Python Exception handling

Exception HandlingErrors and Exceptions

• Error: Syntax issues before the program is run, such as: keywords, indents, parentheses not equal

• Exceptions: Errors that occur during the execution of a program, such as: variable undefined, divisor 0, attribute not present, etc.

Exception Handling

• Description: Exception handling can be considered as a special Process Control statement, which can improve the robustness of the code.

• Syntax: try-except

  try: 
       print ( Normal execution ') 
       # print (a) 
        Print (3/0) 
    except exception as e: 
       # exception is the base class for all exceptions, so you can catch all exceptions here 
       print ( ' exception, processing ') 
       print (e) 
    ? 
    print ( ' other code ')         

• To capture multiple exceptions:

  Try
  # Print (a)
  # Print (3/0)
  FP =Open' 123.txt ')
   except nameerror as e: 
       print ( ' nameerror: ', Span class= "cm-variable" >e) 
    except zerodivisionerror as e: 
       print ( Span class= "cm-string" > ' zerodivisionerror: ', e) 
     Except exception as e: 
       print ( ' other: ', e)  

  can also be used to group multiple exceptions:

   
    try: 
       # print (a) 
       # print (3/0) 
       FP = open ( ' 123.txt ') 
    except (nameerror, zerodivisionerror) as e: 
       # uniformly handle certain exceptions, Write to tuple 
       print (e) 
    except: 
       print ( ' other exceptions ')   

• else and finally

   
    try: 
       print (     print (a) 
    except: 
        print ( ' exception ') 
    else: 
       # an exception is not performed 
       print (  finally: 
        # any error exception will execute 
       print (  

  ELSE: When no exception occurs, the exception is executed.

  Finally: Regardless of whether there is an exception, it will execute

• Throw exception: Raise

   
    try: 
       print (     # according to the needs of the code logic, Manually throwing a specific exception 
       raise exception ( ' manual throw exception ') 
       print ( ' all OK ') 
    except exception as e: 
       print ( "exception occurred: ', e) 
    ? 
    print ( ' end ')         

• Exception nesting: nested try-except structures in try-except structures

  
  Print' I'm going to work, nothing can stop me from working '
  Try
  Print' I'm going to ride an electric car ')
  RaiseException (The wicked guy pulled my charger out last night and couldn't ride a bike.
  Print' Ride the electric car early to reach the company ')
  ExceptExceptionAsE:
  PrintE
  Try
  Print' I'm going to take the bus ')
          raise exception (" waited 20 minutes without a bus, decisively giving up ') 
           print (" take the bus on time to reach the company ') 
       except  Exception as e: 
           print (e) 
           print ( I'm going to take a taxi) 
           print (" taxi or fast, a few will be to the company ') 
    ? 
    print ( ' enthusiastic start of the day's work ')        

• Custom Exception class: base class to inherit from Exception (Exception)

  
  # Custom Exception class, names usually end with exception
  ClassMyException (Exception):
  Def__INIT__ (SelfMSG):
  Self.msg =Msg
  ?
  Def__STR__ (Self):
  # repr key variable is converted to a string, if itself is a string is not necessary
  # return repr (self.msg)
  ReturnSelf.Msg
  ?
      def deal (self): 
           print ( ' exception handled ') 
    ? 
    try: 
       print (      raise myexception (  ' manual throw definition exception ') 
    except myexception as e: 
       print (e) 
       e.deal ()       

• Special use

  • Scene: When doing the file operation, whether read or write in the middle, or whether the error is abnormal, finally must close the file

  • With: using with, there is no need to close the file, the WITH statement block will ensure that the file is closed.

  
  # fp = open (' Test.txt ', ' RB ')
  # various read and write operations
  # Fp.close ()
  ?
  # using with, no need to worry about file closing issues
  Open (fp:
   FP.Read (5)
  Print (content) 

  ?

Python Exception handling