PyQt4 signals and slots

Question

Hello,

I am writing my first Python app with PyQt4. I have a MainWindow and a Dialog class, which is a part of MainWindow class:

self.loginDialog = LoginDialog();

I use slots and signals. Here's a connection made in MainWindow:

QtCore.QObject.connect(self.loginDialog, QtCore.SIGNAL("aa(str)"), self.login)

And I try to emit signal inside the Dialog class (I'm sure it is emitted):

self.emit(QtCore.SIGNAL("aa"), "jacek")

Unfortunately, slot is not invoked. I tried with no arguments as well, different styles of emitting signal. No errors, no warnings in the code. What might be the problem?

Answer 1

I haven't used PyQT4 but take a look at here.

Answer 2

Hey,

Looks like you miss the "SLOT" part in your connect call.

Here is an example :

QtCore.QObject.connect(self.loginDialog, QtCore.SIGNAL("NotifyMySignal(int)"), QtCore.SLOT("onNotifyMySignal(int)"));

then

self.emit(QtCore.SIGNAL('NotifyMySignal(1)'));

Hope this helps !

Answer 3

You don't use the same signal, when emitting and connecting.

QtCore.SIGNAL("aa(str)") is not the same as QtCore.SIGNAL("aa"). Signals must have the same signature. By the way, if you are defining your own signals, don't define parametres. Just write SIGNAL('aa'), because defining parametres is a thing from C++ and Python version of Qt doesn't need this.

So it should look like this:

QtCore.QObject.connect(self.loginDialog, QtCore.SIGNAL("aa"), self.login)

and if you pass any parametres in emit, your login method must accept those parametres. Check, if this helps :-)

Answer 4

As noted by gruszczy you have to use the same QtCore.SIGNAL('xxx') to connect signal and to emit it. Also I think you should use Qt types in the arguments list of signal function. E.g.:

QtCore.QObject.connect(self.loginDialog, QtCore.SIGNAL("aa(QString&)"), self.login)

And then emit with:

self.emit(QtCore.SIGNAL("aa(QString&)"), "jacek")

Sometimes it makes sense to define signal only once as global variable and use it elsewhere:

MYSIGNAL = QtCore.SIGNAL("aa(QString&)")
...
QtCore.QObject.connect(self.loginDialog, MYSIGNAL, self.login)
...
self.emit(MYSIGNAL, "jacek")

Answer 5

What @bialix suggested should have worked, but try an alternative way of connecting:

class Foo(QtCore.QObject):
    mysignal = QtCore.pyqtSignal(str, name='mysignal')

    def connect_to_signal(self):
        # you can use this syntax instead of the 'old' one
        self.mysignal.connect(self.myslot)

        # but this will also work
        self.connect(self, QtCore.SIGNAL('mysignal(QString)'), self.myslot) 

        self.mysignal.emit("hello")

    def myslot(self, param):
        print "received %s" % param

For a more detailed explanation of how signals/slots work in PyQt I'd suggest going through it's documentation, specifically this section.

Answer 6

I checked your code and it looks like the problem is in the way how you're connecting your signal

1. you emit the signal in Ui_Dialog class

   self.emit(QtCore.SIGNAL("aa()"))

2. you connect to the signal in Ui_MainWindow's setupUi method by calling

   QtCore.QObject.connect(self.loginDialog.ui, QtCore.SIGNAL("aa()"), self.login)

notice first parameter is changed to self.loginDialog.ui; your original connect call was using self.loginDialog which is of the LoginDialog type, whereas signal is emitted by the Ui_Dialog class which is ui property of the LoginDialog. After this change login method of the Ui_MainWindow got called

hope this helps, regards