Python Exception Handling

Python Exception Handling

It is good practise to handle exceptions(unexpected data) in the code to avoid arbitrarily termination of the program. Python provides a way to handle unexpected values passed to the code, popular exception example: division by Zero.

• Try
• Except
• Else
• Finally
• Raising Custom Exceptions

• Syntax:

  Exception Handling Block
  try:
     some_code
     some_code
  except [HandleSpecificError]:
     some_code_to_inform_user_of_error_OR_some_action
  except:
     some_code_if_the error_is_different_than_any_of_the_specific_exceptions
     some_code_to_inform_user_of_error_OR_some_action
  [else: 
      optional_else_for_Some_code_if_no_error_occured 
  ]
  [finally: 
      optional_finally_for_Some_code_which will_always_execute_regardless_of_error_occured_or_not. 
  ]
  

• Try Block: 'try' block should be used for a piece of code where exceptions are expected.
  • Example 1: No exception

    
    try:
        print("Hello Dbmstutorials")
    except:
        print("Exception Raised!")
    
    Output: 
    Hello Dbmstutorial
    

  • Example 2: Exception raised

    
    try:
        print("Hello Dbmstutorials"+1)
    except:
        print("Exception Raised!")
    
    Output: 
    Exception Raised!
    

  
  
• Except Block: 'except' block defines whether any action need to be taken when exception occurs or just pass appropriate error message to user.
  • There can be multiple 'except' blocks but one is mandatory for exception handling.
  • 'except' can be defined to handle specific kind of exception. Here are some of the common built in exceptions handling classes, for more classes check out this page.
    • except NameError: This exception is raised when local or global variable is not found

      
      try:
        print(print_undefineVariable)
      except NameError:
        print("NameError: Variable is not defined")
      
      Output: NameError: Variable is not defined
      

    • except TypeError: This exception is raised when improper type value is not used, example: number value used in place of string.

      
      try:
        a=2
        print("num "+a) #trying to append integer to the String
      except NameError:
        print("NameError: Variable is not defined")
      except TypeError:
        print("TypeError: Improper type value used")
      
      Output: TypeError: Improper type value used
      

    • except ZeroDivisionError: This exception is raised when division by zero is encountered.

      
      try:
        print(1/0)
      except ZeroDivisionError:
        print("Division by zero detected")
      
      Output: Division by zero detected
      

    • except IndexError:

      
      try:
        a=[3,4,5,7]
        print(a[4]) #index 4 is not present since Python list starts from 0
      except IndexError:
        print("IndexError: Index not found")
      
      Output: IndexError: Index not found
      

    • except IOError: This exception is raised when Python is not able to open file passed as attribute.

      
      import io
      try:
        a=open("~/test.dml")
      except IOError:
        print("IOError: Error reading file")
      
      Output: IOError: Error reading file
      

    • except AttributeError: This exception is raised when attributes/functions are used which are not defined in module.

      
      import time
      try:
        time.dbmstutorials(100)  # dbmstutorials is not attribute of time module
      except AttributeError:
        print("AttributeError: undefined attribute used")
      
      Output: AttributeError: undefined attribute used
      

    • except KeyboardInterrupt:

      
      import time
      try:
        print("Press Ctrl + C after 10 seconds")
        time.sleep(100) #Sleep for 100 seconds
      except KeyboardInterrupt:
        print("KeyboardInterrupt: Interrupted by user with Ctrl+C")
      
      Output: ^CKeyboardInterrupt: Interrupted by user with Ctrl+C
      

    
    
  • Multiple Excepts: All the specific exception handling classes are child of 'except:'. If 'except:' is specified then it will catch exceptions which were not caught by specific exception handling classes defined in the code.

    
    try:
      a=2
      print("num "+a)
    except ZeroDivisionError:
      print("ZeroDivisionError: Divide by Zero Exception")
    except IndexError:
      print("IndexError: Index not found")
    except:
      print("TypeError Exception handled by Except ")
    
    Output: 
    TypeError Exception handled by Except 
    

    
    
  • Multiple exceptions in single Except: All the specific exception handling classes are child of 'except:'. If 'except:' is specified then it will catch exceptions which were not caught by specific exception handling classes defined in the code.

    
    try:
        print("num "+a) #trying to append undefined variable
    except (NameError, TypeError):
        print("NameError: Variable is not defined OR TypeError: Improper type value used")
    except:
        print("TypeError Exception handled by Except ")
    
    Output: 
    NameError: Variable is not defined OR TypeError: Improper type value used
    

  
  
  
• Else Block: It is optional to specify 'else'. Piece of code specified within 'else' will get executed if no exceptions were raised.
  • Example 1: No exception, else statement got executed

    
    try:
      print("Hello Dbmstutorials")
    except:
      print("Exception Raised")
    else:
      print("No Exception Raised")
    
    Output: 
    Hello Dbmstutorials
    No Exception Raised
    

  • Example 2: Exception raised, else statement not executed

    
    try:
      print("Hello Dbmstutorials"+1)
    except:
      print("Exception Raised")
    else:
      print("No Exception Raised")
      
    Output: Exception Raised!
    

