Flask Cheatsheet

Importing Flask

from flask import Flask

Most Common Imports

Frequently used imports for building Flask applications:

from flask import Flask, render_template, redirect, url_for, request, flash, session, jsonify

Basic Boilerplate Code

A minimal Flask app structure:

from flask import Flask
 
app = Flask(__name__)
 
@app.route("/")
def hello_world():
    return "<p>Hello, World!</p>"
 
if __name__ == "__main__":
    app.run(debug=True)

Tip: Always wrap app.run() inside if __name__ == "__main__": for best practice.

Creating Routes

Create endpoints in your Flask app:

@app.route("/")  # Home page
@app.route("/about")  # About page

Setting Allowed Methods

Specify allowed HTTP methods:

@app.route("/submit", methods=['GET', 'POST'])
def submit():
    if request.method == "POST":
        return "Form submitted!"
    return "Send a POST request!"

Auto Reload During Development

Enable auto-reloading and debugging:

app.run(debug=True)

Changing Host & Port

Change the default host/port:

app.run(host='0.0.0.0', port=5001)

Flask Templates

Render HTML files from templates/ folder:

@app.route("/")
def home():
    return render_template("index.html", name="Harry")

Pass variables to template:

<!-- index.html -->
<h1>Hello, {{ name }}!</h1>

Redirect & URL Building

return redirect(url_for("home"))  # Redirects to home route

Request Data

Access form and query parameters:

request.method
request.form["username"]   # POST form data
request.args.get("page")   # GET query param

Flash Messages

Show temporary messages to users:

app.secret_key = "your-secret-key"
flash("Data saved successfully!", "success")

Use in template:

{% with messages = get_flashed_messages(with_categories=true) %}
  {% for category, message in messages %}
    <div class="{{ category }}">{{ message }}</div>
  {% endfor %}
{% endwith %}

Sessions (Store Data Between Requests)

session["user"] = "Harry"
print(session.get("user"))
session.pop("user", None)

Returning JSON

@app.route("/api")
def api():
    return jsonify({"status": "ok", "data": [1, 2, 3]})

SQLAlchemy Integration

Import & Setup

from flask_sqlalchemy import SQLAlchemy
 
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///test.db'
app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = False
db = SQLAlchemy(app)

Creating a Model

class TableName(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    column_1 = db.Column(db.String(80), nullable=False)
    column_2 = db.Column(db.String(12), nullable=False)
 
    def __repr__(self):
        return f"<TableName {self.column_1}>"

Creating Database

with app.app_context():
    db.create_all()

CRUD Operations

Create / Add Data:

entry = TableName(column_1="Hello", column_2="World")
db.session.add(entry)
db.session.commit()

Read Data:

data = TableName.query.all()        # Get all rows
data = TableName.query.first()      # Get first row
data = TableName.query.filter_by(column_1="Hello").all()  # Filter

Update Data:

entry = TableName.query.first()
entry.column_1 = "Updated Value"
db.session.commit()

Delete Data:

db.session.delete(entry)
db.session.commit()

Error Handling

@app.errorhandler(404)
def page_not_found(e):
    return render_template("404.html"), 404

Flask Docs

Download Flask CheatSheet