Flask — The Python Micro Web Framework

What Flask Does

• Routing — decorator-based URL rules
• Templates — Jinja2 template engine
• Request/response — Werkzeug WSGI utilities
• Blueprints — modular application components
• Extensions — Flask-SQLAlchemy, Flask-Login, Flask-CORS, Flask-Migrate, Flask-RESTful
• CLI — flask run, custom commands
• Sessions — cookie-based, configurable
• Testing — built-in test client
• Context — application and request context management
• Signals — Blinker-based event system

Architecture

WSGI application. Request enters Werkzeug server, Flask routes it by matching URL rules (decorated functions). Request context is pushed per-request, providing access to request, g, session globals. Jinja2 renders HTML. Blueprints organize large apps into reusable modules.

Self-Hosting

# Production (never use flask run in prod)
pip install gunicorn
gunicorn app:app --bind 0.0.0.0:8000 --workers 4

# Docker
FROM python:3.12-slim
COPY requirements.txt .
RUN pip install -r requirements.txt
COPY . .
CMD ["gunicorn", "app:app", "--bind", "0.0.0.0:8000"]

Key Features

• Minimal core (~1500 lines)
• Decorator-based routing
• Jinja2 templates
• Werkzeug request/response
• Blueprints for modularity
• Extensive extension ecosystem
• Built-in debugger
• Test client
• CLI via Click
• Flask 3.0+ requires Python 3.8+

Comparison

FAQ

Q: Flask vs FastAPI? A: FastAPI has native async, built-in OpenAPI docs, and Pydantic validation — good for modern APIs; Flask is simpler with an older ecosystem and more tutorials. For new pure-API projects, FastAPI is recommended; Flask is great for full-stack or existing projects.

Q: Does Flask support async? A: Flask 2.0+ supports async views, but the underlying layer is still WSGI (async runs in a threadpool). For true async I/O, use FastAPI or Quart (Flask's async fork).

Q: How to organize large projects? A: Use Blueprints to split modules by feature. The Application Factory pattern (a create_app() function) supports different environment configurations.

Sources

• Docs: https://flask.palletsprojects.com
• GitHub: https://github.com/pallets/flask
• License: BSD 3-Clause