PySide, Group radioButton in a loop

import sys
from PySide2.QtCore import *
from PySide2.QtGui import *
from PySide2.QtWidgets import *

app = QApplication(sys.argv)
wid = QWidget()

class ui(QWidget):

    def __init__(self):
        super(ui, self).__init__(wid)
        self.ui_layout()

    def ui_layout(self):
        self.main_layout = QVBoxLayout()
        self.setLayout(self.main_layout)

        attrs = ['a', 'b']

        for attr in attrs:

            buttonGroup = QButtonGroup()

            self.attr_layout = QHBoxLayout()
            self.main_layout.addLayout(self.attr_layout)

            self.rb1 = QRadioButton('{}_rb1'.format(attr))
            self.attr_layout.addWidget(self.rb1)
            buttonGroup.addButton(self.rb1)

            self.rb2 = QRadioButton('{}_rb2'.format(attr))
            self.attr_layout.addWidget(self.rb2)
            buttonGroup.addButton(self.rb2)

ui()
wid.show()
wid.resize(250, 150)

sys.exit(app.exec_())