termination of reading data')finally: 
   f.close()print('Close file') 
except:print("No such file")

Five, custom exception

In Python, the syntax for throwing custom exceptions is raise exception class objects.

# Custom exception class, inherit Exception
classShortInputError(Exception): 
  def __init__(self, length, min_len): 
   self.length = length
   self.min_len = min_len

  # Set the description of the exception thrown
  def __str__(self):return f'The length you entered is{self.length},Not less than{self.min_len}Characters' 

def main():try:
   con =input('Please enter your password:')iflen(con)<3: 
     raise ShortInputError(len(con),3) 
  except Exception as result:print(result)else:print('The password has been entered')main()