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.) Language-Specific Patterns Python Error Handling

Custom Exception Hierarchy:

class ApplicationError(Exception): """Base exception for all application errors.""" def init(self, message: str, code: str = None, details: dict = None): super().init(message) self.code = code self.details = details or {} self.timestamp = datetime.utcnow()

class ValidationError(ApplicationError): """Raised when validation fails.""" pass

class NotFoundError(ApplicationError): """Raised when resource not found.""" pass

class ExternalServiceError(ApplicationError): """Raised when external service fails.""" def init(self, message: str, service: str, kwargs): super().init(message, kwargs) self.service = service

Usage

def get_user(user_id: str) -> User: user = db.query(User).filter_by(id=user_id).first() if not user: raise NotFoundError( f"User not found", code="USER_NOT_FOUND", details={"user_id": user_id} ) return user

Context Managers for Cleanup:

from contextlib import contextmanager

@contextmanager def database_transaction(session): """Ensure transaction is committed or rolled back.""" try: yield session session.commit() except Exception as e: session.rollback() raise finally: session.close()

Usage

with database_transaction(db.session) as session: user = User(name="Alice") session.add(user) # Automatic commit or rollback

Retry with Exponential Backoff:

import time from functools import wraps from typing import TypeVar, Callable

T = TypeVar('T')

def retry( max_attempts: int = 3, backoff_factor: float = 2.0, exceptions: tuple = (Exception,) ): """Retry decorator with exponential backoff.""" def decorator(func: Callable[..., T]) -> Callable[..., T]: @wraps(func) def wrapper(args, kwargs) -> T: last_exception = None for attempt in range(max_attempts): try: return func(args, **kwargs) except exceptions as e: last_exception = e if attempt < max_attempts - 1: sleep_time = backoff_factor ** attempt time.sleep(sleep_time) continue raise raise last_exception return wrapper return decorator

Usage

@retry(max_attempts=3, exceptions=(NetworkError,)) def fetch_data(url: str) -> dict: response = requests.get(url, timeout=5) response.raise_for_status() return response.json()

TypeScript/JavaScript Error Handling

Custom Error Classes:

// Custom error classes class ApplicationError extends Error { constructor( message: string, public code: string, public statusCode: number = 500, public details?: Record, ) { super(message); this.name = this.constructor.name; Error.captureStackTrace(this, this.constructor); } }

class ValidationError extends ApplicationError { constructor(message: string, details?: Record) { super(message, "VALIDATION_ERROR", 400, details); } }

class NotFoundError extends ApplicationError { constructor(resource: string, id: string) { super(${resource} not found, "NOT_FOUND", 404, { resource, id }); } }

// Usage function getUser(id: string): User { const user = users.find((u) => u.id === id); if (!user) { throw new NotFoundError("User", id); } return user; }

Result Type Pattern:

// Result type for explicit error handling type Result = { ok: true; value: T } | { ok: false; error: E };

// Helper functions function Ok(value: T): Result { return { ok: true, value }; }

function Err(error: E): Result { return { ok: false, error }; }

// Usage function parseJSON(json: string): Result { try { const value = JSON.parse(json) as T; return Ok(value); } catch (error) { return Err(error as SyntaxError); } }

// Consuming Result const result = parseJSON(userJson); if (result.ok) { console.log(result.value.name); } else { console.error("Parse failed:", result.error.message); }

// Chaining Results function chain( result: Result, fn: (value: T) => Result, ): Result { return result.ok ? fn(result.value) : result; }

Async Error Handling:

// Async/await with proper error handling async function fetchUserOrders(userId: string): Promise { try { const user = await getUser(userId); const orders = await getOrders(user.id); return orders; } catch (error) { if (error instanceof NotFoundError) { return []; // Return empty array for not found } if (error instanceof NetworkError) { // Retry logic return retryFetchOrders(userId); } // Re-throw unexpected errors throw error; } }

// Promise error handling function fetchData(url: string): Promise { return fetch(url) .then((response) => { if (!response.ok) { throw new NetworkError(HTTP ${response.status}); } return response.json(); }) .catch((error) => { console.error("Fetch failed:", error); throw error; }); }

Rust Error Handling

Result and Option Types:

use std::fs::File; use std::io::{self, Read};

// Result type for operations that can fail fn read_file(path: &str) -> Result { let mut file = File::open(path)?; // ? operator propagates errors let mut contents = String::new(); file.read_to_string(&mut contents)?; Ok(contents) }

// Custom error types

[derive(Debug)]

enum AppError { Io(io::Error), Parse(std::num::ParseIntError), NotFound(String), Validation(String), }

impl From for AppError { fn from(error: io::Error) -> Self { AppError::Io(error) } }

// Using custom error type fn read_number_from_file(path: &str) -> Result { let contents = read_file(path)?; // Auto-converts io::Error let number = contents.trim().parse() .map_err(AppError::Parse)?; // Explicitly convert ParseIntError Ok(number) }

// Option for nullable values fn find_user(id: &str) -> Option { users.iter().find(|u| u.id == id).cloned() }

// Combining Option and Result fn get_user_age(id: &str) -> Result { find_user(id) .ok_or_else(|| AppError::NotFound(id.to_string())) .map(|user| user.age) }

