PyQt lazy threading

threading.py

WORKERS = []

class BackgroundWorker(QtCore.QThread):
    result = QtCore.pyqtSignal(object)
    exception = QtCore.pyqtSignal(object)

    def __init__(self, func, fin):
        super().__init__()
        WORKERS.append(self)
        self.finished.connect(lambda: WORKERS.remove(self))
        self.func = func
        if fin:
            self.result.connect(lambda x: fin(x))
        self.exception.connect(self.raise_exception)

    def run(self):
        try:
            result = self.func()
        except Exception as e:
            self.exception.emit(e)
        else:
            self.result.emit(result)

    def raise_exception(self, e):
        """Raise exception in the main thread."""
        raise e


def run_daemon(func, *args, fin=None, **kwargs):
    noargs = lambda: func(*args, **kwargs)
    BackgroundWorker(noargs, fin).start()