为什么 getattr(self, "x") 与本例中的 self.x 不同？

Question

我发现 getattr(self, "x") 的行为与 self.x 不同的情况。抱歉 post 的长度 - 我不知道如何在没有 PySide2 的情况下重现该行为。所以有了这个 example.qml 文件：

import QtQuick 2.10
import QtQuick.Controls 2.1
import QtQuick.Window 2.2
import QtQuick.Controls.Material 2.3

ApplicationWindow {
    id: applicationWindow
    Material.theme: Material.Dark
    title: qsTr("Test Invoke")
    visible: true

    width: 600
    height: 500

    Slider {
        id: slider1
        x: 69
        y: 215
        value: Manager.xyz
        property bool updateValueWhileDragging: true
        onMoved: Manager.xyz = value
    }
}

和这个 example.py 文件：

import sys
import os

from PySide2.QtCore import Qt, QObject, Signal, Slot, Property
from PySide2.QtGui import QGuiApplication
from PySide2.QtQml import QQmlApplicationEngine
from PySide2.QtWidgets import QSlider, QPushButton, QCheckBox

class Manager(QObject):
    xyz_slot = Signal()
    def __init__(self):
        QObject.__init__(self)
        self.__xyz = None
        self.xyz_slot.connect(self.on_xyz_changed)
        self.xyz = 0.3

    @Property(float, notify=xyz_slot) # (6)
    def xyz(self):
        # (1) This works
        # return self.__xyz
        # (2) This doesn't work and produces the error:
        # Error: 'Manager' object has no attribute '__xyz'
        return getattr(self, "__xyz")

    @xyz.setter
    def set_xyz(self, val):
        if self.__xyz == val:
            return
        self.__xyz = val
        self.xyz_slot.emit()

    @Slot()
    def on_xyz_changed(self):
        print(self.__xyz)

if __name__ == "__main__":
    os.environ["QT_QUICK_CONTROLS_STYLE"] = "Material"
    app = QGuiApplication(sys.argv)
    engine = QQmlApplicationEngine()
    manager = Manager()
    ctx = engine.rootContext()
    ctx.setContextProperty("Manager", manager)
    engine.load('example.qml')
    if not engine.rootObjects():
        sys.exit(-1)
    sys.exit(app.exec_())

使用 Python 3.7.

为什么 getattr (2) 版本不工作？

请注意，我需要一个带 getattr 的版本，因为我想装饰其他类。在那种情况下，不可能使用点调用版本。在the documentation中，有一个例子声称两者是一样的。我可以想象可能是 PySide2 的 C++ 代码导致了差异。但是我不知道如何从这里开始。

Answer 1

因为您正在使用双下划线名称修改。

来自the docs

Notice that code passed to exec() or eval() does not consider the classname of the invoking class to be the current class; this is similar to the effect of the global statement, the effect of which is likewise restricted to code that is byte-compiled together. The same restriction applies to getattr(), setattr() and delattr(), as well as when referencing __dict__ directly.

您可能不需要使用双下划线名称修饰，所以不要使用它。

为什么 getattr(self, "x") 与本例中的 self.x 不同？

Why is getattr(self, "x") not the same as self.x in this example?

python

python-3.x

pyside2