Exception Handling in Python Programming.pptx

What is an Exception?
• An exception is an error that happens during execution of a
program.
• When that error occurs, Python generate an exception that
can be handled, which avoids your program to crash.
• Whenever an exception occurs the program halts the
execution and thus further code is not executed.
• Thus exception is that error which python script is unable to
tackle with.

Why use Exceptions?
• Exception in a code can also be handled.
• In case it is not handled, then the code is not executed further
and hence execution stops when exception occurs.

Hierarchy Of Exception
• ZeroDivisionError: Occurs when a number is divided by zero.
• NameError: It occurs when a name is not found. It may be
local or global.
• IndentationError: If incorrect indentation is given.
• IOError: It occurs when Input Output operation fails.
• EOFError: It occurs when end of file is reached and yet
operations are being performed

Exception Handling
• Python provides two very important features to handle any
unexpected error in your Python programs and to add
debugging capabilities in them-
1. Exception Handling.
2. Assertions.

Standard Exceptions
EXCEPTION NAME DESCRIPTION
Exception Base class for all exceptions
ArithmeticError Base class for all errors that occur for numeric
calculation.
ZeroDivisonError Raised when division or modulo by zero takes place for
all numeric types.
EOFError Raised when there is no input from either the
raw_input() or
input() function and the end of file is reached.
ImportError Raised when an import statement fails.
KeyboardInterrupt Raised when the user interrupts program execution,
usually by pressing Ctrl+c.
NameError Raised when an identifier is not found in the local or
global namespace.
SyntaxError Raised when there is an error in Python syntax.

EXCEPTION NAME DESCRIPTION
IndentationError Raised when indentation is not specified properly.
TypeError Raised when an operation or function is attempted that
is invalid for the specified data type.

Exception Handling
• The suspicious code can be handled by using the try block.
• Enclose the code which raises an exception inside the try
block.
• The try block is followed by except clause.
• It is then further followed by statements which are executed
during exception and in case if exception does not occur.

Try •Code in which exception may occur
Raise •Raise the exception
Except •Catch if excepetion occurs

Declaring Multiple Exception
• Multiple Exceptions can be declared using the
same except statement:
try:
code
except (Exception1,Exception2,Exception3,..,ExceptionN):
execute this code in case any Exception of these occur.
else:
execute code in case no exception occurred.

try:
malicious code
except (Exception1):
execute code
except (Exception2):
execute code
....
....
except (ExceptionN):
execute code
else:
In case of no exception, execute the else block code.

try:
a=10/0
print (a)
except (ArithmeticError):
print( "This statement is raising an exception" )
else:
print ("Welcome“)

def fun(a,b):
try:
c=(a+b)/(a-b)
except (ArithmeticError):
print("EXCEPTION OCCUR")
else:
print(c)
finally:
print("End of the world")
# Driver program to test above function
fun(2.0, 3.0) # -5.0 End of the world
fun(3.0, 3.0) # EXCEPTION OCCUR End of the world

Argument of an Exception
• An exception can have an argument, which is a value that gives
additional information about the problem.
• The contents of the argument vary by exception.
• You capture an exception's argument by supplying a variable in
the except clause as follows-
try:
You do your operations here
......................
except ExceptionType as Argument:
You can print value of Argument here...

Example 1
def fun(a):
try:
return int(a)
except ValueError as VE:
print("argument is not a number",VE)
fun(11) # 11
fun("string")
#argument is not a number invalid literal for int() with base 10: 'string’

Example 2
def fun(a,b):
c= a/b
return c
try:
fun("a","b")
except TypeError as VE:
print("Exception: ",VE)
Output:
Exception: unsupported operand type(s) for /: 'str' and 'str'

Raising Exception
• The raise statement allows the programmer to force a specific
exception to occur.
• The sole argument in raise indicates the exception to be raised.
• This must be either an exception instance or an exception class (a class
that derives from Exception).
# Program to depict Raising Exception
try:
raise NameError("Hey! are you sleeping!") # Raise Error
except NameError as NE:
print (NE)

try:
raise Exception("How are you")
except Exception as E:
print(E)
How are you

Explanation:
i) To raise an exception, raise statement is used.
It is followed by exception class name.
ii) Exception can be provided with a value(optional) that can be
given in the parenthesis. (here, Hey! are you sleeping!)
iii) To access the value "as" keyword is used.
“NE" is used as a reference variable which stores the value of
the exception.

User-defined Exceptions in Python
Creating User-defined Exception
• Programmers may name their own exceptions by creating a
new exception class.
• Exceptions need to be derived from the Exception class, either
directly or indirectly.
• Although not mandatory, most of the exceptions are named
as names that end in “Error” similar to naming of the
standard exceptions in python.

# A python program to create user-defined exception
# class MyError is derived from super class Exception
class MyError(Exception):
# Constructor or Initializer
def __init__(self, value):
self.value = value
# __str__ is to print() the value
def __str__(self):
return(repr(self.value))

try:
raise(MyError(3*2))
# Value of Exception is stored in error
except MyError as error:
print('A New Exception occured: ',error.value)

class point(Exception):
def __init__(self,x=0,y=0):
self.x=x
self.y=y
def __str__(self):
return "({0},{1})".format(self.x, self.y)

p=point()
try:
if(p.x==0 and p.y ==0):
raise point()
except point as error:
print("point is ORIGIN",error)
else:
print(p)
OUTPUT:
point is ORIGIN (0,0)

Exception Handling in Python Programming.pptx

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