PyQt Widget connect() and disconnect()

Question

Depending on a conditions I would like to connect/re-connect a button to a different function.

Let's say I have a button:

myButton = QtGui.QPushButton()

For this example let's say I check if there is an internet connection.

if connected == True:
    myButton.clicked.connect(function_A)

elif connected == False:
    myButton.clicked.connect(function_B)

First of all I would like to disconnect a button from any function it was already connected before the button is being re-assigned/re-connected to another function (function_A or function_B). Secondly, I have already noticed that after the button is re-connected it takes an extra click for the button to pick up a new function. After the button is re-connected to another function it still attempts to run a previous function - a function to which a button was connected earlier (before a re-connection). Please advice. Thanks in advance!

EDITED LATER:

It appears a widget's .disconnect() method can be used to disconnect a button from a function it it is connected.

myButton.disconnect()

Unfortunately .disconnect() throws an error if a widget is not connected to any function. To get around it I am using Try/Except. But I would rather use a more elegant solution...

try: myButton.clicked.disconnect() 
except Exception: pass

Mucoprotein

what is the exception raised? bit strange. NOte that myButton.disconnect() is different from myButton.clicked.disconnect(), which are you actually referring to? – Mezereum

@Schollii. It raises a TypeError. And the exception is not any way strange: in fact, it's highly desirable. The reason why it was introduced, is because the old-style method of connecting and disconnecting signals fails silently, which can hide a lot of potential bugs. In short, raising an exception in just much more pythonic. – Scum

Thanks for explanation. I just don't see why a function that is supposed to disconnect() all signals should treat "there are currently no connections" as exceptional behavior. OTOH if you tried disconnect(someSignal) and that signal does not exist, then I could see that as being exceptional condition that should be flagged. But anyhoo, it's neither here nor there, thanks for the info. – Mezereum

Answer 1

If you need to reconnect signals in many places, then you could define a generic utility function like this:

def reconnect(signal, newhandler=None, oldhandler=None):        
    try:
        if oldhandler is not None:
            while True:
                signal.disconnect(oldhandler)
        else:
            signal.disconnect()
    except TypeError:
        pass
    if newhandler is not None:
        signal.connect(newhandler)

...

if connected:
    reconnect(myButton.clicked, function_A)
else:
    reconnect(myButton.clicked, function_B)

(NB: the loop is needed for safely disconnecting a specific handler, because it may have been connected multple times, and disconnect(slot) only removes one connection at a time.).

Scum

So if you do signal.disconnect() - without giving a slot parameter - then this signal will disconnect from all its slots? In other words - no need for looping in such case? – Yardmaster

@Yardmaster That's what the docs say, so if you see different behaviour, it would be a bug. It's easy enough to test this by inspecting the output of sender.receivers(sender.signal). – Scum

Answer 2

Try this:

from PyQt4 import QtGui as gui

app = gui.QApplication([])

myButton = gui.QPushButton()

def function_A():
    myButton.clicked.disconnect() #this disconnect all!
    myButton.clicked.connect(function_B)
    print 'function_A'

def function_B():
    myButton.clicked.disconnect(function_B) #this disconnect function_B
    myButton.clicked.connect(function_A)
    print 'function_B'

myButton.clicked.connect(function_A)
myButton.setText("Click me!")
myButton.show()

app.exec_()

Gurl

Do you know how to check if a widget is connected to a function? I found there is .isSignalConnected() widget method. Unfortunately I don't know how to use it... – Mucoprotein

Well you should read this, I think there is no hope, but indeed if you found a way let me know ;) – Gurl

Answer 3

Concise way for 3.4+ with contextlib.suppress:

with contextlib.suppress(RuntimeError):
    button.clicked.disconnect()
button.connect(func_a if condition else func_b)

Thay

Answer 4

Try like this:

button.doubleClicked.connect(my_function)

It checks whether the button is connected, disconnects it if it is:

if button.doubledClicked.connect(my_function):
     button.doubledClicked.disconnect()

Evars