Question

I am starting to learn Qt4 and Python, following along some tutorial i found on the interwebs. I have the following two files:

lcdrange.py:

from PyQt4 import QtGui, QtCore  class LCDRange(QtGui.QWidget):                             def __init__(self, parent=None):         QtGui.QWidget.__init__(self, parent)          lcd = QtGui.QLCDNumber(2)          self.slider = QtGui.QSlider()         self.slider.setRange(0,99)         self.slider.setValue(0)          self.connect(self.slider, QtCore.SIGNAL('valueChanged(int)'),                      lcd, QtCore.SLOT('display(int)'))         self.connect(self.slider, QtCore.SIGNAL('valueChanged(int)'),                      self, QtCore.SIGNAL('valueChanged(int)'))          layout = QtGui.QVBoxLayout()         layout.addWidget(lcd)         layout.addWidget(self.slider)         self.setLayout(layout)      def value(self):         self.slider.value()      def setValue(self,value):         self.slider.setValue(value) 

main.py:

import sys from PyQt4 import QtGui, QtCore  from lcdrange import LCDRange  class MyWidget(QtGui.QWidget):     def __init__(self, parent=None):         QtGui.QWidget.__init__(self, parent)          quit = QtGui.QPushButton('Quit')         quit.setFont(QtGui.QFont('Times', 18, QtGui.QFont.Bold))         self.connect(quit, QtCore.SIGNAL('clicked()'), QtGui.qApp, QtCore.SLOT('quit()'))          grid = QtGui.QGridLayout()         previousRange = None          for row in range(0,3):             for column in range(0,3):                 lcdRange = LCDRange()                  grid.addWidget(lcdRange, row, column)                 if not previousRange == None:                     self.connect(lcdRange, QtCore.SIGNAL('valueChanged(int)'),                                  previousRange, QtCore.SLOT('setValue(int)'))                 previousRange = lcdRange          layout = QtGui.QVBoxLayout()         layout.addWidget(quit)         layout.addLayout(grid)         self.setLayout(layout)     app = QtGui.QApplication(sys.argv)  widget = MyWidget() widget.show()   sys.exit(app.exec_()) 

When i run this i get the following errors:

Object::connect: No such slot LCDRange::setValue(int) Object::connect: No such slot LCDRange::setValue(int) Object::connect: No such slot LCDRange::setValue(int) Object::connect: No such slot LCDRange::setValue(int) Object::connect: No such slot LCDRange::setValue(int) Object::connect: No such slot LCDRange::setValue(int) Object::connect: No such slot LCDRange::setValue(int) Object::connect: No such slot LCDRange::setValue(int) 

I’ve read that PyQt slots are nothing more than methods, which i have defined, so what am i doing wrong?

I am also learning Qt4 with Ruby which is where this code originates from, i translated it from Ruby to Python. In the Ruby version the LCDRange class is defined as this:

class LCDRange < Qt::Widget   signals 'valueChanged(int)'   slots 'setValue(int)'    def initialize(parent = nil)   ... 

So my guess was that i have to somehow declare the existence of the custom slot?

Answer 1

Try this:

self.connect(lcdRange, QtCore.SIGNAL('valueChanged'), previousRange.setValue) 

What’s the difference?

The PyQt documentation has a section about SIGNALS/SLOTS in PyQt, they work a little differently.

SIGNAL

SIGNAL('valueChanged') is something called a short-circuit signal. They work only for Python-to-Python methods, but they are faster and easier to implement.

SLOT

If you have a python slot, you can specify it just by tipping the method: previousRange.setValue. This works for all methods accessible by Python.

If your slots should be accessible like C++ Qt slots, as you tried in your code, you have to use a special syntax. You can find information about pyqtSignature decorator on the PyQt website.