Change attrs in a CheckboxSelectMultiple widget on every item with django form

I need to build a pre-filled field in a form that have CheckboxSelectMultiple widget. I thought to use different attrs={"checked":""} value on each id of the form filed.

Is possible to loop on items?

from django import forms
from myapp.models import MyModel

class MyForm(forms.Form):
   qs = MyModel.object.value_list("id","value")
   qsa = MyModel.object.value_list("id", "is_checked")
   myfield = forms.ModelMultipleChoiceField(queryset=qs, widget=form.CheckboxSelectMultiple)
   extrafield = forms.CherField(widget=forms.Textarea)

   def __init__(self, *args, **kwargs):
      super(MyForm, self).__init__(*args, **kwargs)
      for id, is_checked in self.qsa:
        if is_checked is true:
         self.fields["myfield"].widget.attrs["checked"] = ""

This will obvoiusly not work, but qhat I need is this.

<form action="some_action">
  <input type="checkbox" id="1" name="1" value="value_1">
  <label for="value_1"> value_1</label><br>
  <input type="checkbox" id="2" name="2" value="value_2">
  <label for="value_2"> value_2</label><br>
  <input type="checkbox" id="3" name="3" value="value_3">
  <label for="value_3" checked="checked"> value_3</label><br><br>
  <input type="submit" value="Submit" >
</form>

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