Home›Flask in 5 Days›Day 1
Day 1 of 5
⏱ ~60 minutes
Flask in 5 Days — Day 1

Routes, Views, and Jinja2 Templates

Install Flask, define your first routes, return HTML templates with Jinja2, and understand the request/response objects.

Your First Flask App

Terminal
pip install flask
mkdir my-app && cd my-app
app.py
from flask import Flask, render_template, request, redirect, url_for

app = Flask(__name__)

@app.route('/')
def index():
    name = request.args.get('name', 'World')
    return render_template('index.html', name=name)

@app.route('/hello/')
def hello(username):
    return f'Hello, {username}!'

@app.route('/submit', methods=['GET', 'POST'])
def submit():
    if request.method == 'POST':
        data = request.form.get('message')
        return redirect(url_for('index'))
    return render_template('form.html')

if __name__ == '__main__':
    app.run(debug=True)
templates/index.html


Flask App

  
Hello, {{ name }}!


  {% if name != 'World' %}
    
Nice to meet you, {{ name }}.

  {% endif %}

  {% for item in items %}
    
  • {{ item }}
    • 
  {% endfor %}

  Home


    
  Finished this lesson?
    💡
    Use url_for('view_name') to generate URLs in templates and code. Never hardcode /submit. If the URL changes, url_for still works. Hardcoded URLs break silently.
    📝 Day 1 Exercise
    Build a Multi-Route App
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    Day 1 Summary

    • @app.route('/path') maps a URL to a Python function.
    • render_template('file.html', key=value) renders Jinja2 templates with context.
    • request.args = query params. request.form = POST form data.
    • url_for('function_name') generates URLs. Always use it instead of hardcoding.
    Next →
    Day 2: SQLAlchemy and Databases
    →