'Subcommand of alter_column that sets default values (overrideable)'

def _alter_set_defaults(self, field, name, params, sqls):

    table_name = self.quote_name(params['table_name'])
    drop_default = self.drop_column_default_sql(table_name, name)
    if drop_default:
        sqls.append((drop_default, []))
