'Changing a NULL field to NOT NULL.'

def ask_not_null_alteration(self, field_name, model_name):

    if (not self.dry_run):
        choice = self._choice_input(("You    are    trying    to    change    the    nullable    field    '%s'    on    %s    to    non-nullable    without    a    default;    we    can't    do    that    (the    database    needs    something    to    populate    existing    rows).\nPlease    select    a    fix:" % (field_name, model_name)), ['Provide    a    one-off    default    now    (will    be    set    on    all    existing    rows    with    a    null    value    for    this    column)', 'Ignore    for    now,    and    let    me    handle    existing    rows    with    NULL    myself    (e.g.    because    you    added    a    RunPython    or    RunSQL    operation    to    handle    NULL    values    in    a    previous    data    migration)', 'Quit,    and    let    me    add    a    default    in    models.py'])
        if (choice == 2):
            return NOT_PROVIDED
        elif (choice == 3):
            sys.exit(3)
        else:
            return self._ask_default()
    return None
