TreeList = ({

    'Header1': ((
        'Item1',
        'Item2',
    )),

    'Header2': ((
        'Item1',
        'Item2'
    ))
})

ROOT
  |
  |-- Header1
  |    |-- Item1
  |    |-- Item2
  |
  |-- Header2
       |-- Item1
       |-- Item2

widget = QTreeWidget()

for i in TreeList:
    val = QTreeWidgetItem([i])
    widget.addTopLevelItem(val)

for i in TreeList['Header1']
    val = QTreeWidgetItem([i])
    widget.Header1.addChild(val)  # DONT THINK THIS IS RIGHT

for i in TreeList['Header2']
    val = QTreeWidgetItem([i])
    widget.Header2.addChild(val)  # DONT THINK THIS IS RIGHT