From flat to nested structure

[nilamo]

That looks super wacky, but sure whatever lol.

I guess a list of dicts, which represents the different sequences of characters might work. As you go through the data, you only ever modify the newest sequence (index -1 is the last index). Then, once a key is found that already exists in that dict, you start a new sequence.

Something like this?

>>> data = '''a : one
...  b : two
...  c : three
...  d : four
...  a : five
...  b : six
...  c : seven
...  d : eight
...  e : nine'''.split('\n')
>>> data
['a : one', ' b : two', ' c : three', ' d : four', ' a : five', ' b : six', ' c : seven', ' d : eight', ' e : nine']
>>> sequences = [{}]
>>> for item in data:
...   pair = item.split(':')
...   key = pair[0].strip()
...   value = pair[1].strip()
...
>>> for item in data:
...   pair = item.split(':')
...   key = pair[0].strip()
...   value = pair[1].strip()
...   if key in sequences[-1]:
...     # this key has already been seen, so we're starting a new sequence
...     sequences.append({})
...   sequences[-1][key] = value
...
>>> sequences
[{'d': 'four', 'b': 'two', 'a': 'one', 'c': 'three'}, {'d': 'eight', 'b': 'six', 'e': 'nine', 'a': 'five', 'c': 'seven'}]
>>> import pprint
>>> pprint.pprint(sequences)
[{'a': 'one', 'b': 'two', 'c': 'three', 'd': 'four'},
 {'a': 'five', 'b': 'six', 'c': 'seven', 'd': 'eight', 'e': 'nine'}]