调用deleteLater（）后直接删除对Qt对象的Python引用安全吗？

Question

请考虑下面的最小例子，它实现了一个自定义的^ {< CD1>}，它维护未完成的^ {< CD2>}实例的列表。

当回复为finished时，它将从unfinished_replies列表中删除

正如在Is deleteLater() necessary in PyQt/PySide?中所讨论的，在finished槽内使用QNetworkReply.deleteLater()来调度Qt对象的删除

但是，我不确定删除对reply对象的Python引用的最佳方法是什么。我可以想出两个（互斥的）选项来删除Python引用，如下例所示：

1. 在调用deleteLater()

2. 当发出QNetworkReply.destroyed信号时删除（docs）

这两种选择似乎都很有效。我更喜欢选项1，但我不确定这是否会在极少数情况下导致意外。哪一个最好？或者还有其他选择吗

import sys
from PyQt5 import QtNetwork, QtWidgets, QtCore


class CustomNetworkAccessManager(QtNetwork.QNetworkAccessManager):
    def __init__(self):
        super(CustomNetworkAccessManager, self).__init__()
        self.unfinished_replies = []
        self.finished.connect(self.slot)

    def get(self, *args, **kwargs):
        reply = super(CustomNetworkAccessManager, self).get(*args, **kwargs)
        reply.index = i  # just for printing
        self.unfinished_replies.append(reply)

    def remove_from_list(self, reply):
        self.unfinished_replies.remove(reply)
        print('{} unfinished replies left'.format(len(self.unfinished_replies)))
        if not self.unfinished_replies:
            QtCore.QCoreApplication.quit()

    def slot(self, reply):
        print('reply {} finished'.format(reply.index))
        # handle the Qt side:
        reply.deleteLater()  
        # handle the Python side:
        # either
        # OPTION 1 - remove now
        self.remove_from_list(reply)
        # or 
        # OPTION 2 - remove when destroyed
        # reply.destroyed.connect(lambda: self.remove_from_list(reply))


if __name__ == '__main__':
    # Initialize
    app = QtWidgets.QApplication(sys.argv)
    manager = CustomNetworkAccessManager()

    # Schedule requests
    url = 'http://httpbin.org/get'
    for i in range(6):
        manager.get(QtNetwork.QNetworkRequest(QtCore.QUrl(url)))

    # Start event loop
    app.exec_()

另外，对于Python2代码，我很抱歉

Answer 1

两者都是等效的，它们只是在被移除时有所不同。但要更详细地理解，您必须了解PyQt5/PySide2绑定是如何工作的。这些库在C++对象周围创建包装器，例如：

class FooWrapper:
    def __new__(self, cls, *args, **kwargs):
         # ...
         instance = ...
         instance._cpp_object = create_cpp_object()
         # ...
         return instance

    def foo_method(self, *args, **kwargs):
        return execute_foo_method(self._cpp_object, *args, **kwargs)

    def __del__(self):
        destroyed_cpp_object(self._cpp_object)

因此，在稍后调用Delete时，仅删除cpp_对象，而不删除包装器，您可以验证是否使用：

reply.destroyed.connect(self.remove_from_list)

Traceback (most recent call last):
  File "main.py", line 32, in <lambda>
    reply.destroyed.connect(self.remove_from_list)
  File "main.py", line 17, in remove_from_list
    self.unfinished_replies.remove(reply)
ValueError: list.remove(x): x not in list

因为destroyed传递的参数是获取上述错误的无效包装。对于这种情况，解决方案是检查是否已使用sip.isdeleted（）删除cpp_对象：

from PyQt5 import QtNetwork, QtWidgets, QtCore
import sip

# ...
class CustomNetworkAccessManager(QtNetwork.QNetworkAccessManager):
    # ...

    def remove_from_list(self):
        self.unfinished_replies = [
            reply for reply in self.unfinished_replies if not sip.isdeleted(reply)
        ]
        print("{} unfinished replies left".format(len(self.unfinished_replies)))
        if not self.unfinished_replies:
            QtCore.QCoreApplication.quit()

    def slot(self, reply):
        print("reply {} finished".format(reply.index))
        # handle the Qt side:
        reply.deleteLater()
        # handle the Python side:
        reply.destroyed.connect(self.remove_from_list)

回到对您的方法的研究，这些方法可以用图表表示如下：

(FIRST METHOD)
      ┬         ┬          -┬           -
            |                  |                     |
 call_deleteLater remove_reply_from_list          destroyed

(SECOND METHOD)
      ┬                    -┬        -┬   
            |                                         |                 |
 call_deleteLater                               destroyed remove_reply_from_list

而且，由于您使用的是原始包装，因此您应该不会有任何问题

结论：两者同等安全