PyQt4 signals and slots with eventFilter

Question

I am having trouble receiving a signal sent from an event filter. In the example below the button pressed signal/slot works fine, and the focus out filter signals emit OK. However the focus out signals are not being intercepted and the slots do not fire. Any idea what I am doing wrong?

from PyQt4.QtCore import SIGNAL, QObject, QEvent
from PyQt4.QtGui import QApplication, QLabel, QWidget, QLineEdit, QPushButton, QTextEdit, QVBoxLayout

class SignalOnFocus(QWidget):
    def __init__(self):
        super(SignalOnFocus, self).__init__()
        layout = QVBoxLayout()
        self.label = QLabel("Type in some text then push button")
        self.inputLineEdit1 = QLineEdit()
        self.inputLineEdit1.setObjectName("inputLineEdit1")

        self.focusOutFilter = FocusOutFilter()
        self.inputLineEdit1.installEventFilter(self.focusOutFilter)
        self.connect(self.inputLineEdit1, SIGNAL("focus_out"),
                 self.focusLost)
        self.inputLineEdit2 = QLineEdit()
        self.inputLineEdit2.setObjectName("inputLineEdit2")
        self.mousePressedFilter = MousePressedFilter()
        self.inputLineEdit2.installEventFilter(self.mousePressedFilter)
        self.connect(self.inputLineEdit2, SIGNAL("mouse_clicked"), self.mouseClicked)
        self.button1 = QPushButton("Press me")
        self.button1.setObjectName("button1")
        self.connect(self.button1, SIGNAL("clicked()"), self.buttonPressed)
        self.textEdit = QTextEdit()
        layout.addWidget(self.label)
        layout.addWidget(self.inputLineEdit1)
        layout.addWidget(self.inputLineEdit2)
        layout.addWidget(self.button1)  
        layout.addWidget(self.textEdit)
        self.setLayout(layout)

    def mouseClicked(self):
        self.textEdit.append(" mouse clicked")

    def buttonPressed(self):
        self.textEdit.append(" button pressed")

    def focusLost(self):
        self.textEdit.append(" focus_out")

class MousePressedFilter(QObject):
    def eventFilter(self, widget, event):
        if event.type() == QEvent.MouseButtonPress:
            print("--eventFilter() mouse_clicked on "+str(widget.objectName()))
            self.emit(SIGNAL("mouse_clicked"))
            return False
        else:
            return False

class FocusOutFilter(QObject):
    def eventFilter(self, widget, event):
        if event.type() == QEvent.FocusOut:
            print("--eventFilter() focus_out on "+str(widget.objectName()))
            self.emit(SIGNAL("focus_out"))
            return False
        else:
            return False

if __name__ == "__main__":
    app = QApplication([])
    form = SignalOnFocus()
    form.show()
    app.exec_()

ekhumoro ekhumoro · Accepted Answer · 2015-01-03T00:13:24

The filter objects are emitting the signals, so that is what you need to specify when connecting them:

    self.connect(self.focusOutFilter, SIGNAL("focus_out"), self.focusLost)
    ...
    self.connect(self.mousePressedFilter, SIGNAL("mouse_clicked"), self.mouseClicked)

But please seriously consider getting rid of that ugly, old-style syntax for connecting signals. Official support for Qt4 is coming to an end this year, and PyQt5 has already made the old-style syntax totally obsolete.

Using the new-style syntax, your example would look like this:

from PyQt4.QtCore import pyqtSignal, QObject, QEvent

class SignalOnFocus(QWidget):
    def __init__(self):
        ...    
        self.focusOutFilter = FocusOutFilter()
        self.inputLineEdit1.installEventFilter(self.focusOutFilter)
        self.focusOutFilter.focusOut.connect(self.focusLost)

class FocusOutFilter(QObject):
    focusOut = pyqtSignal()

    def eventFilter(self, widget, event):
        if event.type() == QEvent.FocusOut:
            print("--eventFilter() focus_out on " + widget.objectName())
            self.focusOut.emit()

which I hope you will agree looks much more readable (and easier to get right).

(Also note that if you are using Python 3 with PyQt, by default, any Qt method that returns a QString is automatically converted to a python string - so you don't need to convert it yourself using str).

PyQt4 signals and slots with eventFilter

1 Answers