def __init__(self, manager):
        self.manager = manager
        self._containers = None
Conserve RangeIndex type for scalar and slice keys.
dataset/ETHPy150Open ContinuumIO/ashiba/enaml/enaml/qt/docking/layout_builder.py/LayoutBuilder.__init__--crossover with--dataset/ETHPy150Open pydata/pandas/pandas/indexes/range.py/RangeIndex.__getitem__