Exception handling Concept in Python:-

Exception Handling in Python – Complete Guide

Exception is an unexpected runtime error that occurs during program execution. Python provides a powerful system to handle such errors using try–except blocks so that the program does not stop unexpectedly.

A real-world project contains multiple modules. If one program fails due to an error, the entire project may crash. To avoid this failure, Python strongly recommends using exception handling in all important code blocks.

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Why Exception Handling Is Important?

• Prevents application crashes
• Allows safe handling of unexpected errors
• Improves user experience with friendly error messages
• Protects data and program flow
• Handles hardware, network, OS, file errors, etc.

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Basic Syntax of Exception Handling

1. Try–Except

try:
    CODE BLOCK
except:
    Error Message

2. Try–Except with Specific Exception Class

try:
    CODE BLOCK
except ExceptionClass:
    Error Message

3. Multiple Except Blocks

try:
    CODE BLOCK
except ExceptionClass1:
    Error Message 1
except ExceptionClass2:
    Error Message 2

4. Try–Except–Finally

try:
    CODE BLOCK
except ExceptionClass:
    Error Message
finally:
    Default block (always runs)

5. Try–Except–Else–Finally

try:
    CODE BLOCK
except ExceptionClass:
    Error Message
else:
    Runs when no exception occurs
finally:
    Runs every time

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Example: Basic Exception Handling

try:
    a = int(input("Enter first number"))
    b = int(input("Enter second number"))
    c = a / b
    print(c)
except ZeroDivisionError:
    print("Denominator cannot be zero")

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Handling Multiple Exceptions

try:
    a = int(input("Enter first number"))
    b = int(input("Enter second number"))
    c = a / b
    print(c)

except ZeroDivisionError:
    print("Denominator cannot be zero")

except ValueError:
    print("Enter only numeric value")

print("Line1")
print("Line2")

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Use of Finally Block

finally block always executes—whether an exception occurs or not. Useful for cleanup operations (closing file, disconnecting DB, etc.)

try:
    a = int(input("Enter first number"))
    b = int(input("Enter second number"))
    print(a/b)

except ZeroDivisionError:
    print("Denominator cannot be zero")

except ValueError:
    print("Enter only numeric value")

finally:
    print("Finally block executed")

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Using Else Block

else executes only when no exception occurs.

try:
    a = int(input("Enter first number"))
    b = int(input("Enter second number"))
    print(a/b)

except ZeroDivisionError:
    print("Denominator cannot be zero")

except ValueError:
    print("Enter only numeric value")

else:
    print("NOT ANY ERROR")

finally:
    print("finally")

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Complete Program Using Try–Except Loop

flag = True
count = 0

while flag:
    try:
        a = int(input("Enter first number"))
        b = int(input("Enter second number"))
        print(a / b)

    except ValueError:
        print("Enter only numeric value")

    except ZeroDivisionError:
        print("Denominator cannot be zero")

    else:
        flag = False

    finally:
        count += 1
        print("Number of attempts:", count)

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User-Defined Exception in Python

Python allows us to create custom exception classes for specific application needs.

Syntax

class ClassName(Exception):
    pass

Example: Salary Exception

class SalaryException(Exception):
    pass

try:
    sal = int(input("Enter employee salary:"))
    if sal < 10000:
        raise SalaryException
    else:
        print("Salary is", sal)

except SalaryException:
    print("Salary should be above 10000")

raise keyword is used to manually throw an exception (similar to throw in Java).

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Common Built-in Exceptions in Python

Sr.No.	Exception Name & Description
1	Exception Base class for all exceptions
2	StopIteration Raised when iterator has no more items.
3	SystemExit Raised by sys.exit().
4	StandardError Base class for built-in exceptions.
5	ArithmeticError Base for numeric operation errors.
6	OverflowError Numeric value exceeds limit.
7	FloatingPointError Floating point calculation failed.
8	ZeroDivisionError Division by zero.
9	AssertionError Assert statement fails.
10	AttributeError Attribute not found.
11	EOFError No input; end of file reached.
12	ImportError Import statement fails.
13	KeyboardInterrupt User presses Ctrl+C.
14	LookupError Base for indexing errors.
15	IndexError Invalid index used.
16	KeyError Dictionary key not found.
17	NameError Identifier not defined.
18	UnboundLocalError Local variable referenced before assignment.
19	EnvironmentError External environment failures.
20	IOError Input/output error (file not found etc.)
21	OS Error Operating system error.
22	SyntaxError Invalid Python syntax.
23	IndentationError Incorrect indentation.
24	SystemError Internal interpreter error.
25	SystemExit Program exit request.
26	TypeError Invalid type operation.
27	ValueError Correct type, wrong value.
28	RuntimeError General runtime failure.
29	NotImplementedError Abstract method not implemented.

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Assignments for Practice

• 1) Create Marksheet Program with all exception handling
• 2) Validate mobile number using user-defined exception
• 3) Calculator program with multiple exception blocks
• 4) Raise exception if student fees > 500000
• 5) Define 10 built-in exceptions with examples