def __init__(self, name=None, children=None, labels=None, sort_by=None):

        # NB: do NOT assign default list/dict arguments in the function
        # declaration itself - these objects are shared between ALL instances
        # of PrettyTree, and by assigning to them it's easy to get into
        # infinite recursions, e.g. when 'self in self.children == True'
        if children is None:
            children = []
        if labels is not None:
            labels = []

        self.name = name
        self.children = children
        self.labels = labels
        self.sort_by = sort_by
dataset/ETHPy150Open PyTables/PyTables/tables/scripts/pttree.py/PrettyTree.__init__/operator `is` -> `is not`