EXCEPTION HANDLING IN PYTHON

The concept of identifying the runtime error representation class object which is created internally at the time of execution of the program, receiving that object and assigning that object to the reference variable of corresponding runtime error representation class is known as exception handling. Exception handling is used to stop the abnormal termination whenever runtime error is occurred. We can implement the exception handling in python by using try and except block.

Try block – The statements which causes to runtime errors and other statements which depends on the execution of the runtime error occur statements are recommended to represent by using try block.

Syntax –

try:

----

----
At the time of execution of the try block statements, if any runtime error representation class object is created then immediately that object is identified by the try block, received by the try block and forward it to the except block without executing remaining statements of the try block.

Note – If there is no runtime error occurs at the time of execution of the try block then control will not go to the except block.

Except block – Except block should be preceded by the try block. Except block receives the runtime error representation class object which is given by the try block and assigns that object to the reference variable of corresponding runtime error representation class. In except block we can define the statements to display the user friendly error message.

Example –

print 'hi'
i=input('Enter num1 : ')
j=input('Enter num2 : ')
try:
    k=i/j
    print k
except(ZeroDivisionError):
    print 'num2 cannot be zero'
print 'bye'

Output –

hi

Enter num1 : 10

Enter num2 : 0

num2 cannot be zero

bye


Single try with multiple except blocks – Whenever runtime error is occurred while executing try block statements then control will go to the first except block. If the first except block is not able to handle that runtime error then control will go to the second except block. If all except blocks cannot handle that runtime error then program will be terminated abnormally.

Example –

print 'hi'
try:
    i = input('Enter num1 : ')
    j = input('Enter num2 : ')
    k=i/j
    print k

except(ZeroDivisionError):
    print 'num2 cannot be zero'

except(NameError):
    print 'Do not enter string'

print 'bye'

Output –

hi

Enter num1 : 10

Enter num2 : abc

Do not enter string

bye

Default except block – Default block is used to handle any type of runtime error. In default except block we write the statements to display the common error messages (General error messages).

Example –

print 'hi'
try:
    i = raw_input('Enter num1 : ')
    j = raw_input('Enter num2 : ')
    a=int(i)
    b=int(j)
    k=i/j
    print k

except(ZeroDivisionError):
    print 'num cannot be zero'

except(NameError):
    print 'do not enter string values'

except:
    print 'error occurred'

print 'bye'

Output –

hi

Enter num1 : abc

Enter num2 : 100

error occurred

bye

Example –

print 'hi'
try:
    i = raw_input('Enter num1 : ')
    j = raw_input('Enter num2 : ')
    a=int(i)
    b=int(j)
    k=i/j
    print k

except:
    print 'error occurred'

print 'bye

Output –

hi

Enter num1 : 10

Enter num2 : 0

error occurred

bye

Note – Default except block should be the last except block whenever we define single try with multiple except block, otherwise we will get syntax error.

Example –

print 'hi'
try:
    i = raw_input('Enter num1 : ')
    j = raw_input('Enter num2 : ')
    a=int(i)
    b=int(j)
    k=i/j
    print k

except:
    print 'error occurred'

except(ZeroDivisionError):
    print 'num2 cannot be zero'

except(NameError):
    print 'Do not enter string value'

print 'bye'

Output –

  File "C:/Users/om/PycharmProjects/Pythonn/if3.py", line 8

    print k

SyntaxError: default 'except:' must be last

Finally block – The set of statements which are compulsory to execute whether exception is occurred or not occurred. Even though exception is occurred whether it is handled or not handled recommended to represent in finally block. Resource releasing statements (file closing statements, database connection closing statements) are recommended to represent in finally block. Finally block should be preceded by either try or except block.

Syntax 1 –

try :

          --------

          --------

finally :

          --------

          --------

Syntax 2 –

try :

          --------

          --------

except() :

          -------

          -------

finally :

          -------

          -------

Example –

try :
    x=input("Enter num1 : ")
    y=input("Enter num2 : ")
    z=x/y
    print z

except(ZeroDivisionError):
    print 'Num2 cannot be zero'

finally:
    print 'end'
    print 'bye'

Output 1 –

Enter num1 : 100

Enter num2 : 20

5

end

bye

Output 2 –

Enter num1 : 100

Enter num2 : 0

Num2 cannot be zero

end

bye

Output 3 –

Enter num1 : 100

Enter num2 : abc

end

bye

Traceback (most recent call last):

  File "C:/Users/om/PycharmProjects/Pythonn/if3.py", line 3, in <module>

    y=input("Enter num2 : ")

  File "<string>", line 1, in <module>

NameError: name 'abc' is not defined

Nested try blocks – The concept of defining one try block inside another try block is known as a nested try block.

Syntax –

try :

          ----

          ----

          try :

                   ----

                   ----

          except :

                   ----

                   ----

except :

          ----

          ----

A try block which contains an another try block is known as outer try block. A try block which is defined in another try block is known as a inner try block. If exception is occurred in the outer try block then control will go to the outer try except block. If outer try except block is not able to handle that exception then program will be terminated abnormally. If exception is occurred in inner try block then control will go to the inner try except block. If inner try except block is not able to handle that exception then control will go to the outer try except block.

Example –

try:
    print 'in try1'
    try :
        print 'in try2'
        try :
            print 'in try3'
        except :
            print 'in except3'
    except :
        print 'in except2'

except :
    print 'in except1'

Output –

in try1

in try2

in try3

Example –

try :
    print 'in try1'
    try :
        x=input('Enter num1 : ')
        y=input('Enter num2 : ')
        z=x/y
        print z
    except(ZeroDivisionError):
        print 'Num2 cannot be zero'

except :
    print 'in except1'

Output 1 –

in try1

Enter num1 : 10

Enter num2 : 0

Num2 cannot be zero

Output 2 –

in try1

Enter num1 : 10

Enter num2 : abc

in except1

Example –

try :
    print 'in try1'
    try :
        print 'in try2'
    except:
        print 'in except2'
    finally:
        print 'in finally2'

except :
    print 'in except1'
    try :
        print 'in try3'
    except :
        print 'in except3'
    finally:
        print 'in finally3'

finally:
    print 'in finally1'

try :
    print 'in try4'

except:
    print 'in except4'

finally:
    print 'in finally4'

Output –

in try1

in try2

in finally2

in finally1

in try4

in finally4