Splitting list of dictionary into sublists after the occurence of particular key of dictionary

You can use a generator that collects elements and yields when the condition is met:

def split_by_key(lst, key):
    collected = []
    for d in lst:
        collected.append(d)
        if key in d:
            yield collected
            collected = []
    if collected:  # yield any remainder
        yield collected

final_lst = list(split_by_key(lst, 'a'))

Demo:

>>> lst = [{'x': 23}, {'y': 23432}, {'z': 78451}, {'a': 564}, {'x': 45},
...        {'y': 7546}, {'a': 4564}, {'x': 54568}, {'y': 4515}, {'z': 78457},
...        {'b': 5467}, {'a': 784}]
>>> list(split_by_key(lst, 'a'))
[[{'x': 23}, {'y': 23432}, {'z': 78451}, {'a': 564}], [{'x': 45}, {'y': 7546}, {'a': 4564}], [{'x': 54568}, {'y': 4515}, {'z': 78457}, {'b': 5467}, {'a': 784}]]
>>> pprint(_)
[[{'x': 23}, {'y': 23432}, {'z': 78451}, {'a': 564}],
 [{'x': 45}, {'y': 7546}, {'a': 4564}],
 [{'x': 54568}, {'y': 4515}, {'z': 78457}, {'b': 5467}, {'a': 784}]]

Here is a straightforward solution:

result = []

for item in lst:
    if not result or 'a' in result[-1][-1]:
        result.append([])

    result[-1].append(item)