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What is Exception?
Advantages of Exception Handling:
Unexpected situation or errors occur during the program execution is known as
Exception. This will lead to the termination of program execution. For this programmer
have no control on.
Egs. ATM machine running out of cash, When you try to open a file that does not exit in
that path. These type of anomalous situations are generally called exceptions and the way
to handle them is called exception handling.
Exception handling separates error handling code from normal code
It enhances readability
It stimulates the error handling to take place one at a time in one manner.
It makes for clear, robust, fault tolerant programs.

Some common examples of Exception – disturb normal flow of execution during run
time.
• Divide by zero errors
• Accessing th elements of an array/list beyond its range
• Index Error
• Invalid input
• Hard disk crash
• Opening of non-existing file
• Heap memory exhausted
Example:
>>>3/0
Traceback (most recent call last):
File "<pyshell#0>", line 1, in <module>
3/0
ZeroDivisionError: division by zero
This is generated by default exception handler of
python.
(i) Print out exception description
(ii) Prints the stack trace (where the exception
occurred
(iii) Causes the program termination.

Example:
>>>l1=[1,2,3]
l1[4]
IndexError: list index out of range
Example:
>>>l1=[1,2,3]
>>>li[4]
li[4]
NameError: name 'li' is not defined. Did you mean: 'l1'?

Concept of Exception handling:
1. Write a code in such a way that it raises some error flag every time something goes
wrong
2. Then trap this error flag and if this is spotted, call the error handling routine.
• Raising an imaginary error flag is called throwing or raising an error
• When error is thrown, the overall system responds by catching the error.
• The surrounding a block of error-sensitive-code-with-exception-handling is called trying
to execute a block.
Write code such that it
raises an error-flag
every time something
goes wrong
-------------------------------
Throw exception
If error flag
Is raised then
Call the error handling
routine
-------------------------------
Catch exception

Description Python terminology
An unexpected error that occurs during
runtime
Exception
A set of code that might have an
exception thrown in it
Try block
The process by which an exception is
generated and executing atatements that
try to resolve the problem
Catching
The block of code that attempts to deal
with the exception (ie. Problem)
Except clause for except/exception block
or catch block
The sequence of method calls that
brought control to the point where the
exception occurred
Stack trace
Terminology used with exception handling:

When to use Exception Handling:
 Processing exception situation
 Processing exception for components that cannot handle them directly
 Large projects that require uniform error-processing.
The segment of code where there is any possibility of error or exception, is placed
inside one block. The code to be executed in case the exception has occurred, is placed
inside another block.
Syntax of Exception Handling:
try:
#write here the code that may generate an exception
except:
#write code here about what to do when the exception has occurred

Example:
try:
a,b=int(input("enter a no")),int(input("enter a divisor"))
print(a/b)
except:
print("Division by Zero ! denominator must not be zero")
Output 1:
enter a no6
enter a divisor3
result of a/b 2.0
Output2:
enter a no6
enter a divisor0
Division by Zero ! denominator must not be zero

General built-in Python Exception:

Program to handle exception while opening a file
try:
myf=open("d:/myf.txt",'r')
print(myf.read())
except:
print("Error opening a file")
OUTPUT:
Hello python
The above output is displayed if the file has been opened for reading successfully.
Otherwise it shows ‘Error opening a file’

Exception raised because of TypeError and ValueError:
import traceback
try:
a=5
b=int(input("enter a no."))
print(a+b)
print(a+"five")
except ValueError:
print("arithmetic is not possible between a no. and string")
except TypeError:
print("cannot convert a string to a number")

Second argument of the except block:
try:
# code
Except <ExceptionName> as <exArgument>:
# handle error file
try:
print("result of 5/0=",5/5)
print("result of 5/0=",5/0)
except ZeroDivisionError as e:
print("Exception raised",e)
Output:
result of 5/0= 1.0
Exception raised division by zero

Handling multiple errors:
Multiple exception blocks – one for each exception raised.
This will give exact error message for a specific exception than having one common
error message for all exception.
try:
#:
except <exceptionName1>:
#:
except <exceptionName1>:
#:
except :
#:
else:
#if there is no exception then the statements in this block get executed.

Example:
try:
my_file=open('d:myf.txt')
my_line=my_file.readline()
my_int=int(s.strip())
my_calc=101/my_int
print(my_calc)
except IOError:
print('I/O error occurred')
except ValueError:
print('Division by zero error')
except ZeroDivisionError:
print("unexpected error")
except:
print('unexpected error')
else:
print('No exceptions')
When <try suite> is executed, an exception
occurs, the <except suite> is executed with
name bound if found; otherwise unnamed
except suite is executed.

Finally block:
Finally block can be used just like except block but any code placed inside finally block
must execute, whether the try block raised an exception or not.
try:
#statements that may raise exception
[except:
# handle exception here]
finally:
#statements that will always run
Example:
try:
fh=open(“poems.txt” , ”r+”)
fh.write(“Adding new line”)
except:
print(“Exception has occurred”)
finally:
print(“goodbye!!!”)
The except block is executed only when
exception has occurred but finally block
will be executed always in the end.

Raising/Forcing an Exception:
Raise keyword can be used to raise/force an exception.
The programmer can force an exception to occur through raise keyword with a custome
message to the exception handling module.
Example:
try:
a=int(input(“enter numerator”))
b=int(input(“enter denominator”))
if b==0:
raise ZeroDivisionError(str(a)+”/0 not possible”)
print(a/b)
except ZeroDivisionError as e:
print(“Exception”, e)
Raise <exception>(<message>)
It is a predefined built-in exception.

Exception handling.pptxnn h

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Exception handling.pptxnn h

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