Error lambda missing 1 required positional argument when using with QPushButton

This is entire my code:

import sys

from PySide2.QtCore import Qt
from PySide2.QtWidgets import (
    QApplication,
    QHBoxLayout,
    QLabel,
    QMainWindow,
    QPushButton,
    QVBoxLayout,
    QWidget,
)

class MainWindow(QMainWindow):
    def __init__(self):
        super().__init__()

        v = QVBoxLayout()
        h = QHBoxLayout()

        for a in range(10):
            button = QPushButton(str(a))
            button.clicked.connect(lambda checked, a=a: self.button_clicked(a)) # error here

            h.addWidget(button)

        v.addLayout(h)

        self.label = QLabel("")
        v.addWidget(self.label)
        
        w = QWidget()
        w.setLayout(v)

        self.setCentralWidget(w)

    def button_clicked(self, n):
        self.label.setText(str(n))

app = QApplication(sys.argv)
window = MainWindow()
window.show()
app.exec_()

When I run this code, I get a window like this:

Below the buttons, there is a QLabel, and I want when I click on any button, the button’s label will refer to this QLabel, but I get a bunch of confusing errors in the terminal. What’s wrong with my code, help me, thanks.

Answer

The clicked signal is overload so it accepts 2 signatures where it can send a bool or not. The default signature depends on the library, in this case it seems that PySide2 by default does not send the “checked” parameter, unlike PyQt5 that does.

The solution is to indicate the signature:

button.clicked[bool].connect(lambda checked, a=a: self.button_clicked(a))