QQmlPropertyMap 插入；神秘地插入无，或删除其他值

Question

我在 PySide2 和 QML 工作，而 QQmlPropertyList 正在做两件我觉得很奇怪的事情。第一个是如果 QObject 在同一语句中创建，则将其分配为值不起作用：from PySide2.QtCore import QObjectfrom PySide2.QtQml import QQmlPropertyMapqmap = QQmlPropertyMap()qmap.insert('test_key', QObject())print(qmap.value('test_key'))输出：None也许这与 QObject 的初始化时间有关？它可以通过首先将它分配给一个变量来修复：qmap = QQmlPropertyMap()obj = QObject()qmap.insert('test_key', obj)print(qmap.value('test_key'))<PySide2.QtCore.QObject(0x7fe8f26b5eb0) at 0x107075ec0>但它变得陌生。忍受我。分配两个 QObjects 工作：qmap = QQmlPropertyMap()obj = QObject()qmap.insert('test_key', obj)obj2 = QObject()qmap.insert('test_key2', obj2)print(qmap.value('test_key'))print(qmap.value('test_key2'))<PySide2.QtCore.QObject(0x7f85f7f269e0) at 0x10d32b080><PySide2.QtCore.QObject(0x7f85f7f26b70) at 0x10d32b100>我什至可以将同一个对象分配给多个键：qmap = QQmlPropertyMap()obj = QObject()qmap.insert('test_key', obj)qmap.insert('test_key2', obj)print(qmap.value('test_key'))print(qmap.value('test_key2'))<PySide2.QtCore.QObject(0x7fde37d7d9d0) at 0x105cdc140><PySide2.QtCore.QObject(0x7fde37d7d9d0) at 0x105cdc140>但是如果我为两个不同的对象重用相同的变量名，第一个键的值会以某种方式被删除：qmap = QQmlPropertyMap()obj = QObject()qmap.insert('test_key', obj)obj = QObject()qmap.insert('test_key2', obj)print(qmap.value('test_key'))print(qmap.value('test_key2'))None<PySide2.QtCore.QObject(0x7fcecaf072f0) at 0x111ba60c0>怎么会这样？PySide 是否对运行时变量名做了某种隐藏的魔法？在这个例子中它是微不足道的，但它在我的代码中导致了一个很难隔离的错误，我想更好地理解这一点。

Answer 1

当您创建一个对象并为其分配相同的变量时，前一个对象被销毁，可以通过以下测试观察到：

class Foo:

    def __del__(self):

        print("destroyed")

print("before creating Foo object: 1")

foo = Foo()

print("after creating Foo object: 1")

print("before creating Foo object: 2")

foo = Foo()

print("after creating Foo object: 2")

输出

before creating Foo object: 1

after creating Foo object: 1

before creating Foo object: 2

destroyed

after creating Foo object: 2

destroyed

正如您所看到的，当第二个对象被分配给同一个变量时，第一个销毁被调用。所以这不是 PySide2 的魔力而是 python 的魔力，同样可以用被破坏的信号来验证。

from PySide2.QtCore import QObject

print("before creating QObject object: 1")

qobject = QObject()

qobject.destroyed.connect(lambda _id=id(qobject): print("destroyed1: ", _id))

print("after creating QObject object: 1")

print("before creating QObject object: 2")

qobject = QObject()

qobject.destroyed.connect(lambda _id=id(qobject): print("destroyed2: ", _id))

print("after creating QObject object: 2")

输出：

before creating QObject object: 1

after creating QObject object: 1

before creating QObject object: 2

destroyed1:  <PySide2.QtCore.QObject(0x562c1b6ee2f0) at 0x7f4a1faece40>

after creating QObject object: 2

如果你不希望这个问题发生，那么使用 Qt 的超能力，你可以将对象作为“qmap”的父对象传递，这样所有权将由 QObject 持有，它将不再由 python 处理：

from PySide2.QtCore import QObject

from PySide2.QtQml import QQmlPropertyMap

qmap = QQmlPropertyMap()

obj = QObject(qmap)

qmap.insert("test_key", obj)

obj = QObject(qmap)

qmap.insert("test_key2", obj)

print(qmap.value("test_key"))

print(qmap.value("test_key2"))

输出：

<PySide2.QtCore.QObject(0x559629d1e230) at 0x7fef951f01c0>

<PySide2.QtCore.QObject(0x559629d1eae0) at 0x7fef951f0240>

更新：

为了更好地理解，我们必须知道PySide2（PyQt5也适用）的QObject是Qt的QObject的包装器，也就是说，它是这样的：

class PyQObject:

    def __init__(self, *args):

        self._qobject_cpp = CPPQObject(*args)

        # ...

    def __del__(self):

        delete self._qobject_cpp