<form method="GET" action="">
  <label for="id_context">Context:</label>
  <select id="context" name="context" 
  onchange="this.form.elements[1].disabled=true; this.form.submit()">
    {%for val in context_values%}
      <option value="{{val}}"
      {%ifequal val context%} selected="selected"{%endifequal%}>
      {{val}} lines</option>
    {%endfor%}
  </select>
  <input type="submit" value="Adjust context" />
</form>