Master error handling patterns across languages including exceptions, Result types, error propagation, and graceful degradation to build resilient applications. Use when implementing error handling, designing APIs, or improving application reliability.
apm install @wshobson/error-handling-patterns
[](https://apm-p1ls2dz87-atlamors-projects.vercel.app/packages/@wshobson/error-handling-patterns)---
name: error-handling-patterns
description: Master error handling patterns across languages including exceptions, Result types, error propagation, and graceful degradation to build resilient applications. Use when implementing error handling, designing APIs, or improving application reliability.
---
# 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:**
```python
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:**
```python
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:**
```python
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:**
```typescript
// Custom error classes
class ApplicationError extends Error {
constructor(
message: string,
public code: string,
public statusCode: number = 500,
public details?: Record<string, any>,
) {
super(message);
this.name = this.constructor.name;
Error.captureStackTrace(this, this.constructor);
}
}
class ValidationError extends ApplicationError {
constructor(message: string, details?: Record<string, any>) {
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:**
```typescript
// Result type for explicit error handling
type Result<T, E = Error> = { ok: true; value: T } | { ok: false; error: E };
// Helper functions
function Ok<T>(value: T): Result<T, never> {
return { ok: true, value };
}
function Err<E>(error: E): Result<never, E> {
return { ok: false, error };
}
// Usage
function parseJSON<T>(json: string): Result<T, SyntaxError> {
try {
const value = JSON.parse(json) as T;
return Ok(value);
} catch (error) {
return Err(error as SyntaxError);
}
}
// Consuming Result
const result = parseJSON<User>(userJson);
if (result.ok) {
console.log(result.value.name);
} else {
console.error("Parse failed:", result.error.message);
}
// Chaining Results
function chain<T, U, E>(
result: Result<T, E>,
fn: (value: T) => Result<U, E>,
): Result<U, E> {
return result.ok ? fn(result.value) : result;
}
```
**Async Error Handling:**
```typescript
// Async/await with proper error handling
async function fetchUserOrders(userId: string): Promise<Order[]> {
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<Data> {
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:**
```rust
use std::fs::File;
use std::io::{self, Read};
// Result type for operations that can fail
fn read_file(path: &str) -> Result<String, io::Error> {
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<io::Error> for AppError {
fn from(error: io::Error) -> Self {
AppError::Io(error)
}
}
// Using custom error type
fn read_number_from_file(path: &str) -> Result<i32, AppError> {
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<User> {
users.iter().find(|u| u.id == id).cloned()
}
// Combining Option and Result
fn get_user_age(id: &str) -> Result<u32, AppError> {
find_user(id)
.ok_or_else(|| AppError::NotFound(id.to_string()))
.map(|user| user.age)
}
```
### Go Error Handling
**Explicit Error Returns:**
```go
// 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.
```python
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.
```typescript
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.
```python
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
1. **Fail Fast**: Validate input early, fail quickly
2. **Preserve Context**: Include stack traces, metadata, timestamps
3. **Meaningful Messages**: Explain what happened and how to fix it
4. **Log Appropriately**: Error = log, expected failure = don't spam logs
5. **Handle at Right Level**: Catch where you can meaningfully handle
6. **Clean Up Resources**: Use try-finally, context managers, defer
7. **Don't Swallow Errors**: Log or re-throw, don't silently ignore
8. **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