通过构建垂直布局，然后将上半部和需要放在下方的按钮，设置收缩量，一个收缩量为0， 按钮的收缩量为1

RightBottomButton.py 

"""
让按钮永远在窗口右下角
"""

import sys, math
from PyQt5.QtWidgets import *
from PyQt5.QtCore import Qt

class RightBottomButton(QWidget):
    def __init__(self):
        super(RightBottomButton, self).__init__()
        self.setWindowTitle("让按钮永远在右下角")
        self.resize(400, 300)

        okButton = QPushButton("确定")
        cancelButton = QPushButton("取消")
        hbox = QHBoxLayout()
        hbox.addStretch(1)
        hbox.addWidget(okButton)
        hbox.addWidget(cancelButton)

        vbox = QVBoxLayout()
        btn1 = QPushButton("按钮1")
        btn2 = QPushButton("按钮2")
        btn3 = QPushButton("按钮3")

        vbox.addStretch(0)
        vbox.addWidget(btn1)
        vbox.addWidget(btn2)
        vbox.addWidget(btn3)
        vbox.addStretch(1)
        vbox.addLayout(hbox)

        self.setLayout(vbox)


if __name__ == "__main__":
    app = QApplication(sys.argv)

    main = RightBottomButton()
    main.show()

    sys.exit(app.exec_())