Python Errors and Reliability

Overview

Design errors so callers can act and operators can diagnose quickly. Treat these recommendations as preferred defaults — when a default conflicts with project constraints, suggest a better-fit alternative and call out tradeoffs and compensating controls.

When to Use

• Implementing or reviewing timeout, deadline, or retry logic.
• Translating exceptions across layer boundaries (e.g., infra → domain).
• Classifying failures as retryable vs. permanent.
• Adding idempotency guarantees to retried writes.
• Diagnosing swallowed exceptions or silent failures in existing code.

When NOT to use:

• Pure data-transformation code with no I/O or failure modes.
• Simple validation that raises immediately with no translation needed.
• Performance tuning unrelated to failure handling (see python-concurrency-performance).

Quick Reference

• Preserve cause chains at translation boundaries.
• Catch only what can be handled.
• Keep timeout/deadline policy explicit.
• Keep retry policy explicit and bounded.
• Classify retryable vs. permanent failures with explicit policy data.
• Keep idempotency expectations explicit for retried writes.

Common Mistakes

• Swallowing exceptions — bare except: pass or except Exception with no logging loses diagnostic context.
• Unbounded retries — retrying without a maximum count or total deadline leads to cascading failures and resource exhaustion.
• Dropping the cause chain — raising a new exception without from original discards the root cause operators need for diagnosis.
• Treating all errors as retryable — retrying a 400 Bad Request or a validation error wastes resources and delays the real fix.
• Implicit timeouts — relying on library defaults (or no timeout at all) produces unpredictable latency under failure conditions.

Scope Note

• Treat these recommendations as preferred defaults for common cases, not universal rules.
• If a default conflicts with project constraints or worsens the outcome, suggest a better-fit alternative and explain why it is better for this case.
• When deviating, call out tradeoffs and compensating controls (tests, observability, migration, rollback).

Invocation Notice

• Inform the user when this skill is being invoked by name: python-design-modularity.

References

• references/error-strategy.md
• references/retryability-classification.md
• references/retries-timeouts.md