import QtQuick 2.12
import QtQuick.Controls 2.12
ApplicationWindow {
  id: window
  width: 250
  height: 500
  visible: true
  // ...
}

engine = QQmlApplicationEngine()
engine.load('qml-test.qml')
root_obj = engine.rootObjects()[0]

import QtQuick 2.12
import QtQuick.Controls 2.12
ApplicationWindow {
  id: window
  width: 250
  height: 500
  visible: true
  Text {
    id: txt
    objectName: "txt"
    text: "Click Me"
    font.pixelSize: 20
    anchors.centerIn: parent
  }
}

engine = QQmlApplicationEngine()
engine.load('qml-test.qml')
txt_obj = engine.rootObjects()[0].findChild(QObject, "txt")

txt_obj = engine.rootObjects()[0].findChild(QObject, "txt")

txt_value = txt_obj.property("text")

txt_obj.setProperty("text", "Clicked!")

import sys
from PyQt5.QtCore import QObject
from PyQt5.QtGui import QGuiApplication
from PyQt5.QtQml import QQmlApplicationEngine
app = QGuiApplication(sys.argv)
engine = QQmlApplicationEngine()
engine.load('qml-test.qml')

# 根对象
root_obj = engine.rootObjects()[0]

# Text对象
txt_obj = engine.rootObjects()[0].findChild(QObject, "txt")

# 读取属性值
txt_value = txt_obj.property("text")

# 设置属性值
txt_obj.setProperty("text", "Clicked!")

sys.exit(app.exec())