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Chapter 1
Exception handling
Ne
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syllabu
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2024-25

Exception
Handling
As a student of computer science, you know that many
times while executing the program we are getting
some errors.
Errors are problems in a program due to
which will stop the program from execution.
These errors are categorized as follows:

Syntax Errors : occures when the program is not written
correctly as per the format required
Example : x =+ 2
Semantic Errors : occur when the statement has no
meaning in the program.
Example: a=5 #Sattement 1
b=7 #Statement 2
a+b=res #Statement 3

Exceptions
An exception is a program event that occurs during
program execution and disrupts the flow of a program.
When a python program cannot Handle with a situation, it
raises an exception.
An exception is a Python object that represents an error.
Some common Python Build-in exceptions are as follows:
1) SyntaxError: This exception is raised when the interpreter
encounters a syntax error the code, such as a misspelled
keyword, a missing colon, or an unbalanced parenthesis.
2) TypeError: This exception is raised when an operation or
function is applied to an object of the wrong type, such as
adding a string to an integer.

3) NameError: This exception is raised when a variable or
function name is not found in the current scope.
4) IndexError: This exception is raised when an index is out of
range for a list, tuple, or other sequence types.
5) KeyError: This exception is raised when a key is not found in a
dictionary.
6) ValueError: This exception is raised when a function or method
is called with an invalid argument or input, such as trying to
convert a string to an integer when the string does not represent
a valid integer.
7) AttributeError: This exception is raised when an attribute or
method is not found on an object, such as trying to access a non-
existent attribute of a class instance.

8) IOError: This exception is raised when an I/O operation,
such as reading or writing a file, fails due to an input/output
error.
9) ZeroDivisionError: This exception is raised when an attempt is
made to divide a number by zero.
10) ImportError: This exception is raised when an import
statement fails to find or load a module.

Raising Exceptions
Exceptions are raised when the program is
syntactically correct, but the code results in an error.
This error does not stop the execution of the
program, however, it changes the normal flow of
the program.
Example: a = 10 / 0
print(a)

User-defined exceptions
The exception created by the programmer according to
the requirement of the program is called user-defined
exceptions.
The user defined-exception can be created using two
methods:
1) raise statement
2) assert statement

raise statement
It is used to throw an exception. The syntax is as follows:
raise exception-name[(optional argument)]
Note: The optional argument is a string passed to the
exception, that displays the message.
Example:

assert statement
An assert statement is used to check a condition in the
program code. If the result after evaluation is false, then
the exception is raised.
The syntax for the assert statement is:
If this expression is false, an AssertionError exception is
raised which can be handled like any other exception.
Example:

What is Exception Handling in Python
The process of catching and preventing errors when they
occurred is called exception handling. It is a mechanism
to overrule the exceptions using some blocks.
It is the concept of error handling when something goes
wrong, tracking the error, and calling the handling code.
The following terms are used for exception handling:

handling exceptionsusing
try-except-finally blocks
try:
# Some Code....which may have runtime error
except:
# optional block
# Handling of exception (if required)
else:
# execute if no exception
finally:
# Some code .....(always executed)

try:
k = 9//0 # raises divide by zero
exception.
print(k)
# handles zerodivision
exception except
ZeroDivisionError:
print("Can't divide by zero")
finally:
# this block is always
executed
# regardless of exception
generation. print('This is always
executed')

Lets see some example’s for better
understanding of Try and Except block

Use of
try-except-
else-finally

Process of Handling
Exception Step 1: The exception object is created by a
Python interpreter that contains information
related to the error such as type, file name,
and position where an error has occurred.
Step 2: The object is handed over to the
runtime system to find an appropriate code
to handle exceptions. This process is called
throwing an exception.
Step 3: The runtime system searches for a
block of code known as an exception handler
that handles the raised error. First, it searches
for the method by which the error has
occurred. If not found then it search method
from which this method is called. This
process continues till the exception handler is
found. When it found a handler it will be
executed. This process is known as catching.
Step 4: Finally the program gets terminated

Advantages of Exception Handling:
• Improved program reliability
• Simplified error handling
• Cleaner code
• Easier debugging
Disadvantages of Exception
Handling:
• Performance overhead
• Increased code complexity
• Possible security risks

Assignment Questions
Q.1 What is exception handling explain with example.
Q.2 What are the advantages and Disadvantages of
Exception handling.
Q.3 Explain the use of try-except-else-finally blocks.
Q.4 Write a short note on assert and raise statment
Q.5 Explain the process of Handling Exception.

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Exception handling with python class 12.pptx

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