Exception Handling in Python

Learn how to handle runtime errors and keep your Python programs running safely.

What is an Exception?

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

When an exception occurs and is not handled, the program can stop and display an error message.

Why Do We Need Exception Handling?

Real applications can face many unexpected situations.

• A file may not exist.
• A user may enter invalid input.
• A calculation may cause an error.
• A database or network operation may fail.

Exception handling allows us to handle these situations in a controlled way instead of letting the program stop unexpectedly.

Exception Handling Diagram

The basic flow of exception handling can be understood like this:

Simple Example

Consider a program that divides two numbers.

number1 = 10
number2 = 0

result = number1 / number2

print(result)

This program raises a ZeroDivisionError because a number cannot be divided by zero.

Handling the Exception

We can use try and except to handle the problem.

try:

    number1 = 10
    number2 = 0

    result = number1 / number2

    print(result)

except ZeroDivisionError:

    print("Cannot divide by zero")

Output:

Cannot divide by zero

Instead of stopping the program with an unhandled exception, we display a meaningful message.

Common Python Exceptions

Example: ValueError

A ValueError can occur when the data type is acceptable but the actual value is not suitable for the operation.

try:

    age = int("abc")

    print(age)

except ValueError:

    print("Invalid number")

Output:

Invalid number

Example: TypeError

try:

    result = "10" + 5

    print(result)

except TypeError:

    print("Incompatible data types")

Output:

Incompatible data types

Example: IndexError

numbers = [10, 20, 30]

try:

    print(numbers[5])

except IndexError:

    print("Invalid list index")

Output:

Invalid list index

Example: KeyError

student = {
    "name": "Rahul",
    "course": "Python"
}

try:

    print(student["fees"])

except KeyError:

    print("Key does not exist")

Output:

Key does not exist

Example: FileNotFoundError

try:

    with open(
        "students.txt",
        "r"
    ) as file:

        data = file.read()

        print(data)

except FileNotFoundError:

    print("File not found")

This is useful when a program tries to read a file that does not exist.

Exception Handling Flow

Benefits of Exception Handling

• Prevents unexpected program termination.
• Provides meaningful error messages.
• Makes applications more reliable.
• Helps separate normal logic from error handling.
• Makes debugging and maintenance easier.

Best Practices

Accessing the Exception Message

We can store the exception object using as and print its message.

try:

    number = int("abc")

except ValueError as error:

    print("Error:", error)

The variable error contains information about the exception.

Handling a General Exception

Python also allows a general Exception handler.

try:

    result = 10 / 0

except Exception as error:

    print("Error:", error)

A broad handler should be used carefully because it can catch many different exception types.

Example 📩

Suppose a student enters a number for marks.

try:

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

    print("Marks:", marks)

except ValueError:

    print("Please enter a valid number")

If the user enters text instead of a number, the program handles the problem gracefully.

Without vs With Exception Handling

Common Mistakes

• Putting the wrong code inside the try block.
• Catching the wrong exception type.
• Using an overly broad exception handler everywhere.
• Hiding the real problem with an unclear message.
• Using exceptions to replace normal program logic.

Practice Programs

1. Handle division by zero.
2. Handle invalid integer input.
3. Handle invalid list index.
4. Handle missing dictionary keys.
5. Handle a missing file.
6. Display the exception message using as error.

Exception Handling Topics in This Module