django admin how to display widget on readonly field

I would say it's a feature.

When field is set to readonly, Django uses display_for_field function which hardcodes the result, you can't customize it.

Another approach would be not to set the field as readonly and override formfield_for_dbfield, an undocumented ModelAdmin method and act accordingly to your needs.

An alternative and more flexible solution is to set the "disabled" attribute of those fields you want to render in read-only mode, overriding the get_form(...) method of your admin class:

def get_form(self, *args, **kwargs):    form = super(SupplierAdmin, self).get_form(*args, **kwargs)    for field_name in self.fake_readonly_fields:        form.base_fields[field_name].disabled = True    return form

note that I take the name of readonly fields not from the usual self.readonly_fields but from a different array that I named self.fake_readonly_fields, in order to avoid name overlaps

The limit of the proposed solution (using formfield_for_dbfield) is that in that way you render at the same way all the database fields of a given type, wether it should be read-only or not. That way works if your entire form should be read-only, but if you deals with forms that are half read-only and half not, it is better to rely on the disabled attribute.

Further details on my answer here: django modeladmin, readonly_fields and boolean fields