Exception in Python

1. Introduction

Why Study This Topic?
Exceptions occur when an error disrupts the normal flow of a program. Python provides built-in mechanisms to handle exceptions, preventing crashes and improving program reliability. Understanding exception handling is crucial for writing robust and error-free code.
What Will Be Covered?
- What are exceptions?
- Common built-in exceptions in Python.
- Handling exceptions with try-except.
- Using finally and else with exceptions.
- Raising custom exceptions.

2. Detailed Content

What are Exceptions?

Exceptions are runtime errors that occur due to invalid operations, such as dividing by zero or accessing an undefined variable. Without proper handling, exceptions cause program termination. Understanding exceptions is vital for debugging.

2.1 Common Built-in Exceptions

Exception	Description
`ZeroDivisionError`	Raised when a number is divided by zero.
`ValueError`	Occurs when an invalid value is passed to a function.
`TypeError`	Raised when an operation is applied to an incompatible data type.
`IndexError`	Occurs when trying to access an invalid index in a list.
`KeyError`	Raised when trying to access a non-existent dictionary key.
`FileNotFoundError`	Occurs when attempting to open a non-existent file.
`NameError`	Raised when referencing an undefined variable.
`AttributeError`	Occurs when an invalid attribute is accessed on an object.

2.2 Handling Exceptions with Try-Except

pythonCopyEdittry:
    num = int(input("Enter a number: "))
    result = 10 / num
    print(f"Result: {result}")
except ZeroDivisionError:
    print("Error: Cannot divide by zero.")
except ValueError:
    print("Error: Invalid input. Please enter a number.")

2.3 Using Finally for Cleanup

pythonCopyEdittry:
    file = open("example.txt", "r")
    content = file.read()
finally:
    file.close()
    print("File closed.")

The finally block ensures that the file closes regardless of errors. This handling of exceptions guarantees resource release.

2.4 Using Else with Try-Except

pythonCopyEdittry:
    num = int(input("Enter a number: "))
    result = 10 / num
except ZeroDivisionError:
    print("Error: Cannot divide by zero.")
else:
    print(f"Result: {result}")  # Executes if no exception occurs

2.5 Raising Custom Exceptions

pythonCopyEditclass NegativeNumberError(Exception):
    """Exception raised for negative numbers."""
    pass

num = -5
if num < 0:
    raise NegativeNumberError("Negative numbers are not allowed.")

3. Summary

Exceptions help in handling errors gracefully without program crashes.
Python provides built-in exceptions, and users can define custom exceptions for better error management.
try-except-finally-else structures allow efficient error handling through exceptions management.

4. Learning Outcomes

After completing this topic, learners will be able to:

Identify and handle common Python exceptions.
Use try-except-finally to prevent program crashes. This is essential for exceptions.
Raise and define custom exceptions.

5. Common Interview Questions

What is an exception in Python?
- A runtime error that disrupts program execution.
What is the difference between syntax errors and exceptions?
- Syntax errors occur during compilation, while exceptions occur at runtime.
Can we have multiple except blocks?
- Yes, multiple except blocks can handle different exception types.

6. Practice Exercises

Write a Python script to handle ZeroDivisionError and ValueError. These are common exceptions.
Implement a function that raises a FileNotFoundError when a file is missing.
Create a custom exception for handling invalid user input.

7. Additional Resources

Python Exception Handling Documentation
Tutorials on Real Python and GeeksforGeeks, offering deeper insights into exceptions handling.