Find the index related to the widgets inside a layout PyQt

I have a central widget(self.widget) and inside it a QHBoxLayout(self.box)

And I have placed three buttons inside this layout

What I want is to print the button index by clicking on each button

For example, when I click on button 3, the index of this button will be printed

But when I click on one of the buttons, none is returned for me

and index -1 is returned

Because the button becomes none, it naturally gets the wrong index

Note that I know that I can find the clicked button in other ways

But I want to find the clicked button using self.widget.childAt(e.pos())

Where is the problem?

code:

from PyQt6.QtWidgets import *
from PyQt6.QtCore import *
from PyQt6.QtGui import *

class CustomPushButton(QPushButton):
    signal = pyqtSignal(QEvent)
    def __init__(self, name):
        super().__init__(name)

    def mousePressEvent(self, event):
        self.signal.emit(event)
        

class main(QMainWindow):
    def __init__(self):
        super().__init__()
        self.box = QHBoxLayout()
        self.widget = QWidget()
        self.widget.setLayout(self.box)

        self.button = CustomPushButton('button1')
        self.button2 = CustomPushButton('button2')
        self.button3 = CustomPushButton('button3')

        self.box.addWidget(self.button)
        self.box.addWidget(self.button2)
        self.box.addWidget(self.button3)


        self.button.signal.connect(self.findIndex)
        self.button2.signal.connect(self.findIndex)
        self.button3.signal.connect(self.findIndex)

        self.setCentralWidget(self.widget)
        self.show()
        
    def findIndex(self,e):
        button = self.widget.childAt(e.pos())
        print(button)
        print('button',self.box.indexOf(button))


app = QApplication([])
m = main()
m.show()
app.exec()

Each widget has it's own local coordinate system, and the QMouseEvent.pos() method returns a QPoint in that widgets local coordinate system. However the QWidget.childAt() method expects the parameter to be in it's own coordinate system.

So when you send the event generated by the button and then apply its position to the parent widgets coordinate system, it is unlikely that the button will be found at those coordinates.

What you can do instead is map the coordinates to the buttons parent using the mapToParent method, prior to emitting the signal, and sending the QPoint result for the mapped position as the signal parameter.

For example:

from PyQt6.QtWidgets import *
from PyQt6.QtCore import *
from PyQt6.QtGui import *

class CustomPushButton(QPushButton):
    signal = pyqtSignal(QPoint)
    def __init__(self, name):
        super().__init__(name)

    def mousePressEvent(self, event):
        self.signal.emit(self.mapToParent(event.pos()))
        

class main(QMainWindow):
    def __init__(self):
        super().__init__()
        self.box = QHBoxLayout()
        self.widget = QWidget()
        self.widget.setLayout(self.box)

        self.button = CustomPushButton('button1')
        self.button2 = CustomPushButton('button2')
        self.button3 = CustomPushButton('button3')

        self.box.addWidget(self.button)
        self.box.addWidget(self.button2)
        self.box.addWidget(self.button3)


        self.button.signal.connect(self.findIndex)
        self.button2.signal.connect(self.findIndex)
        self.button3.signal.connect(self.findIndex)

        self.setCentralWidget(self.widget)

        
    def findIndex(self,e):
        button = self.widget.childAt(e)
        print(button)
        print('button',self.box.indexOf(button))


app = QApplication([])
m = main()
m.show()
app.exec()

After applying the above changes, after pushing each button the terminal output looks like:

<__main__.CustomPushButton object at 0x000001FEFF4CA440>
button 0
<__main__.CustomPushButton object at 0x000001FEFF4CA4D0>
button 1
<__main__.CustomPushButton object at 0x000001FEFF4CA560>
button 2

P.S. You are also calling show() twice. once inside the constructor and again in afterwards toward the bottom, which is unnecessary.

While the documentation warns about doing it for strictly OOP related aspects, as long as the object structure is contained and known, you can use QObject.sender():

    def findIndex(self,e):
        button = self.sender()
        ...

In that way you don't even need to send the event within the signal, which is not forbidden, but conceptually and potentially dangerous, and normally discouraged as such.

An even more simpler approach would be to let the signal emit the instance itself:

class CustomPushButton(QPushButton):
    signal = pyqtSignal(object)
    def __init__(self, name):
        super().__init__(name)

    def mousePressEvent(self, event):
        self.signal.emit(self)


class main(QMainWindow):
    ...
    def findIndex(self, button):
        print('button', self.box.indexOf(button))

This may not look completely elegant, and it could be an issue in some very special, fringe cases (threading, multiprocessing), but it's fine enough for simpler situations like these, in which we can assume that the object is still existing when the signal is received.

Note that the object argument of the signal is to simplify some aspects related to the Python/C++ layering of the binding. If you do know what you're doing, you can use QWidget or QPushButton instead.