PyQt Checkbox List with lambda signal

Question

I create some QCheckBoxes from a dict using the dict keys as checkbox text. Then I connect the signal to the update function using a lambda expression. What I want to pass to this update function is the text of the checkbox so each lambda has its specific argument. Nevertheless I always get the text of the last added widget, no matter which checkbox I click. Do I miss something crucial about the concept of signal passing using lambda?

import sys
from PyQt5.QtWidgets import QDialog, QCheckBox, QVBoxLayout, QApplication, QSlider, QLabel


class Window(QDialog):
    def __init__(self, parent=None):
        super(Window, self).__init__(parent)

        layout = QVBoxLayout()

        cb_dict = {"cb1": False, "cb2": True, "cb3": False}
        cb_widget_list = list()

        for checkbox in cb_dict:
            widget = QCheckBox(checkbox)
            widget.stateChanged.connect(lambda: self.update(checkbox))
            cb_widget_list.append(widget)

        for widget in cb_widget_list:
            layout.addWidget(widget)
        self.setLayout(layout)

    def update(self, origin: str):
        print("Checkbox {} has changed".format(origin))


if __name__ == '__main__':
    app = QApplication(sys.argv)

    main = Window()
    main.show()

    sys.exit(app.exec_())

So the output is always Checkbox cb3 has changed.

score 2 · Accepted Answer · edited Jun 20 '20 at 09:12

void QCheckBox::stateChanged(int state)

This signal is emitted whenever the checkbox's state changes, i.e., whenever the user checks or unchecks it.

state contains the checkbox's new Qt::CheckState.

Since you want to pass another parameter, you should do this:

        widget.stateChanged.connect(
            lambda state, checkbox=checkbox: self.update(state, checkbox)  # +++
        )

import sys
from PyQt5.QtWidgets import QDialog, QCheckBox, QVBoxLayout, QApplication, QSlider, QLabel


class Window(QDialog):
    def __init__(self, parent=None):
        super(Window, self).__init__(parent)

        layout = QVBoxLayout()

        cb_dict = {"cb1": False, "cb2": True, "cb3": False}
        cb_widget_list = list()

        for checkbox in cb_dict:
            widget = QCheckBox(checkbox)
            widget.stateChanged.connect(
                lambda state, checkbox=checkbox: self.update(state, checkbox)  # +++
            )
            cb_widget_list.append(widget)

        for widget in cb_widget_list:
            layout.addWidget(widget)
        self.setLayout(layout)

    def update(self, state, origin: str):                                      # + state
        print("Checkbox {} has changed: {}".format(origin, state))


if __name__ == '__main__':
    app = QApplication(sys.argv)

    main = Window()
    main.show()

    sys.exit(app.exec_())

That works, could you quickly explain why? – po.pe Nov 21 '19 at 12:43 — po.pe, Nov 21 '19 at 12:43

PyQt Checkbox List with lambda signal

1 Answers1