Launch an App
- Since Flask 2.2, threading is by default:
app.run(threaded=True) # default since Flask 2.2- the
Werkzeug dev serverstarts one OS‑level process and spawns a new thread for every concurrent request.
- the
Jinja
Jinja is a templating language, not a general-purpose programming language. It lives on the server inside Flask (or other Python frameworks) and provides you with:
- Template syntax
({{ … }}for expressions,{% … %}for control structures like loops/ifs. -
Filters and tests to transform or inspect data (e.g. {{ user.name upper }}, {% if items length > 0 %}).
When you call Flask’s render_template("page.html", foo=bar), Jinja runs on the server, stitches your Python data into the template, and emits plain HTML (with CSS, etc.) to the browser.
Where does JavaScript come in?
- Server-side rendering (Jinja)
- All the dynamic bits you write in
{{ … }} or {% … %}are resolved before the browser ever sees the page. The client gets a finalized HTML document.
- All the dynamic bits you write in
- Client-side dynamics (JavaScript)
- If you want interactive behavior after the page loads—DOM updates on clicks, real-time data without full reloads, animations, form validation—you write JavaScript (vanilla JS or a framework like React/Vue). That JS runs in the browser and can fetch new data (via fetch, WebSockets, SSE, etc.) and manipulate the DOM.
Quick Example:
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<!-- templates/hello.html -->
<!doctype html>
<html>
<head><title>Hello</title></head>
<body>
<h1>Hello, {{ user_name }}!</h1> {# Jinja: filled in on the server #}
<button id="btn">Click me</button>
<p id="msg"></p>
<script>
// JavaScript: runs in the browser after the HTML arrives
document.getElementById('btn').onclick = () => {
document.getElementById('msg').textContent = 'You clicked!';
};
</script>
</body>
</html>
- The
{{ user_name }}bit is Jinja, resolved by Flask before shipping the HTML. - The
<script>block is pure JavaScript, responsible for client-side interactivity after the page loads.
