@app.route('/signup/', methods=['GET', 'POST'])
def signup():
    form = RegisterForm()

    if form.validate_on_submit():
        hashed_password = generate_password_hash(form.password.data, method='sha256')
        new_user = User(username=form.username.data, email=form.email.data, password=hashed_password)
        db.session.add(new_user)
        db.session.commit()

        return '<h1>New user has been created!</h1>'

    return render_template('signup.html', form=form, errors=form.errors.items())

{% block body_content %}
<div class="container">

<form method="POST" action="/signup">
    {{ form.csrf_token }}
    <div class="form-group">
       {{ form.username }}
    </div>
    <div class="form-group">
        {{ form.email }}
    </div>
    <div class="form-group">
        {{ form.password }}
    </div>
    <button type="submit" class="btn btn-primary">Sign Up</button>
    </form>

      <div class="errors">
        {% for field, err in errors %}
            <p>{{field}} : {{err|join(', ')}}</p>
        {% endfor %}
      </div>
</div>
{% endblock %}