Error Handling: Graceful Failure and Recovery Patterns

Why Error Handling Matters

Errors happen. Good error handling:

• Prevents crashes


• Provides useful information


• Enables recovery


• Maintains user trust

Types of Errors

Expected Errors

Things that might reasonably fail:

• Network requests


• File operations


• User input validation


• External API calls

Unexpected Errors

Bugs and edge cases:

• Null pointer exceptions


• Type errors


• Logic errors


• Resource exhaustion

Fatal Errors

Unrecoverable situations:

• Out of memory


• Disk full


• Critical dependency missing

Basic Error Handling

Try/Catch

try:
    result = risky_operation()
except SpecificError as e:
    handle_specific_error(e)
except Exception as e:
    handle_general_error(e)
finally:
    cleanup()

Error Types

Catch specific errors when possible:

try:
    data = json.loads(text)
except json.JSONDecodeError as e:
    logger.error(f"Invalid JSON: {e}")
    return None

Don't Swallow Errors

# Bad - hides problems
try:
    do_something()
except:
    pass

# Good - at least log it
try:
    do_something()
except Exception as e:
    logger.error(f"Operation failed: {e}")

Recovery Strategies

Retry with Backoff

def retry_with_backoff(func, max_retries=3):
    for attempt in range(max_retries):
        try:
            return func()
        except TransientError:
            wait = 2 ** attempt
            time.sleep(wait)
    raise Exception("Max retries exceeded")

Fallback Values

def get_config(key):
    try:
        return fetch_remote_config(key)
    except NetworkError:
        return DEFAULT_CONFIG.get(key)

Graceful Degradation

def get_user_data(user_id):
    try:
        return get_full_profile(user_id)
    except ProfileServiceError:
        # Return basic data instead
        return get_cached_profile(user_id)

Circuit Breaker

class CircuitBreaker:
    def __init__(self, threshold=5):
        self.failures = 0
        self.threshold = threshold
        self.open = False
    
    def call(self, func):
        if self.open:
            raise CircuitOpenError()
        try:
            result = func()
            self.failures = 0
            return result
        except Exception:
            self.failures += 1
            if self.failures >= self.threshold:
                self.open = True
            raise

Communicating Errors

To Logs

logger.error(f"Failed to process order {order_id}: {e}", exc_info=True)

To Users/Humans

"I encountered an error while processing that request.
The file couldn't be read - it may be corrupted or missing.
Would you like me to try an alternative approach?"

Error Messages

Bad:

"Error: NullPointerException at line 42"

Good:

"Couldn't load user profile - the user may not exist.
Check the user ID and try again."

Error Patterns

Validation First

def process_order(order):
    # Validate before processing
    errors = validate_order(order)
    if errors:
        raise ValidationError(errors)
    
    # Now safe to process
    return do_processing(order)

Error Aggregation

def validate_form(data):
    errors = []
    
    if not data.get('email'):
        errors.append("Email is required")
    if not data.get('name'):
        errors.append("Name is required")
    
    if errors:
        raise ValidationError(errors)

Context Enrichment

try:
    process_item(item)
except ProcessingError as e:
    raise ProcessingError(
        f"Failed to process item {item.id}: {e}"
    ) from e

Best Practices

Be Specific

# Too broad
except Exception:
    pass

# Better
except (ConnectionError, TimeoutError) as e:
    handle_network_error(e)

Include Context

# Not helpful
raise ValueError("Invalid value")

# Helpful
raise ValueError(f"Age must be positive, got {age}")

Clean Up Resources

file = None
try:
    file = open(path)
    process(file)
finally:
    if file:
        file.close()

# Or use context managers
with open(path) as file:
    process(file)

Fail Fast

def process(data):
    if data is None:
        raise ValueError("Data cannot be None")
    # Continue with valid data

Conclusion

Good error handling:

• Catches specific errors


• Provides useful context


• Enables recovery when possible


• Communicates clearly


• Cleans up resources

Expect errors. Handle them gracefully. For related patterns, see debugging strategies when things go wrong.

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