Go Error Handling

Explicit Error Returns:

// Basic error handling func getUser(id string) (*User, error) { user, err := db.QueryUser(id) if err != nil { return nil, fmt.Errorf("failed to query user: %w", err) } if user == nil { return nil, errors.New("user not found") } return user, nil }

// Custom error types type ValidationError struct { Field string Message string }

func (e *ValidationError) Error() string { return fmt.Sprintf("validation failed for %s: %s", e.Field, e.Message) }

// Sentinel errors for comparison var ( ErrNotFound = errors.New("not found") ErrUnauthorized = errors.New("unauthorized") ErrInvalidInput = errors.New("invalid input") )

// Error checking user, err := getUser("123") if err != nil { if errors.Is(err, ErrNotFound) { // Handle not found } else { // Handle other errors } }

// Error wrapping and unwrapping func processUser(id string) error { user, err := getUser(id) if err != nil { return fmt.Errorf("process user failed: %w", err) } // Process user return nil }

// Unwrap errors err := processUser("123") if err != nil { var valErr *ValidationError if errors.As(err, &valErr) { fmt.Printf("Validation error: %s\n", valErr.Field) } }

Universal Patterns Pattern 1: Circuit Breaker

Prevent cascading failures in distributed systems.

from enum import Enum from datetime import datetime, timedelta from typing import Callable, TypeVar

T = TypeVar('T')

class CircuitState(Enum): CLOSED = "closed" # Normal operation OPEN = "open" # Failing, reject requests HALF_OPEN = "half_open" # Testing if recovered

class CircuitBreaker: def init( self, failure_threshold: int = 5, timeout: timedelta = timedelta(seconds=60), success_threshold: int = 2 ): self.failure_threshold = failure_threshold self.timeout = timeout self.success_threshold = success_threshold self.failure_count = 0 self.success_count = 0 self.state = CircuitState.CLOSED self.last_failure_time = None

def call(self, func: Callable[[], T]) -> T:
    if self.state == CircuitState.OPEN:
        if datetime.now() - self.last_failure_time > self.timeout:
            self.state = CircuitState.HALF_OPEN
            self.success_count = 0
        else:
            raise Exception("Circuit breaker is OPEN")

    try:
        result = func()
        self.on_success()
        return result
    except Exception as e:
        self.on_failure()
        raise

def on_success(self):
    self.failure_count = 0
    if self.state == CircuitState.HALF_OPEN:
        self.success_count += 1
        if self.success_count >= self.success_threshold:
            self.state = CircuitState.CLOSED
            self.success_count = 0

def on_failure(self):
    self.failure_count += 1
    self.last_failure_time = datetime.now()
    if self.failure_count >= self.failure_threshold:
        self.state = CircuitState.OPEN

Usage

circuit_breaker = CircuitBreaker()

def fetch_data(): return circuit_breaker.call(lambda: external_api.get_data())

Pattern 2: Error Aggregation

Collect multiple errors instead of failing on first error.

class ErrorCollector { private errors: Error[] = [];

add(error: Error): void { this.errors.push(error); }

hasErrors(): boolean { return this.errors.length > 0; }

getErrors(): Error[] { return [...this.errors]; }

throw(): never { if (this.errors.length === 1) { throw this.errors[0]; } throw new AggregateError( this.errors, ${this.errors.length} errors occurred, ); } }

// Usage: Validate multiple fields function validateUser(data: any): User { const errors = new ErrorCollector();

if (!data.email) { errors.add(new ValidationError("Email is required")); } else if (!isValidEmail(data.email)) { errors.add(new ValidationError("Email is invalid")); }

if (!data.name || data.name.length < 2) { errors.add(new ValidationError("Name must be at least 2 characters")); }

if (!data.age || data.age < 18) { errors.add(new ValidationError("Age must be 18 or older")); }

if (errors.hasErrors()) { errors.throw(); }

return data as User; }

Pattern 3: Graceful Degradation

Provide fallback functionality when errors occur.

from typing import Optional, Callable, TypeVar

T = TypeVar('T')

def with_fallback( primary: Callable[[], T], fallback: Callable[[], T], log_error: bool = True ) -> T: """Try primary function, fall back to fallback on error.""" try: return primary() except Exception as e: if log_error: logger.error(f"Primary function failed: {e}") return fallback()

Usage

def get_user_profile(user_id: str) -> UserProfile: return with_fallback( primary=lambda: fetch_from_cache(user_id), fallback=lambda: fetch_from_database(user_id) )

Multiple fallbacks

def get_exchange_rate(currency: str) -> float: return ( try_function(lambda: api_provider_1.get_rate(currency)) or try_function(lambda: api_provider_2.get_rate(currency)) or try_function(lambda: cache.get_rate(currency)) or DEFAULT_RATE )

def try_function(func: Callable[[], Optional[T]]) -> Optional[T]: try: return func() except Exception: return None

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

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 Resources references/exception-hierarchy-design.md: Designing error class hierarchies references/error-recovery-strategies.md: Recovery patterns for different scenarios references/async-error-handling.md: Handling errors in concurrent code assets/error-handling-checklist.md: Review checklist for error handling assets/error-message-guide.md: Writing helpful error messages scripts/error-analyzer.py: Analyze error patterns in logs