How do I iterate over the options of a SelectField in a template?

I have a select field in the form and now I need to iterate over options in this field.

{{ form.myselect }} gives me this:

<select name="myselect" id="id_myselect">
    <option value="" selected="selected">---------</option>
    <option value="2">Item 1</option>
    <option value="3">Item 2</option>
    ...
</select>

Now I need to add some attributes to the options and because of that what I need is:

<select name="myselect" id="id_myselect">
{% for x in form.myselect %}
    <option value="{{ x.id }}">{{ x.name }}</option>
{% endfor %}
</select>

but there is an error:

Caught TypeError while rendering: 'BoundField' object is not iterable

I tried form.myselect.all, form.myselect.option_set but it gives nothing