[FIXED] pyqt signal emit with object instance or None

Issue

I would like to have a pyqt signal, which can either emit with a python "object" subclass argument or None. For example, I can do this:

valueChanged = pyqtSignal([MyClass], ['QString'])

but not this:

valueChanged = pyqtSignal([MyClass], ['None'])
TypeError: C++ type 'None' is not supported as a pyqtSignal() type argument type

or this:

valueChanged = pyqtSignal([MyClass], [])
TypeError: signal valueChanged[MyObject] has 1 argument(s) but 0 provided

I also tried None without the quotes and the c++ equivalent "NULL". But neither seems to work. What is it I need to do to make this work?

Solution

There are a couple of reasons why what you tried didn't work.

Firstly, the type argument of pyqtSignal accepts either a python type object or a string giving the name of a C++ object (i.e. a Qt class).

So, to use None as an argument type, you would have to pass type(None) rather than the string "None".

Secondly, None is special-cased to allow the default overload of the signal to be used without explicitly selecting the signature.

So if you created the signal like this:

valueChanged = QtCore.pyqtSignal([MyClass], [type(None)])

and then attempted to emit the signal like this:

self.valueChanged[type(None)].emit(None)

you would get an error like this:

TypeError: Window.valueChanged[MyClass].emit():
argument 1 has unexpected type 'NoneType'

So using valueChanged[type(None)] would actually result in the default signature ([MyClass]) being selected, which would then receive a mismatched argument of None.

Answered By - ekhumoro