Why can't you use a generator function as a slot?

Question

I'm trying to iterate through a list in a PySide2 application. So each time a "Next" button is pressed, the next item is returned from the list and displayed. I could keep track of the index of the entry in my list which was most recently read and manually increment the index on each slot call, but I thought it might be more elegant to turn the slot into a generator function. But it doesn't work.

Minimal (not) working example follows.

import sys
from PySide2.QtWidgets import QApplication, QPushButton
from PySide2.QtCore import SIGNAL, QObject

def func():
    stringEntries=["One", "Two", "Three"]
    for item in stringEntries:
        # In the application this sets the values of a numpy array 
        # and fires a signal which updates a matplotlib canvas but meh, whatever
        print("func ", item, " has been called!")
        # This sort of works without the following yield statement
        yield

app = QApplication(sys.argv)
button = QPushButton("Next")
QObject.connect(button, SIGNAL ('clicked()'), func)
button.show()                                                                                             

sys.exit(app.exec_())

I was sort of expecting a different string to be printed each time I pressed the "Next" button, but instead it just sits there mocking me...

Is anyone able to point out the thing that I've fundamentally misunderstood please?

Answer 1

As @jasonharper points out in a comment you are creating a new iterator every time the button is pressed that generates the problem, a possible solution is to create a class that has the iterator as an attribute and using the __call__ method that looks over it, to make it simple and elegant I have created a decorator:

from PySide2.QtCore import QObject
from PySide2.QtWidgets import QApplication, QPushButton


class decorator:
    def __init__(self, f):
        self._f = f
        self._iterator = None

    def __call__(self, *args, **kwargs):
        if self._iterator is None:
            self._iterator = self._f(*args, **kwargs)
        try:
            return next(self._iterator)
        except StopIteration:
            pass


@decorator
def func():
    stringEntries = ["One", "Two", "Three"]
    for item in stringEntries:
        print("func ", item, " has been called!")
        yield


if __name__ == "__main__":
    import sys

    app = QApplication(sys.argv)
    button = QPushButton("Next")
    button.clicked.connect(func)
    button.show()

    sys.exit(app.exec_())