Python 3 – Handling Exceptions

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Python

Exceptions

  • An exception is an error that occurs during execution of a program.
  • Python allows a programmer to handle such exceptions using try ... except clauses, thus avoiding the program to crash.
  • Some of the python expressions, though written correctly in syntax, result in error during execution. Such scenarios have to be handled.

Sample Exceptions

  • Try executing the following two expressions

10 + ( 1 / 0)
36 + '20'

  • The first one throws error message, ZeroDivisionError : division by zero, and
  • Second one throws, TypeError : unsupported operand type(s) for +: 'int' and 'str'.

In Python, every error message has two parts. The first part tells what type of exception it is and second part explains the details of error.

Handling Exception

  • Python handles exceptions using code written inside try ... except blocks.
  • A try block is followed by one or more except clauses.
  • The code to be handled is written inside try clause and the code to be executed when an exception occurs is written inside except clause.

Sample Exception Handling

try:
    a = pow(2, 4)
    print("Value of 'a' :", a)
    b = pow(2, 'hello')   # results in exception
    print("Value of 'b' :", b)
except TypeError as e:
    print('oops!!!')
print('Out of try ... except.')

Output


Value of 'a' : 16
oops!!!
Out of try ... except.
  • You can observe from the output that execution of statements, present, after exception are skipped.

Raising Exceptions

  • raise keyword is used when a programmer wants a specific exception to occur.
try:
    a = 2; b = 'hello'
    if not (isinstance(a, int)
            and isinstance(b, int)):
        raise TypeError('Two inputs must be integers.')
    c = a**b
except TypeError as e:
    print(e)

Output


Two inputs must be integers.
  • Above example raises TypeError when both a or b are not integers.

User Defined Functions

  • There are many built-in exceptions in Python, which are directly or indirectly derived from Exception class.
  • Python also allows a programmer to create custom exceptions, derived from base Exception class.
class CustomError(Exception):
    def __init__(self, value):
        self.value = value
    def __str__(self):
        return str(self.value)
try:
    a = 2; b = 'hello'
    if not (isinstance(a, int)
            and isinstance(b, int)):
        raise CustomError('Two inputs must be integers.')
    c = a**b
except CustomError as e:
    print(e)

Output


Two inputs must be integers.
  • CustomError is raised in above example, instead of TypeError.

Using ‘finally’ clause

  • finally clause is an optional one that can be used with try ... except clauses.
  • All the statements under finally clause are executed irrespective of exception occurrence.
def divide(a,b):
    try:
        result = a / b
        return result
    except ZeroDivisionError:
        print("Dividing by Zero.")
    finally:
        print("In finally clause.")
print('First call')
print(divide(14, 7))
print('Second call')
print(divide(14, 0))

Output


First call
In finally clause.
2.0
Second call
Dividing by Zero.
In finally clause.
None
  • Statements inside finally clause are executed in both function calls.

Using ‘else’ clause

  • else clause is also an optional clause with try ... except clauses.
  • Statements under else clause are executed only when no exception occurs in tryclause.
try:
    a = 14 / 7
except ZeroDivisionError:
    print('oops!!!')
else:
    print('First ELSE')
try:
    a = 14 / 0
except ZeroDivisionError:
    print('oops!!!')
else:
    print('Second ELSE')

Output


First ELSE
oops!!!

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