Why does the data from the dropdown list disappear after a post request in FLASK?

Question

Flask

Hello.

There is a dropdown list in html template:

<label>
    Выбери продавца:
      <select name="seller" class="seller form-select">
        <option value=""></option>
        {% for name_item in name_seller %}
          {% for seller in name_item %}
            <option value="{{ seller }}">{{ seller }}</option>
          {% endfor %}
        {% endfor %}
      </select>
    </label>

This list is in a form with action="POST", when you click on the submit button, the data disappears from the list.

The function looks like this, that is, the data is added to the list from the database:

def get_name_seller():
        cursor.execute("SELECT name_seller FROM receipt GROUP BY name_seller ORDER BY name_seller")
        name_sellers = cursor.fetchall()
        return render_template("index.html", name_seller=name_sellers)

The question is why and how to overcome it?

The problem occurs both locally and on the server.

Answer 1

Because when forming a drop-down list, you do not mark the selected element in any way.