Exception Handling in Python – Detailed Notes

Exception Handling in Python – Detailed Notes

-What is an Exception?

An exception is an error that occurs while a program is running.

Example:
If you divide a number by 0, Python shows an error:

print(10 / 0)

Error: ZeroDivisionError

-Why Do Exceptions Occur?

Exceptions happen when your program does something that is not allowed, such as:

• Division by zero
• Using a variable that is not defined
• Opening a non-existent file
• Invalid input

-Why Do We Need Exception Handling?

Without exception handling, the program stops immediately when an error occurs.

Exception handling allows the program to:

-Continue running

-Separates error handling code from normal code

-Enhances readability

 -Keywords used in Exception Handling:-

Keyword	Meaning
try	The block of code where an exception may occur
except	Code that runs when an exception occurs
else	Runs when no exception occurs
finally	Always runs (with or without exception)

 

- Broadly classified in two types:

1. Compile-time Error: Errors that occur due to the violation of programming language's grammar rules. 

Example- Syntax errors like:   print('X'+4)

2. Run-time Error: Errors that occur during runtime because of unexpected situations. This type of error is handled through exception handling.

- Syntax

try:

    # code that may cause an error

except:

    # code that runs if error occurs

 1. Example

try:

    a = int(input("Enter a no.="))

    print("No. entered=", a)

except:

    print("Invalid input. Please enter a no.")

Explanation:

If the user enters a string or tuple or list or anything other than a number, error occurs.

Here except block handles the error and prints a message without stopping the program.

 2. Handling Specific Exceptions

Example:

try:

    a = int(input("Enter a number: "))

    b = int(input("Enter another number: "))

    print(a / b)

 

except ValueError:

    print("Please enter NUMBERS only!")

 

except ZeroDivisionError:

    print("You cannot divide by zero!")

 3. Using 'else' Block

else runs only if no exception occurs.

try:

    num = int(input("Enter a number+ "))

except:

    print("Error! Not a number!")

else:

    print("Success! You entered=", num)

 4. Using finally Block

finally runs every time, even if there is an error.

try:

    print("Trying to divide...")

    result = 10 / 0

except:

    print("An error occurred.")

finally:

    print("End of program.")

Output:

Trying to divide...

An error occurred.

End of program.

 5. Multiple Exceptions Together

try:

    x = int(input("Enter a number="))

    y = int(input("Enter another number="))

    print("Result=", x / y)

 

except (ValueError, ZeroDivisionError):

    print("Error!Either wrong input or division by zero!")

 6. Raising an Exception externally

You can create your own errors using raise.

age = int(input("Enter age: "))

if age < 0:

    raise ValueError("Age cannot be negative!")

else:

    print("Valid age:", age)

- Summary Table

Keyword	Purpose
try	Place code that may cause error
except	Handle the error
else	Runs if no error occurs
finally	Always runs

 Program Using Exception Handling

try:

   num = int(input("Enter a number: "))

    print("Square:", num * num)

except ValueError:

    print("Please enter a valid number only!")

-Exception Types in Python – Detailed Notes

Python has many built-in exception types, each describing a different kind of error.
When something goes wrong during program execution, Python raises (creates) an exception object of a particular type.

 - What is an Exception Type?

An exception type tells us what kind of error occurred.

Example:

ZeroDivisionError = Error caused by dividing by zero
ValueError = Error caused by wrong type of input

Each error has a name and purpose.

- Most Common Exception Types in Python

Below are the most important exception types, explained with simple examples.

- SyntaxError

Occurs when Python code has wrong syntax (structure).

Example:

print("Hello"

Error:

SyntaxError: unexpected EOF while parsing

This happens before the program runs.

- NameError

Occurs when you use a variable that does not exist.

Example:

print(x)

Error:

NameError: name 'x' is not defined

- TypeError

Occurs when an operation is used on the wrong type of data.

Example:

print("5" + 5)

Error:

TypeError: can only concatenate str (not "int") to str

-  ValueError

The data type is correct, but the value is wrong.

Example:

num = int("hello")

Error:

ValueError: invalid literal for int()

"hello" is a string but cannot be converted to a number.

- IndexError

Occurs when accessing a list index that does not exist.

Example:

fruits = ["apple", "banana"]

print(fruits[5])

Error:

IndexError: list index out of range

-  KeyError

Occurs when accessing a dictionary key that is not present.

Example:

student = {"name": "Raj"}

print(student["age"])

Error:

KeyError: 'age'

-  ZeroDivisionError

Occurs when dividing by zero.

Example:

print(10 / 0)

Error:

ZeroDivisionError: division by zero

- FileNotFoundError

Occurs when trying to open a file that does not exist.

Example:

f = open("abc.txt", "r")

Error:

FileNotFoundError: [Errno 2] No such file or directory

- AttributeError

Occurs when an object has no such attribute or function.

Example:

x = 10

x.append(5)

Error:

AttributeError: 'int' object has no attribute 'append'

- ImportError

Occurs when Python cannot import a module.

Example:

import mathsss

Error:

ImportError: No module named 'mathsss'

- Other Important Exception Types

Exception	Meaning
IndentationError	Incorrect indentation (spaces)
RuntimeError	Generic error during execution
MemoryError	Program runs out of memory
OverflowError	Number too large to handle
StopIteration	Happens in loops or iterators
KeyboardInterrupt	When user stops program (Ctrl+C)

Example Showing Multiple Exceptions Together

try:

    a = int(input("Enter number: "))

    b = int(input("Enter divisor: "))

    print(a / b)

except ZeroDivisionError:

    print("Error: Cannot divide by zero.")

except ValueError:

    print("Error: Please enter numbers only.")

except Exception as e:

    print("Some other error occurred:", e)

# Exception is the base class of all exceptions

- Summary Table of Common Exceptions

Exception	Cause	Example
SyntaxError	Wrong code syntax	Missing bracket
NameError	Variable not defined	print(x)
TypeError	Wrong data type	"5" + 5
ValueError	Wrong value	int("abc")
ZeroDivisionError	Divide by zero	10/0
IndexError	Wrong list index	mylist[10]
KeyError	Missing dictionary key	dict["age"]
FileNotFoundError	File missing	open("abc.txt")
AttributeError	Missing attribute	10.append()