Python Error Handling

Modern error handling, logging, and debugging patterns.

Core Principles

1. Specific exceptions - Catch specific types, not Exception
2. Fail fast - Raise errors early at source
3. Context in errors - Include relevant information
4. Log, don't just print - Structured, searchable logs

Exception Handling Patterns

Be specific about what you catch

# BAD - Too broad
try:
    result = process_data()
except:
    print("Error occurred")

# GOOD - Specific handling
try:
    result = process_data()
except FileNotFoundError as e:
    logger.error(f"Config file not found: {e}")
    use_defaults()
except ValueError as e:
    logger.error(f"Invalid data: {e}")
    raise
except Exception as e:
    logger.error(f"Unexpected error: {e}", exc_info=True)
    raise

See exception-patterns.md.

Custom Exceptions

Create domain-specific exceptions

class DataValidationError(ValueError):
    """Raised when data validation fails."""
    pass

class APIError(Exception):
    """Base exception for API errors."""
    def __init__(self, message: str, status_code: int):
        self.message = message
        self.status_code = status_code
        super().__init__(self.message)

class APITimeout(APIError):
    """API request timeout."""
    def __init__(self, message: str):
        super().__init__(message, status_code=504)

See custom-exceptions.md.

Context Managers for Cleanup

from contextlib import contextmanager

@contextmanager
def managed_resource(name):
    resource = acquire_resource(name)
    try:
        yield resource
    finally:
        release_resource(resource)  # Always called

with managed_resource("database") as db:
    db.query("SELECT * FROM users")
    # Even if exception, resource released

Logging with Structlog

import structlog

logger = structlog.get_logger()

def process_order(order_id: str):
    log = logger.bind(order_id=order_id)
    
    try:
        log.info("processing_order_started")
        result = process(order_id)
        log.info("processing_order_completed", result=result)
        return result
    except Exception as e:
        log.error("processing_order_failed", error=str(e))
        raise

See logging-patterns.md.

Debugging Tools

Use pdb for interactive debugging

def complex_function(data):
    # Set breakpoint
    import pdb; pdb.set_trace()
    
    # Or in Python 3.7+
    breakpoint()
    
    result = process(data)
    return result

See debugging-tools.md.

Error Recovery Patterns

from tenacity import retry, stop_after_attempt, wait_exponential

@retry(
    stop=stop_after_attempt(3),
    wait=wait_exponential(multiplier=1, min=4, max=10)
)
def unreliable_api_call():
    """Retry with exponential backoff."""
    response = requests.get("https://api.example.com")
    response.raise_for_status()
    return response.json()

See retry-patterns.md.

source: Python error handling best practices