Can signal handlers memory leak in PyQt?

Question

Short question:

Can signal handlers memory leak.

Long question:

In C#, if I attach a handler to an event

left_object.left_event += right_object.right_handler

Then I need to remove the handler when I get rid of right_object or the garbage collector will never dispose of it (since left_object.left_event retains a pointer to right_object)

Is the same also true of PyQt signals and slots.

left_object.left_signal.connect( right_object.right_handler )

I see from this question that Qt automatically delinks signals and slots, when the destructor of either left_object or right_object is called, but in Python I cannot explicitly call the destructor, and right_handler is a plain-old-function.

Do I need to remove the handler to prevent right_objects memory-leaking, or does PyQt use some sort of weak-referencing?

While one of the answers touches on this, this similar question asks about how PyQt handles objects inside lambda expressions, not how PyQt handles signals.

Possible duplicate: https://stackoverflow.com/q/47941743/984421. — ekhumoro, Nov 17 '19 at 22:34

score 1 · Answer 1 · answered Nov 19 '19 at 13:03

It is possible to have a memory leak with this design. You need to disconnect the signals and slots if those connections contain references which are keeping objects alive.

Whether or not this actually happens depends on what exactly right_handler is. If right_handler is a closure with a reference to self then you have this problem.

score 0 · Answer 2 · answered Nov 20 '19 at 05:42

but in Python I cannot explicitly call the destructor,

You can call destructor in python. Check del and __del__

Do I need to remove the handler to prevent right_objects memory-leaking,

No, you don't need to do that. python will handle that. Run below code check your self.

In this code signal of startButton1 and stopButton1 are connected to methods of class object of Hello and this object(hello1) is attribute of Widget class. So hello1 will live until life object of Widget OR we delete it in method Widget.onDelete with Delete button. Once you click Delete button, destructor of hello1 is called and it will goes out of scope. Now signal of startButton1 and stopButton1 don't work as before.

Now in second row there are buttons startButton2 and stopButton2, whose signal are connected with object of hello2. But as soon as construct of Widget is over hello2 life time is finish. So there is not slot connected for signal of those buttons after Widget construct is over.

from PyQt5.QtWidgets import QApplication,QPushButton,QWidget,QHBoxLayout,QVBoxLayout

class Hello():
    def __init__(self):
        print("init")

    def onStart(self):
        print("onStart");

    def onStop(self):
        print("onStop");

    def __del__(self):
        print("del")

class Widget(QWidget):
    def __init__(self,parent=None):
        QWidget.__init__(self,parent);
        vLayout = QVBoxLayout()
        self.setLayout(vLayout)
        buttons1 = QWidget()
        hLayout1 = QHBoxLayout()
        buttons1.setLayout(hLayout1)
        vLayout.addWidget(buttons1)
        startButton1 = QPushButton("Start");
        stopButton1 = QPushButton("Stop");
        deleteButton1 = QPushButton("Delete");
        self.hello1 = Hello();
        startButton1.clicked.connect(self.hello1.onStart)
        stopButton1.clicked.connect(self.hello1.onStop)
        deleteButton1.clicked.connect(self.onDelete)
        hLayout1.addWidget(startButton1);
        hLayout1.addWidget(stopButton1);
        hLayout1.addWidget(deleteButton1);

        buttons2 = QWidget()
        hLayout2 = QHBoxLayout()
        buttons2.setLayout(hLayout2)
        vLayout.addWidget(buttons2)
        startButton2 = QPushButton("Start");
        stopButton2 = QPushButton("Stop");
        deleteButton = QPushButton("Delete");
        hello2 = Hello();
        startButton2.clicked.connect(hello2.onStart)
        stopButton2.clicked.connect(hello2.onStop)
        hLayout2.addWidget(startButton2);
        hLayout2.addWidget(stopButton2);


    def onDelete(self):
        if hasattr(self,"hello1"):
            del self.hello1

app = QApplication([])
w = Widget();
w.show()
app.exec_()

Hope this will clear you doubt.

Can signal handlers memory leak in PyQt?

2 Answers2

Linked