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

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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