About Preselecting All Multiple-Choice Values

To preselect all values of a multiple-choice checkbox list, pass the list of choice values to the initial attribute:

class FilterForm(forms.Form):
    categories = forms.MultipleChoiceField(
        label=_("Categories"),
        required=False,
        widget=forms.CheckboxSelectMultiple(),
        choices=CATEGORY_CHOICES,
        initial=[value for value, title in CATEGORY_CHOICES],
    )