FastAPI is a modern Python web framework built on Starlette that prioritizes speed and automatic API documentation with async support by default, while Flask is a lightweight, minimalist framework that emphasizes simplicity and flexibility with synchronous request handling. FastAPI's built-in features result in ~3x faster request handling and automatic OpenAPI/Swagger documentation, whereas Flask requires manual setup for these capabilities.
FastAPI vs Flask — Side-by-side comparison on A Versus B
Choose FastAPI if you're building modern REST APIs that require high performance, automatic documentation, async operations, or type safety with built-in validation—ideal for microservices, real-time applications, and teams prioritizing developer experience. Choose Flask if you prioritize simplicity, have an existing codebase, need a lightweight framework for small projects, or prefer minimal dependencies and maximum flexibility in architecture decisions.
Modern, fast Python web framework for building APIs with automatic documentation and async-first design.
Lightweight Python WSGI web framework for building traditional web applications and APIs.
Ecosystem Maturity (years since release)
Structured comparison data for FastAPI vs Flask with 161 attributes.
Production Applications (market estimate)
Production Readiness Without External Server
Average Community Response Time (GitHub Issues)
Third-party Packages Required (typical CRUD)
Job Market Demand (LinkedIn postings 2026)
FastAPI vs Flask — Attribute Category Vocabulary
Taxonomy of attribute categories used to classify comparison data for FastAPI vs Flask.
Which framework is faster, FastAPI or Flask?
FastAPI is approximately 3x faster than Flask. Benchmark testing shows FastAPI handles ~12,000 requests/second while Flask handles ~4,000 requests/second on comparable hardware. This performance advantage comes from FastAPI's async-first design built on Starlette, whereas Flask handles requests synchronously by default. For most small projects, this difference is negligible, but for high-traffic APIs or real-time systems, FastAPI's throughput advantage is significant.
Flask 2.0+ (released 2021) added native async/await support for view functions, but it remains limited. Flask's async implementation doesn't provide the same concurrency benefits as FastAPI because Flask's ecosystem (middleware, extensions) is designed synchronously. To use async effectively in Flask, you typically need to use external libraries like gevent or implement async manually. FastAPI's async support is first-class and deeply integrated throughout the framework, making it the better choice for async-heavy applications.
Do I need to manually document my API with FastAPI?
No. FastAPI automatically generates interactive OpenAPI (Swagger UI) and ReDoc documentation from your code's type hints and docstrings. Simply define your endpoints with Pydantic models and type annotations, and documentation is instantly available at `/docs` (Swagger) and `/redoc` (ReDoc). Flask requires manual documentation setup using tools like Flasgger, Connexion, or manually writing OpenAPI/Swagger YAML files. This saves significant development time with FastAPI.
Yes, Flask remains highly relevant and production-ready, especially for small-to-medium projects, prototypes, and teams that value simplicity. Flask's mature ecosystem (18+ years), extensive community resources, and proven track record make it ideal for educational projects and systems where flexibility matters more than raw performance. However, for new API-heavy projects, teams increasingly prefer FastAPI's built-in features and modern async support. Many organizations continue maintaining Flask applications successfully.
Which framework should a beginner choose?
Flask is better for beginners due to its minimal learning curve (5-10 hours to first API) and straightforward syntax. FastAPI's type hints, async concepts, and automatic validation—while powerful—require more foundational Python knowledge. Beginners typically learn Flask faster and can build functional applications immediately. Once comfortable with Python and async concepts, graduating to FastAPI is straightforward. Starting with Flask is a reasonable path for long-term learning.