'Check a value of `prepopulated_fields` dictionary, i.e. it\'s an
iterable of existing fields.'

def _check_prepopulated_fields_value(self, obj, model, val, label):

    if (not isinstance(val, (list, tuple))):
        return must_be('a    list    or    tuple', option=label, obj=obj, id='admin.E029')
    else:
        return list(chain.from_iterable((self._check_prepopulated_fields_value_item(obj, model, subfield_name, ('%s[%r]' % (label, index))) for (index, subfield_name) in enumerate(val))))