  
  
• Finally Block: It is optional to specify 'finally' but piece of code specified within 'finally' will always execute regardless of whether exception were raised or not.
  • Example 1: No exception raised but still finally statement got executed.

    
    try:
        print("Hello Dbmstutorials")
    except:
        print("Exception Raised")
    finally:
        print("Finally is executed")
    
    Output: 
    Hello Dbmstutorials
    Finally is executed
    

  • Example 2: Exception raised but still finally statement got executed.

    
    try:
        print("Hello Dbmstutorials"+1)
    except:
        print("Exception Raised!")
    finally:
        print("Finally is executed")
    Output: 
    Exception Raised!
    Finally is executed
    

  
  
• Raising Custom Exceptions: Python provides a way to raise user defined exception using 'raise Exception'. Here are
  • Example 1: Raising Custom exception without any parameter.

    
    try:
        a=6
        if (a==6):
            raise Exception
    except Exception:
        print("Custom Exception Raised")
    
    Output: 
    Custom Exception Raised
    

    
    
  • Example 2: Raising Custom exception with single parameter. Type of exception variable 'except_val' catching custom exception in the below example is 'exceptions.Exception'.

    
    Python 3:
    
    try:
        a=6
        if (a==6):
            raise Exception(a)
    except Exception as except_val:
        print("Type of the exception variable "+str(type(except_val)))
        print("This is value for custom raised exception variable "+str(except_val))
    
    Output: 
    Type of the exception variable <type 'exceptions.Exception'>
    This is value for custom raised exception variable 6
    
    
    Python 2.7:
    
    try:
        a=6
        if (a==6):
            raise Exception(a)
    except Exception, except_val:
        print("Type of the exception variable "+str(type(except_val)))
        print("This is value for custom raised exception variable "+str(except_val))
    
    Output: 
    Type of the exception variable <type 'exceptions.Exception'>
    This is value for custom raised exception variable 6
    

    Note: Syntax is different in old versions of Python(2.7) and in Python 3
    
    
  • Example 3: Raising Custom exception with 2 parameters

    
    Python 3:
    
    try:
        a=6;b=3
        if (a==6 and b < a):
            raise Exception(a , b)
    except Exception as except_val:
        print("Value for custom raised exception variables "+str(except_val))
        c,d= except_val.args
        print("Value of c ="+str(c)+ " and value of d ="+str(d))
    
    Output: Value for custom raised exception variables (6, 3)
    Value of c =6 and value of d =3
    
    
    Python 2.7:
    
    try:
        a=6;b=3
        if (a==6 and b < a):
            raise Exception(a , b)
    except Exception, except_val:
        print("Value for custom raised exception variables "+str(except_val))
        c, d = except_val
        print("Value of c ="+str(c)+ " and value of d ="+str(d))
    
    Output: Value for custom raised exception variables (6, 3)
    Value of c =6 and value of d =3
    

    Note: Syntax is different in old versions of Python(2.7) and in Python 3
    
    
  • Example 4: Raising Custom exception with multiple parameters. Multi parametered exception variable can be converted to Tuple using 'args' attribute.

    
    Python 3:
    
    try:
        a=6;b=3;c=39
        if (a==6 and b < a):
            raise Exception(a , b, c)
    except Exception as except_val:
        print("Type of the exception variable "+str(type(except_val)))
        dTuple = except_val.args
        print("Type of the dTuple variable "+str(type(dTuple)))
        print("Value of the dTuple variable "+str(dTuple))
    
    Output: 
    Type of the exception variable <type 'exceptions.Exception'>
    Type of the dTuple variable 
    Value of the dTuple variable (6, 3, 39)
    
    
    
    Python 2.7:
    
    try:
        a=6;b=3;c=39
        if (a==6 and b < a):
            raise Exception(a , b, c)
    except Exception, except_val:
        print("Type of the exception variable "+str(type(except_val)))
        dTuple = except_val.args
        print("Type of the dTuple variable "+str(type(dTuple)))
        print("Value of the dTuple variable "+str(dTuple))
    
    Output: 
    Type of the exception variable <type 'exceptions.Exception'>
    Type of the dTuple variable 
    Value of the dTuple variable (6, 3, 39)
    

    Note: Syntax is different in old versions of Python(2.7) and in Python 3