Error Handling Patterns

Build resilient applications with robust error handling strategies that gracefully handle failures and provide excellent debugging experiences.

When to Use This Skill

Implementing error handling in new features
Designing error-resilient APIs
Debugging production issues
Improving application reliability
Creating better error messages for users and developers
Implementing retry and circuit breaker patterns
Handling async/concurrent errors
Building fault-tolerant distributed systems

Core Concepts

1. Error Handling Philosophies

Exceptions vs Result Types:

Exceptions: Traditional try-catch, disrupts control flow
Result Types: Explicit success/failure, functional approach
Error Codes: C-style, requires discipline
Option/Maybe Types: For nullable values

When to Use Each:

Exceptions: Unexpected errors, exceptional conditions
Result Types: Expected errors, validation failures
Panics/Crashes: Unrecoverable errors, programming bugs

2. Error Categories

Recoverable Errors:

Network timeouts
Missing files
Invalid user input
API rate limits

Unrecoverable Errors:

Out of memory
Stack overflow
Programming bugs (null pointer, etc.)

Detailed patterns and worked examples

Detailed pattern documentation lives in references/details.md. Read that file when the navigation tier above is insufficient.

Best Practices

Fail Fast: Validate input early, fail quickly
Preserve Context: Include stack traces, metadata, timestamps
Meaningful Messages: Explain what happened and how to fix it
Log Appropriately: Error = log, expected failure = don't spam logs
Handle at Right Level: Catch where you can meaningfully handle
Clean Up Resources: Use try-finally, context managers, defer
Don't Swallow Errors: Log or re-throw, don't silently ignore
Type-Safe Errors: Use typed errors when possible

python

# Good error handling example
def process_order(order_id: str) -> Order:
    """Process order with comprehensive error handling."""
    try:
        # Validate input
        if not order_id:
            raise ValidationError("Order ID is required")

        # Fetch order
        order = db.get_order(order_id)
        if not order:
            raise NotFoundError("Order", order_id)

        # Process payment
        try:
            payment_result = payment_service.charge(order.total)
        except PaymentServiceError as e:
            # Log and wrap external service error
            logger.error(f"Payment failed for order {order_id}: {e}")
            raise ExternalServiceError(
                f"Payment processing failed",
                service="payment_service",
                details={"order_id": order_id, "amount": order.total}
            ) from e

        # Update order
        order.status = "completed"
        order.payment_id = payment_result.id
        db.save(order)

        return order

    except ApplicationError:
        # Re-raise known application errors
        raise
    except Exception as e:
        # Log unexpected errors
        logger.exception(f"Unexpected error processing order {order_id}")
        raise ApplicationError(
            "Order processing failed",
            code="INTERNAL_ERROR"
        ) from e

Common Pitfalls

Catching Too Broadly: except Exception hides bugs
Empty Catch Blocks: Silently swallowing errors
Logging and Re-throwing: Creates duplicate log entries
Not Cleaning Up: Forgetting to close files, connections
Poor Error Messages: "Error occurred" is not helpful
Returning Error Codes: Use exceptions or Result types
Ignoring Async Errors: Unhandled promise rejections