User Registration Flow

A typical registration flow:

1. User fills a registration form
2. Backend validates fields (username/email uniqueness)
3. Password is hashed
4. User row is created
5. Optionally log the user in

diagram the registration flow, and where it can refuse mermaid

  flowchart TD
  A["GET /register"] --> B["render the form"]
  B --> C["POST /register"]
  C --> D{"all fields present?"}
  D -->|no| E["flash error, redirect back"]
  D -->|yes| F{"username already taken?"}
  F -->|yes| E
  F -->|no| G{"email already registered?"}
  G -->|yes| E
  G -->|no| H["generate_password_hash(password)"]
  H --> I["db.session.add(user)"]
  I --> J["db.session.commit()"]
  J --> K["login_user(user)"]
  K --> L["redirect to dashboard"]
  E --> B

Every branch that sends the user back is a validation the server must do itself -- the browser cannot be trusted to have done it. Note the order: the password is hashed before anything is written, and the row is committed before the user is logged in.

Model fields (example)

• username (unique)
• email (unique)
• password_hash

Example (simplified)

python

from flask import request, redirect, url_for, flash
from werkzeug.security import generate_password_hash
from flask_login import login_user
 
@app.route("/register", methods=["GET", "POST"])
def register():
    if request.method == "POST":
        username = request.form.get("username", "").strip()
        email = request.form.get("email", "").strip().lower()
        password = request.form.get("password", "")
 
        if not username or not email or not password:
            flash("All fields are required", "error")
            return redirect(url_for("register"))
 
        if User.query.filter_by(username=username).first():
            flash("Username already taken", "error")
            return redirect(url_for("register"))
 
        if User.query.filter_by(email=email).first():
            flash("Email already registered", "error")
            return redirect(url_for("register"))
 
        user = User(
            username=username,
            email=email,
            password_hash=generate_password_hash(password),
        )
        db.session.add(user)
        db.session.commit()
 
        login_user(user)
        flash("Welcome! Your account was created.", "success")
        return redirect(url_for("dashboard"))
 
    return "Register page"

Next improvements

For production-quality registration:

• use Flask-WTF validators
• password policy (length/complexity)
• email verification
• rate limiting
• audit logging