Python Flask Cheat Sheet
Covers Flask app setup, routing, request and response handling, Jinja2 templates, and organizing routes with blueprints.
1 PageIntermediateMar 30, 2026
Minimal Flask App
The smallest possible Flask application.
python
from flask import Flaskapp = Flask(__name__)@app.route("/")def index(): return "Hello, Flask!"if __name__ == "__main__": app.run(debug=True) # Dev server with auto-reload# Run with: flask --app app run --debug# or: python app.py
Routes & Request Handling
Read query strings, JSON bodies, and path parameters.
python
from flask import request, jsonify@app.route("/users/<int:user_id>")def get_user(user_id): return jsonify({"id": user_id})@app.route("/search")def search(): query = request.args.get("q", "") # Query string param return jsonify({"query": query})@app.route("/users", methods=["POST"])def create_user(): data = request.get_json() # Parse JSON body name = data.get("name") return jsonify({"created": name}), 201
Templates (Jinja2)
Render server-side HTML with Jinja2.
python
from flask import render_template@app.route("/profile/<username>")def profile(username): return render_template("profile.html", username=username)# templates/profile.html# <h1>Hello, {{ username }}!</h1># {% if username == "admin" %}<p>Welcome back</p>{% endif %}# {% for item in items %}<li>{{ item }}</li>{% endfor %}
Blueprints
Organize routes into reusable modules.
python
# blog/routes.pyfrom flask import Blueprintbp = Blueprint("blog", __name__, url_prefix="/blog")@bp.route("/")def index(): return "Blog home"# app.pyfrom blog.routes import bpapp.register_blueprint(bp)
Flask Essentials
Frequently used objects and helpers.
- app.config- Dict-like object for app configuration (e.g. SECRET_KEY, DEBUG)
- request.form- Access form-encoded POST data
- request.args- Access URL query string parameters
- abort(404)- Short-circuit a view with an HTTP error response
- url_for('view_name')- Build a URL by endpoint name instead of hardcoding paths
- flash('message')- Queue a one-time message to show on the next rendered page
- g- Application context object for storing per-request data
Application Factory & Extensions
Avoid global app state so you can create multiple configured instances (testing, prod) cleanly.
python
from flask import Flaskfrom flask_sqlalchemy import SQLAlchemydb = SQLAlchemy()def create_app(config_object="config.ProductionConfig"): app = Flask(__name__) app.config.from_object(config_object) db.init_app(app) from .blog.routes import bp as blog_bp app.register_blueprint(blog_bp) return app# wsgi.py# from myapp import create_app# app = create_app()## Testing: app = create_app("config.TestConfig")
Application/Request Context & Hooks
Run code around every request and understand when Flask's context globals are valid.
python
from flask import g, requestimport time@app.before_requestdef start_timer(): g.start = time.monotonic()@app.after_requestdef log_duration(response): elapsed_ms = (time.monotonic() - g.start) * 1000 app.logger.info("%s %s took %.1fms", request.method, request.path, elapsed_ms) return response@app.teardown_appcontextdef close_db(exception=None): db_conn = g.pop("db_conn", None) if db_conn is not None: db_conn.close()# request/g are only valid DURING an active request/app context —# accessing them from a background thread raises RuntimeError.
Custom Error Handlers & JSON APIs
Turn exceptions into consistent, structured error responses instead of default HTML pages.
python
from flask import jsonifyfrom werkzeug.exceptions import HTTPExceptionclass APIError(Exception): def __init__(self, message, status_code=400): super().__init__(message) self.message = message self.status_code = status_code@app.errorhandler(APIError)def handle_api_error(err): return jsonify({"error": err.message}), err.status_code@app.errorhandler(HTTPException)def handle_http_error(err): return jsonify({"error": err.name, "description": err.description}), err.code@app.errorhandler(500)def handle_unexpected(err): app.logger.exception("Unhandled error") return jsonify({"error": "internal_server_error"}), 500
Testing with the Test Client & Custom CLI Commands
Exercise routes without a running server and script maintenance tasks via 'flask' commands.
python
# test_app.pydef test_create_user(client): resp = client.post("/users", json={"name": "Ada"}) assert resp.status_code == 201 assert resp.get_json()["created"] == "Ada"# conftest.pyimport pytestfrom myapp import create_app@pytest.fixturedef client(): app = create_app("config.TestConfig") with app.test_client() as c: yield c# Custom CLI command@app.cli.command("seed-db")def seed_db(): """flask seed-db -- populate initial data.""" db.session.add_all([...]) db.session.commit() print("Seeded.")
Advanced Flask Concepts
Patterns that come up once an app grows past a single-file prototype.
- async def view- Flask supports 'async def' route handlers directly (via ASGI-compatible servers); useful for awaiting I/O-bound calls
- Flask-Login- Extension providing session-based user authentication, current_user, and login_required
- Application Context vs Request Context- App context (current_app, g) can outlive a request; request context (request, session) exists only per-request
- before_first_request (removed 2.3+)- Deprecated hook; use app-factory init code or 'with app.app_context():' at startup instead
- Flask-Migrate- Wraps Alembic to provide 'flask db migrate/upgrade' commands for SQLAlchemy schema changes
- send_from_directory- Safely serve files from a directory while preventing path-traversal outside it
- SERVER_NAME config- Enables url_for(..., _external=True) to build absolute URLs outside a request context
Pro Tip
Never run app.run(debug=True) in production — the interactive debugger can execute arbitrary code from anyone who reaches an error page. Serve with a WSGI server like gunicorn behind a reverse proxy instead.
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