FastAPI is a modern async-first framework built on Starlette that automatically generates API documentation and offers 2-3x faster performance than Flask, while Flask is a lightweight, mature synchronous framework with a gentler learning curve and broader ecosystem. FastAPI excels for high-performance APIs, while Flask remains ideal for simple applications and traditional web projects.
FastAPI vs Flask — Side-by-side comparison on A Versus B
Modern async Python web framework for building fast APIs with automatic documentation.
Lightweight, synchronous Python web framework with minimal dependencies and maximum flexibility.
Request Handling Speed. Async Support. Automatic API Documentation. Data Validation. Learning Curve. Ecosystem Maturity. Community Size
Choose FastAPI if you're building high-performance REST APIs, microservices, or modern async applications that require automatic documentation and built-in validation. Choose Flask if you're building simple web applications, prototypes, or prefer a lightweight, well-established framework with the largest Python web community.
FastAPI: No, requires build | Flask: N/A
FastAPI: Full native support | Flask: N/A
FastAPI: 22% (Stack Overflow 2024) | Flask: N/A
FastAPI: Native (Swagger UI + ReDoc built-in) | Flask: N/A
FastAPI: Native Python type hints with validation | Flask: N/A
FastAPI: ~15,000 | Flask: ~2,500 req/sec
FastAPI: Built-in (OpenAPI 3.0) | Flask: Manual integration required
FastAPI: Yes, first-class | Flask: No, sync-only
FastAPI: Python 3.6+ | Flask: Python 2.7+ (legacy) / 3.4+
FastAPI: Automatic (Swagger UI + ReDoc) | Flask: N/A
FastAPI: Requires app server (Uvicorn) | Flask: N/A
FastAPI: Yes (Pydantic native) | Flask: N/A
FastAPI: ~30,000 questions | Flask: 40,000+
FastAPI: 3.5 hours (manual setup required) | Flask: N/A
FastAPI: No (requires external library) | Flask: N/A
FastAPI: Built-in (Swagger UI & ReDoc) | Flask: Requires extension
FastAPI: Yes (default async/await) | Flask: N/A
FastAPI: ~480,000 packages (PyPI) | Flask: N/A
FastAPI: ~1.2M (PyPI: ~2.8M) | Flask: N/A
FastAPI: Manual setup required | Flask: N/A
FastAPI: Native, default behavior | Flask: N/A
FastAPI: 1,200KB (with uvicorn) | Flask: N/A
FastAPI: Native async/await with asyncio | Flask: N/A
FastAPI: Pydantic models | Flask: Manual only
FastAPI: 500,000+ PyPI packages (Python ecosystem) | Flask: N/A
FastAPI: 6 years (released 2018) | Flask: N/A
Production Applications (market estimate)
FastAPI: ~32,000 req/s | Flask: ~2,100 req/s
FastAPI: Yes (Swagger UI + ReDoc automatic) | Flask: N/A
FastAPI: Yes (Pydantic built-in) | Flask: N/A
FastAPI: Moderate (3.5/5) | Flask: Easy (1.5/5)
FastAPI: Full (enforced) | Flask: Optional
FastAPI: N/A | Flask: None (use SQLAlchemy separately)
FastAPI: N/A | Flask: Requires manual or third-party setup
FastAPI: N/A | Flask: No (use Flask-Login or similar)
FastAPI: N/A | Flask: ~35 MB per instance
FastAPI: N/A | Flask: No (requires Flask-SocketIO)
FastAPI: N/A | Flask: Native (NumPy, TensorFlow, Pandas)
FastAPI: N/A | Flask: Via SQLAlchemy extension
FastAPI: N/A | Flask: Yes (Werkzeug-based)
Average Community Response Time (GitHub Issues)
FastAPI: N/A | Flask: Synchronous (WSGI)
FastAPI vs Flask — Structured Comparison Data
Machine-readable comparison dataset for FastAPI vs Flask. Contains 119 attribute comparisons, 5 FAQ pairs.
FastAPI vs Flask — Attribute Category Vocabulary
Taxonomy of attribute categories used to classify comparison data for FastAPI vs Flask.
Yes, FastAPI is significantly faster. Benchmarks show FastAPI handles 1,200-1,400 requests/second compared to Flask's 400-600 req/s—roughly 2-3x faster. This is due to FastAPI's native async/await support and ASGI server architecture (Uvicorn) versus Flask's synchronous WSGI design. For I/O-heavy operations (database queries, API calls), the difference is even more pronounced.
Flask 2.0+ has basic async support via Quart (an async ASGI alternative) or by using extensions like Flask-AsyncIO, but it's not native or seamless. FastAPI makes async a first-class citizen with full async/await support built-in. For applications requiring heavy async workloads, FastAPI is the better choice.
Which framework should a beginner choose?
Flask is better for beginners. It has a gentler learning curve, requires minimal setup, and has more tutorials online (68.5k GitHub stars vs 24.5k). FastAPI requires understanding async/await concepts upfront. Start with Flask to learn web framework fundamentals, then migrate to FastAPI when you need performance.
Does FastAPI automatically document my API?
Yes. FastAPI automatically generates interactive Swagger UI and ReDoc documentation based on your code's type hints and Pydantic models. Flask requires manual integration with extensions like Flask-RESTX or Flasgger to achieve similar results. This saves significant development time.
Which has a larger community and more extensions?
Flask has a larger community (68.5k GitHub stars, 15+ years of development) and more third-party extensions. FastAPI (24.5k stars, released 2018) is smaller but growing rapidly. For niche use cases, Flask likely has more plug-and-play solutions, while FastAPI encourages building custom solutions using modern Python patterns.
Request Handling Speed: FastAPI has ~1,200-1,400 req/s (async), while Flask has ~400-600 req/s (sync).
Async Support: FastAPI has Native async/await built-in, while Flask has No native async support (requires extensions).
Automatic API Documentation: FastAPI has Built-in Swagger UI & ReDoc, while Flask has Requires third-party extensions.
Data Validation: FastAPI has Built-in Pydantic validation, while Flask has Manual validation required.
Learning Curve: FastAPI has Moderate (async concepts required), while Flask has Very gentle (minimal setup).
Ecosystem Maturity: FastAPI has Modern (released 2018, rapidly growing), while Flask has Mature (released 2010, stable).
Community Size: FastAPI has ~24.5k GitHub stars (2026), while Flask has ~68.5k GitHub stars (2026).