Exceptions & Logging

Good error handling makes your code reliable. Good logging makes debugging easy.

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Exception Handling Basics

try:
    result = 10 / 0
except ZeroDivisionError as e:
    print(f"Error: {e}")

Full Try/Except Structure

import json

try:
    data = load_config("config.json")
except FileNotFoundError:
    print("Config file not found")
except json.JSONDecodeError:
    print("Config file has invalid JSON")
else:
    print("Config loaded successfully")
finally:
    print("This always runs")

Block	When It Runs
try	Always — the main code
except	Only if an error happens
else	Only if no error happens
finally	Always — cleanup code

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Common Built-in Exceptions

Exception	When It Happens
ValueError	Wrong value (e.g., int("abc"))
TypeError	Wrong type (e.g., "a" + 1)
KeyError	Missing dictionary key
IndexError	List index out of range
FileNotFoundError	File does not exist
AttributeError	Object has no such attribute
TimeoutError	Operation took too long
ConnectionError	Network connection failed

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Custom Exceptions

Create your own exceptions for specific error cases:

class ApiError(Exception):
    def __init__(self, status_code: int, message: str) -> None:
        self.status_code = status_code
        self.message = message
        super().__init__(f"API Error {status_code}: {message}")

class NotFoundError(ApiError):
    def __init__(self, resource: str) -> None:
        super().__init__(404, f"{resource} not found")

Using Custom Exceptions

def get_user(user_id: int) -> dict:
    response = fetch(f"/users/{user_id}")
    if response.status_code == 404:
        raise NotFoundError(f"User {user_id}")
    return response.json()

try:
    user = get_user(999)
except NotFoundError as e:
    print(e)  # "API Error 404: User 999 not found"

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Logging

Basic Setup

import logging

logging.basicConfig(
    level=logging.DEBUG,
    format="%(asctime)s [%(levelname)s] %(name)s: %(message)s",
)
logger = logging.getLogger(__name__)

Logging Levels

Level	Use For
DEBUG	Detailed info for debugging
INFO	General events (test started, step completed)
WARNING	Something unexpected but not an error
ERROR	An error happened but the program continues
CRITICAL	A serious error — program may stop

Using the Logger

logger.debug("Connecting to database at %s", db_host)
logger.info("Test suite started with %d tests", test_count)
logger.warning("Retry attempt %d of %d", attempt, max_retries)
logger.error("Failed to load config: %s", error_message)

Use lazy formatting

Use logger.info("Value: %s", value) instead of logger.info(f"Value: {value}"). This avoids formatting the string if the log level is disabled.

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Logging in Tests

import logging

logger = logging.getLogger(__name__)

def test_user_creation():
    logger.info("Creating test user")
    user = create_user("Alice")
    logger.debug("User created: %s", user)
    assert user["name"] == "Alice"
    logger.info("Test passed")

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Best Practices

• Never use bare except: — always catch specific exceptions
• Never silently ignore exceptions (empty except block)
• Log error context, not just the error message
• Use custom exceptions for your application's error cases
• Use finally for cleanup (close files, connections)
• Set log level to DEBUG during development, INFO in production
• Add meaningful log messages at key points in your code
• Use structured logging for easier analysis