QDialog: how to use question mark (?) button?

Question

By default, QDialog windows have a question mark pushbutton in the upper-right corner. When I press it, the mouse cursor is changed to the \'Forbidden\' cursor,

Answer 1

The other answers were a bit misleading for me, focusing only on catching the question mark event, but not explaining the normal usage.

When this button is clicked and WhatsThisMode is triggered, the elements of the dialog are supposed to give info about themselves. And if mouse hovers over an element that supports this info then the pointer will become a pointing arrow with a question mark (on Windows at least), with a tooltip message displayed on mouse click.

Here's how to achieve it in PySide:

someWidget.setWhatsThis("Help on widget")

QWhatsThis documentation for PySide and Qt5 is also available.