Flask Best Practices
Keep Flask 3.1 apps testable and production-safe.
How to Use This List
- Review during design reviews and before production deploys.
- Convert repeat failures into automated CI policy where possible.
- Revisit after framework minor upgrades.
Structure
- Use application factory and blueprints.
- Keep routes thin; services own logic.
- Avoid circular imports with a core package.
HTTP
- Validate JSON with Pydantic at the edge.
- Return stable error JSON shapes.
- Set body size limits at the proxy.
State
- Store per-request data on
g, not globals. - Configure sessions with secure cookie flags.
- Do not run SQLite on multi-worker writes.
Ops
- Run behind Gunicorn with multiple workers.
- Externalize secrets to env vars.
- Add health checks and structured logs.
FAQs
When should I adopt Flask best practices?
Use it when the patterns and trade-offs on this page match your API or data boundary.
What is the top production mistake with Flask best practices?
Skipping validation, timeouts, or explicit error contracts at the HTTP edge.
How do I test Flask best practices?
Use the framework test client, override dependencies, and assert status plus JSON shape.
Does Flask best practices work with Python 3.14?
Yes - examples target Python 3.14 with pinned framework versions from the stack footer.
How does Flask best practices relate to Pydantic 2?
Validate and serialize at boundaries; keep services working with typed domain objects.
Sync or async?
Prefer async routes when I/O dominates; keep CPU work small or offload to workers.
Where should business logic live?
Thin handlers; services own rules; repositories own queries.
How do I document APIs?
Publish OpenAPI or schema docs that match response models in code.
How do I handle versioning?
Explicit URL or header versioning with deprecation windows - avoid silent breaks.
What should I read next?
Follow the Related links for the next layer of depth in this section.
How do I stay secure?
Authenticate callers, authorize per resource, rate-limit, and never log secrets.
Performance first step?
Measure DB and upstream latency before swapping frameworks.
Related
- Flask Basics - Routes and requests
- Application Factories & Blueprints - Structure
- Building REST APIs - JSON endpoints
- Deploying Flask - WSGI production
Stack versions: This page was written for Python 3.14.0 (stable 3.14, maintenance 3.13), FastAPI 0.115+, Django 5.2, Flask 3.1, Pydantic 2, PyTorch 2.6+, pandas 2.2+, Polars 1.x, ruff 0.9+, and uv 0.6+.