Python Exceptions

Introduction

Python exceptions are events that occur during program execution that disrupt the normal flow. Proper exception handling is crucial for building robust data science applications. Understanding how to catch, handle, and raise exceptions helps create resilient code that gracefully handles unexpected situations.

Key Concepts

• Exception types: Built-in exceptions like ValueError, TypeError, KeyError
• Try-except blocks: Catching and handling exceptions
• Finally clause: Code that runs regardless of exceptions
• Raising exceptions: Signaling errors with raise keyword
• Custom exceptions: Creating user-defined exception classes
• Exception hierarchy: Understanding parent-child relationships

Python Implementation

# Basic try-except
try:
    result = 10 / 0
except ZeroDivisionError:
    print("Cannot divide by zero")

# Multiple exceptions
try:
    value = int("abc")
    data = {"key": "value"}
    item = data["missing"]
except ValueError:
    print("Invalid value")
except KeyError:
    print("Key not found")
except Exception as e:
    print(f"Unexpected error: {e}")

# Try-except-else-finally
try:
    file = open("data.txt", "r")
    content = file.read()
except FileNotFoundError:
    print("File not found")
else:
    print("File read successfully")
finally:
    if 'file' in locals():
        file.close()

# Raising exceptions
def divide(a, b):
    if b == 0:
        raise ValueError("Divider cannot be zero")
    return a / b

# Custom exception
class DataValidationError(Exception):
    def __init__(self, message, field=None):
        super().__init__(message)
        self.field = field

# Assert for debugging
assert len(data) > 0, "Data cannot be empty"

When to Use

• Handling missing files and data
• Validating user input and data
• Managing API errors and network issues
• Graceful handling of division by zero
• Working with missing dictionary keys
• Creating custom error handling for specific domains

Key Takeaways

1. Catch specific exceptions rather than using broad except clauses
2. Use finally to ensure cleanup code runs regardless of exceptions
3. Custom exceptions help create domain-specific error handling
4. Always log exceptions for debugging and monitoring
5. Avoid catching exceptions unless you can handle them meaningfully