Flask Quick Reference

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How to Use This Quick Reference

This page is organized as a scannable cheat sheet rather than a tutorial: each section groups closely related Flask APIs (routing, request/response, context, CLI) so you can jump straight to the syntax you half-remember instead of re-reading full explanations, which is exactly the kind of lookup you need mid-coding rather than while first learning the concept.

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Cricket analogy: A batter glances at a laminated card of field placements taped to the dressing room wall for a quick reminder rather than rereading a full coaching manual mid-innings, the same fast-lookup purpose this cheat sheet serves.

Routing and Views Cheat Sheet

Routes are registered with @app.route('/path', methods=['GET', 'POST']), URL converters like <int:id>, <string:slug>, or <path:subpath> constrain and type-convert dynamic segments, and url_for('endpoint_name', id=5) generates URLs by endpoint name rather than hardcoded strings so routes can change without breaking every link. Blueprints register their own routes with @bp.route(...) and are mounted onto the app via app.register_blueprint(bp, url_prefix='/api').

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Cricket analogy: A scoreboard operator uses a standard coded system (like '4' or '6' for boundaries) rather than free-text descriptions, similar to how URL converters like <int:id> constrain and standardize what a route segment can accept.

Request, Response, and Context Cheat Sheet

Inside a view, request.args gets query string parameters, request.form gets submitted form fields, request.get_json() parses a JSON body, and request.files handles uploads. For responses, return a string for a simple 200 OK, return a tuple (body, status_code) or (body, status_code, headers) for custom status/headers, jsonify(data) for JSON responses with the correct Content-Type, and abort(404) to short-circuit with an HTTP error.

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Cricket analogy: An umpire reads different signals depending on the situation — a raised finger for out, crossed arms for a dead ball, arms out for a wide — each a distinct, purpose-built signal, similar to request.args, request.form, and request.get_json() each reading a different part of the request.

CLI and Configuration Cheat Sheet

flask run starts the dev server (reads FLASK_APP and FLASK_DEBUG from environment or a .flaskenv file), flask shell opens a Python shell with the app context already pushed, and flask db migrate / flask db upgrade run Flask-Migrate/Alembic database migrations. Custom commands are defined with @app.cli.command('seed') and become available as flask seed once the app factory registers them.

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Cricket analogy: A team has a standard set of pre-match rituals — toss, warm-up, national anthem — each triggered by a known cue, similar to how flask run, flask shell, and flask db upgrade are standard, named commands with predictable behavior.

python

from flask import Flask, request, jsonify, abort, url_for

app = Flask(__name__)

@app.route("/users/<int:user_id>", methods=["GET"])
def get_user(user_id):
    user = db.session.get(User, user_id)
    if user is None:
        abort(404)
    return jsonify({"id": user.id, "name": user.name})

@app.route("/users", methods=["POST"])
def create_user():
    data = request.get_json()
    name = data.get("name")
    if not name:
        return jsonify({"error": "name is required"}), 400
    user = User(name=name)
    db.session.add(user)
    db.session.commit()
    return jsonify({"id": user.id, "url": url_for("get_user", user_id=user.id)}), 201

@app.cli.command("seed")
def seed():
    """Usage: flask seed"""
    db.session.add(User(name="Demo User"))
    db.session.commit()
    print("Seeded database.")

Set FLASK_APP=myapp and FLASK_DEBUG=1 in a .flaskenv file (loaded automatically if python-dotenv is installed) so you don't have to export them manually in every terminal session during local development.

@app.route() with URL converters like <int:id> defines and type-constrains dynamic route segments.
url_for('endpoint_name') generates URLs by endpoint name so routes can change without breaking links.
request.args, request.form, request.get_json(), and request.files each read a distinct part of a request.
jsonify(), tuples like (body, status), and abort(404) are the standard ways to shape a response.
Blueprints are mounted with app.register_blueprint(bp, url_prefix='/api') to modularize routes.
flask run, flask shell, and flask db migrate/upgrade are the core built-in CLI commands.
@app.cli.command('name') registers a custom command runnable as flask name.