QWidget::setLayout: Attempting to set QLayout "" on MainWindow "", which already has a layout

You can't set a QLayout directly on the QMainWindow. You need to create a QWidget and set it as the central widget on the QMainWindow and assign the QLayout to that.

wid = QtGui.QWidget(self)
self.setCentralWidget(wid)
layout = QtGui.QVBoxLayout()
wid.setLayout(layout)

Just an update to Brenden Abel's answer:

QWidget and QVBoxLayout (for Python3, PyQt5) are now contained in the PyQt5.QtWidgets module and not the PyQt5.QtGui module.

So updated code:

wid = QtWidgets.QWidget(self)
self.setCentralWidget(wid)
layout = QtWidgets.QVBoxLayout()
wid.setLayout(layout)