Pyqt QScrollArea not scrolling

I want to put some elements of my UI in a scroll area as there can be a lot of them. I tried the following peice of code, but the area just keeps growing as I put more elements on it.

In the first part I set up a scroll area, a widget and a layout. I apply the layout to the widget and I set the widget to the scrollarea. Then I fill in my layout in an external function. The button under all of it allows to fill more elements in the layout.

Code:

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scrollRow = QtGui.QScrollArea()
    scrollRow.setMaximumSize(600, 400)
    self.rowAssetWidget = QtGui.QWidget()
    self.rowAssetLayout = QtGui.QGridLayout()
    self.rowAssetLayout.setSpacing(20)
    self.rowAssetWidget.setLayout(self.rowAssetLayout)
    scrollRow.setWidget(self.rowAssetWidget)
    #self.mainLayout.addLayout(self.rowAssetLayout, 2, 0)
    self.mainLayout.addWidget(self.rowAssetWidget, 2, 0)
    self.assetRow()
 
    self.addAssetRowBtn = QtGui.QPushButton("+")
    self.addAssetRowBtn.setFixedSize(20, 20)
    self.mainLayout.addWidget(self.addAssetRowBtn, 3, 0)
    self.connect(self.addAssetRowBtn, QtCore.SIGNAL("clicked()"), self.addAssetRow)

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My elements appear fine, but it is not scrolling. Any idea ?