Simple lazy model

simple_lazy_model.py

from PyQt5 import QtWidgets, QtCore
import gevent
import random
import weakref


def getData(key):
    data = "{}_{}_{}".format(key[0], key[1], random.choice("abcdefg"))
    gevent.sleep(random.random() * 5 + 2)
    return data


class Model(QtCore.QAbstractItemModel):
    def __init__(self, parent=None):
        super(Model, self).__init__(parent)
        self._data = {}
        self._procs = weakref.WeakSet()

    def columnCount(self, index, parent=None):
        return 3

    def rowCount(self, index, parent=None):
        if not index.isValid():
            return 2
        return 0

    def parent(self, index):
        return QtCore.QModelIndex()

    def index(self, row, col, parent=None):
        return self.createIndex(row, col)

    def reset(self):
        self.beginResetModel()
        while self._procs:
            proc = self._procs.pop()
            proc.kill()
        self._data.clear()
        self.endResetModel()

    def data(self, index, role=QtCore.Qt.DisplayRole):
        if role != QtCore.Qt.DisplayRole:
            return None
        key = index.row(), index.column()
        try:
            return self._data[key]
        except KeyError:
            proc = gevent.spawn(getData, key)
            proc.link(lambda p: self._dataUpdate(key, p.get()))
            self._procs.add(proc)
            self._data[key] = None
        return None

    def _dataUpdate(self, key, value):
        self._data[key] = value
        index = self.index(*key)
        self.dataChanged.emit(index, index)


class Main(QtWidgets.QMainWindow):
    def __init__(self):
        super(Main, self).__init__(None)

        layout = QtWidgets.QVBoxLayout()

        self._tree = QtWidgets.QTreeView()
        self._tree.setModel(Model())
        layout.addWidget(self._tree)

        self._reset = QtWidgets.QPushButton("Reset")
        self._reset.clicked.connect(self._handleReset)
        layout.addWidget(self._reset)

        container = QtWidgets.QWidget()
        container.setLayout(layout)
        self.setCentralWidget(container)

    def _handleReset(self):
        self._tree.model().reset()


if __name__ == "__main__":

    app = QtWidgets.QApplication([])
    win = Main()
    win.show()
    loop = QtCore.QTimer()
    loop.setInterval(10)
    loop.timeout.connect(lambda: gevent.sleep(0.01))
    loop.start()
    app.exec_()