Python code to parse a (sub)tree starting from a node nid to the horizontal format. None elements

tree_to_hformat.py

# Python code to parse a (sub)tree starting from a node nid to the horizontal format. 
# Returns a python list with node IDs. None elements indicates backtracking
def tree_to_hformat(tree, nid = 0, list_hformat=None):
    if list_hformat is None:
        list_hformat = []
    list_hformat.append(nid)
    for child in tree.successors(nid):
        tree_to_hformat(tree, child, list_hformat)
        list_hformat.append(None)
    return(list_hformat)