def groups(stream, is_group):
    group, items = None, []
    for item in stream:
        if is_group(item):
            if group:
                yield group, items
            group, items = item, []
        else:
            items.append(item)
    if group:
        yield group, items

test_data = ('A', 1, 2, 3, 'B', 4, 5, 'C', 7)

for name, items in groups(test_data, lambda x: isinstance(x, str)):
    print name, items
==============
A [1, 2, 3]
B [4, 5]
C [7]

def groupper(d, x):
    if isinstance(x, str):
       d.append((x, []))
   else:
       d[-1][1].append(x)
    return d

reduce(groupper, test_data, [])

from itertools import groupby
it = groupby(test_data, lambda x: isinstance(x, str))
for item in it:
    print item[1].next(), list(it.next()[1])

test_data = ('A', 1, 2, 3, 'B', 4, 5, 'C', 7, '#')
letters = [i for i in test_data if isinstance(i, str)]
print dict(zip(letters,
    [test_data[test_data.index(letters[i]) + 1 : test_data.index(letters[i + 1])] for i in xrange(len(letters) - 1)])) 
---------------------
{'A': (1, 2, 3), 'C': (7,), 'B': (4, 5)}