Flask — Forms & POST Handling

• Write an HTML form that POSTs to a route.
• Read submitted fields with request.form.
• Validate user input; redisplay the form on error.
• Follow Post/Redirect/Get with redirect + flash messages.

from flask import Flask, render_template, request

app = Flask(__name__)

@app.route("/contact", methods=["GET", "POST"])
def contact():
    if request.method == "POST":
        name = request.form["name"]
        return f"Thanks, {name}!"
    return render_template("contact.html")

Same route, two HTTP methods. GET shows the form; POST processes it. Cleanest pattern in Flask.

The HTML form

<form action="/contact" method="post">
  <label>Name <input name="name" required></label>
  <label>Email <input type="email" name="email" required></label>
  <label>Message <textarea name="msg"></textarea></label>
  <button>Send</button>
</form>

• Each input's name= becomes a key in request.form.
• required gets free browser-side validation.
• action= can be the same path you're on (Flask will route by method).

Read + validate

@app.route("/contact", methods=["GET", "POST"])
def contact():
    if request.method == "POST":
        name  = request.form.get("name", "").strip()
        email = request.form.get("email", "").strip()
        msg   = request.form.get("msg", "").strip()

        errors = []
        if not name:  errors.append("Name is required.")
        if "@" not in email: errors.append("Email looks wrong.")

        if errors:
            return render_template("contact.html",
                                   errors=errors,
                                   name=name, email=email, msg=msg)

        save_message(name, email, msg)
        flash("Message sent 🎉")
        return redirect(url_for("contact"))

    return render_template("contact.html")

flash + secret_key

app.secret_key = "change-me-in-prod"   # required for flash

# in template:
{% with messages = get_flashed_messages() %}
  {% for m in messages %}<p class="flash">{{ m }}</p>{% endfor %}
{% endwith %}

Re-fill form fields after error

Always pass the user's values back so they don't have to retype everything. In the template:

<input name="name" value="{{ name or '' }}" required>

# app.py
from flask import Flask, render_template, request, redirect, url_for, flash

app = Flask(__name__)
app.secret_key = "demo-secret"

USERS = []   # in-memory list, fine for the demo

@app.route("/signup", methods=["GET", "POST"])
def signup():
    if request.method == "POST":
        name  = request.form.get("name", "").strip()
        email = request.form.get("email", "").strip().lower()
        errors = []

        if not name:                    errors.append("Name is required.")
        if "@" not in email:             errors.append("Bad email.")
        if any(u["email"] == email for u in USERS):
                                          errors.append("Email already used.")

        if errors:
            return render_template("signup.html",
                                   errors=errors, name=name, email=email)

        USERS.append({"name": name, "email": email})
        flash(f"Welcome, {name}!")
        return redirect(url_for("members"))
    return render_template("signup.html")

@app.route("/members")
def members():
    return render_template("members.html", users=USERS)

<!-- templates/signup.html -->
{% extends "base.html" %}
{% block content %}
  <h1>Sign up</h1>
  {% for e in errors %}<p class="err">{{ e }}</p>{% endfor %}
  <form method="post">
    <label>Name  <input name="name"  value="{{ name or '' }}" required></label>
    <label>Email <input name="email" value="{{ email or '' }}" type="email" required></label>
    <button>Create account</button>
  </form>
{% endblock %}

<!-- templates/members.html -->
{% extends "base.html" %}
{% block content %}
  <h1>Members ({{ users|length }})</h1>
  {% with msgs = get_flashed_messages() %}
    {% for m in msgs %}<p class="flash">{{ m }}</p>{% endfor %}
  {% endwith %}
  <ul>{% for u in users %}<li>{{ u.name }} — {{ u.email }}</li>{% endfor %}</ul>
  <p><a href="{{ url_for('signup') }}">+ Add another</a></p>
{% endblock %}

Read the diff

Full Post/Redirect/Get: bad input redisplays the form with errors + previous values; good input redirects to the members page. Refresh after success doesn't re-add anyone. That's production-quality form handling in 40 lines.

Same route, both methods. request.form["name"] reads inputs. Validate, redisplay on error, redirect on success. Use flash + secret_key for one-shot messages. Tomorrow we plug a real database under it.