source = [
    (None, 'a'),
    (None, 'b'),
    (None, 'c'),
    ('a', 'a1'),
    ('a', 'a2'),
    ('a2', 'a21'),
    ('a2', 'a22'),
    ('b', 'b1'),
    ('b1', 'b11'),
    ('b11', 'b111'),
    ('b', 'b2'),
    ('c', 'c1'),
]
a = {}

def appending(root_id, child_name, a):
    if root_id in a:
        a[root_id][child_name] = {}
    else:
        for el in a:
            el = appending(root_id, child_name, a[el])
    return a

for el in source:
    if not el[0]:
        a[el[1]] = {}
    else:
        a = appending(el[0], el[1], a)
print(a)