from itertools import groupby, chain
 
data = [
    -1, 0,
    'a','b', 1, 2, 3,
    'c', 6,
    'd', 'e', .4, .5,
    'a', 'b', 4,
    'f', 'g'
]
output = {}
groups = groupby(data, lambda i: isinstance(i, str))
try:
    is_str, group = next(groups)
    if not is_str:
        is_str, group = next(groups)
except StopIteration:
    pass
else:
    for is_str, group in chain([(is_str, group)], groups):
        if is_str:
            key = '/'.join(group)
        else:
            output.setdefault(key, []).extend(group)
    if is_str:
        output.setdefault(key, [])
print(output)

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