Question

#!/usr/bin/python
# -*- coding: utf-8 -*-

import sys
from PyQt4 import QtGui, QtCore

class Example(QtGui.QMainWindow):
    def __init__(self):
        super(Example, self).__init__();
        self.initUI()


    def initUI(self):
        self.button = QtGui.QPushButton("print clicked",self)
        self.clicked='not_clicked'
        self.button.clicked.connect(lambda opt='clicked': self.option(opt)) 

    def option(self,opt):
        self.clicked=opt
        print opt



def main():
    app = QtGui.QApplication(sys.argv)
    ex = Example()
    ex.show()
    sys.exit(app.exec_())

if __name__=='__main__':
    main()

Consider this code. Now, when I click the ‘print clicked’ button. ‘False’ get printed( in the option function). Why is this happening?

Answer 1

QPushButton inherits the signal clicked from QAbstractButton.
Qt documentation states:

void QAbstractButton::clicked ( bool checked = false ) [signal]

This signal is emitted when the button is activated (i.e. pressed down then released while the mouse cursor is inside the button), when the shortcut key is typed, or when click() or animateClick() is called. Notably, this signal is not emitted if you call setDown(), setChecked() or toggle().

If the button is checkable, checked is true if the button is checked, or false if the button is unchecked.

The argument the slot received indicates whether the button is checked. Since QPushButton is by default not checkable. the argument is always False. That is why ‘False’ got printed.

If you want the print result varies, printing ‘True’ or ‘False’, you can either set the button checkable or change it to QCheckBox.
For example, initUI should be

def initUI(self):
    self.button = QtGui.QCheckBox("print clicked",self)
    self.button.setCheckable(True)
    self.clicked='not_clicked'
    self.button.clicked.connect(lambda opt='clicked': self.option(opt)) 

or

def initUI(self):
    #self.button = QtGui.QPushButton("print clicked",self)
    self.button = QtGui.QCheckBox("print clicked",self)
    self.clicked='not_clicked'
    self.button.clicked.connect(lambda opt='clicked': self.option(opt))