Exception Handling

Exception Handling

• It is a technique of handling runtime errors in structured way.
• An exception is an error that happens during execution of a program.
• When an error occurs, Python generate an exception type of object representing that error and throws it.
• Our program must catch that exception and handle the error accordingly, which avoids your program from being crash.
• In case if exception is not handled, then the code is not executed further and hence execution stops.

Common Exceptions

Exception Handling

try:
    Statement
    ---------
except exceptionX as e:
    Statement
    ---------
except exceptionY as e:
    Statement 
    ---------     
...
else:
    statement
    ---------      
finally: 
    statement
    ---------    

• The code where run time error may occur must be enclosed in the try block.

• Except statement is used to catch Exception.
• In case that exception is occurred, the corresponding statement will be executed.
• There can be multiple except statement to catch different Exception.

• this code will be executed if no exception occurs in the try block.

• this code will be executed, whether exception occurs or not.

try:
    print("start")
    z=4/2
    print(z)
except ZeroDivisionError:
    print("divide by zero")
except ValueError:
    print("Value Error")
else:
    print("No error")
finally:
    print("Always Executed...")

start
2.0
No error
Always Executed...

• try: The code where run time error may occur must be enclosed in the try block.
• except: Except statement is used to catch Exception.
• else : this code which will be executed if no exception occurs in the try block.
• finally : this code which be executed, whether exception occurs or not.

try:
    print("Start")
    z=4/0
    print(z)
except ZeroDivisionError:
    print("divide by zero")
except ValueError:
    print("Value Error")
else:
    print("No error")
finally:
    print("Always Executed...")

Start
divide by zero
Always Executed..

Exception Propogation

• If some run time error is occurred in a function and in that function that exception is not handled then that exception is thrown in the function from where that function is called.
• This is called as Exception Propogation.

def test():
    try: 
        print("open file")
        z=4/0
        print(z)
    finally:
        print("close file")

try:
    test( )
except ZeroDivisionError:
    print("divide by zero")
except ValueError:
    print("Value Error")
else:
    print("No error")
finally:
    print("Always Executed...")

open file
close file
divide by zero
Always Executed...

Raise statement

• An exception can be raised by using the raise statement.

  try: 
age = int(input("Enter age:")) 
if age<0: 
 raise ValueError 
else: 
 print("the age is valid") 
except ValueError: 
 print("The age is not valid")

Enter age: 23
The age is valid
Enter age: -25 
The age is not valid

Custom Exception

• We can create our exceptions that can be raised from the program and caught using the except clause.
• The custom exception class must be created from base class Exception.

class AgeError(Exception):
    pass
try: 
    age = int(input("Enter age:")) 
    if age<0: 
        raise AgeError()
    else: 
        print("The age is valid") 
except AgeError: 
    print("The age is not valid")

Enter age: 23
The age is valid
Enter age: -25 
The age is not valid