line edit event filter

class LineEditStyleEventFilter(QObject):

    def __init__(self, lineEdit, defaultStyle, placeholderStyle, **kwargs):
        super().__init__(parent=lineEdit, **kwargs)
        self.defaultStyle = defaultStyle
        self.placeholderStyle = placeholderStyle
        lineEdit.setStyleSheet(placeholderStyle)
        lineEdit.installEventFilter(self)


    def eventFilter(self, obj, event):
        if isinstance(event, (QFocusEvent, QInputEvent)):
            if obj.text():
                obj.setStyleSheet(self.defaultStyle)
            else:
                obj.setStyleSheet(self.placeholderStyle)
        return super().eventFilter(obj, event)


# ...
# LineEditStyleEventFilter(someLineEdit, 'color: black', 'color: red')