Reboot Academy | Exception Handling in Python

02 Nov Exception Handling in Python

Posted at 10:16h in Python by Shivani Agrawal 0 Comments

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What is exception?

An exception is an error that occurs while a program is running, causing the program to abruptly halt.
Below are some common exceptions in Python:

IOError

If the file cannot be opened.

ImportError

If python cannot find the module

ValueError

Raised when a built-in operation or function receives an argument that has the right type but an inappropriate value

KeyboardInterrupt

Raised when the user hits the interrupt key (normally Control-C or Delete)

For example:



#this program check voting eligibility
def main():
    #get the age
    age=int(input("Enter your age"))
    if age>18:
        print("Eligible to vote")
    else:
        print("Not eligible to vote")    
main()



Output:
Enter your age thirty
Traceback (most recent call last):  
  File "F:/../exception.py", line 19, in 
    main()
  File "F:/../exception.py", line 10, in main
    age=int(input("Enter your age"))

ValueError: invalid literal for int() with base 10: 'thirty'

So what all information we get from this message:

The lengthy error message that is shown in the sample run is called a traceback.
The traceback gives information regarding the line number(s) that caused the exception.
The last line shows the name of the exception (ZeroDivisionError) and a brief description of the error that caused the exception to be raised (integer division or modulo by zero).

How to write Exception Handler?



try:
 #this program check voting eligibility
def main():
    #get the age    
    try:
        age=int(input("Enter your age"))
        if age>18:
            print("Eligible to vote")
        else:
            print("Not eligible to vote")    
    except:
        print("age must be a valid number")    
main()

try-except clause with multiple except clause.



#this program check voting eligibility
def main():
    #single try statement can have multiple except statements.   
    try:
        age=int(input("Enter your age"))
        if age>18:
            print("Eligible to vote")
        else:
            print("Not eligible to vote")    
    except ValueError:
        print("age must be a valid number")
    except IOError:
        print("Enter correct value")
    #generic except clause, which handles any exception.
    except:
        print("An Error occured")    
main()

How does it work?

If exceptions will occur in the code written in try blocks then the execution will stopped in try block and control will jump down to the except block.

How to display an Exception’s Default Error Message



def main():    
    try:
        age=int(input("Enter your age"))
        if age>18:
            print("Eligible to vote")
        else:
            print("Not eligible to vote") 
    #display exception's default error message
    except ValueError as err:
        print(err)    
    except:
        print("An Error occured") 
	print("rest of the code...")
main()



Output:
Enter your age thirty
invalid literal for int() with base 10: ' thirty'
rest of the code...

**The else clause**

else block executed after the statements in the try block, only if no exceptions were raised. If an exception is raised, the else block is skipped.



#this program check voting eligibility
def main():    
    try:
        age=int(input("Enter your age"))
        if age>18:
            print("Eligible to vote")
        else:
            print("Not eligible to vote") 
    #display exception's default error message
    except ValueError as err:
        print(err)    
    else:
        # code to be excuted when there is no exception
        print("Thank you, you have successfully checked the voting eligibility")
main()



Output:
Enter your age 24
Eligible to vote
Thank you, you have successfully checked the voting eligibility



Output:
Enter your age thirty
invalid literal for int() with base 10: ' thirty'

**The finally clause**

The statements in the finally block are always executed whether an exception occurs or not.
The purpose of the finally block is to perform clean-up operations, such as closing files or other resources.



#this program check voting eligibility
def main():    
    try:
        age=int(input("Enter your age"))
        if age>18:
            print("Eligible to vote")
        else:
            print("Not eligible to vote") 
    #display exception's default error message
    except ValueError as err:
        print(err)    
    finally:
        # code to be excuted whether exception occurs or not
        #typically for closing files and other resources
        print("Thank you")
main()



#in case of no exception
Output:
Enter your age 34
Eligible to vote
Thank you



#in case of exception
Output:
Enter your age fifty
invalid literal for int() with base 10: ' fifty'
Thank you

Raising an Exception

we can raise exception whenever our program attempts to do something erroneous or meaningless.



#this program check voting eligibility
def main():       
    age=int(input("Enter your age "))        
    if age>18:
        print("Eligible to vote")            
    else:
        # raising exception if age is not valid
        # The code below to this would not be executed
        # if we raise the exception
        raise ValueError("Invalid age")
    print("rest of the code...")
main()



Output:
Enter your age 13
Traceback (most recent call last):
File "F:/REBOOT/exception.py", line 17, in main
      raise ValueError("Invalid age")

ValueError: Invalid age

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Exception Handling in Python