QGraphicsItem.parentItem() fails when parent has __getitem__ defined

When using subclasses of QGraphicsItem, I have found that defining _getitem_ on a subclass will cause parentItem() to fail on a child item. For example:

from PySide2.QtWidgets import QGraphicsItem, QGraphicsRectItem

class GraphicsItem(QGraphicsItem):
    def __getitem__(self, item):
        return None

parent = GraphicsItem()
child = QGraphicsRectItem(parent)

print("Parent:", child.parentItem())

Running the above code generates the following error:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: object of type 'GraphicsItem' has no len()

The above example might seem contrived, but it was the result of about an hour of stripping down a larger application until it became clear what the problem was. So although the solution here is simple (remove _getattr_ from parent class), it is not at all obvious when this is encountered in a real application, and the result is very difficult to debug.