You could join on the punctuation first:
def join_punctuation(seq, characters='.,;?!'):
characters = set(characters)
seq = iter(seq)
current = next(seq)
for nxt in seq:
if nxt in characters:
current += nxt
else:
yield current
current = nxt
yield current
c = ' '.join(join_punctuation(b))
The join_punctuation
generator yields strings with any following punctuation already joined on:
>>> b = ['Hello', ',', 'who', 'are', 'you', '?']
>>> list(join_punctuation(b))
['Hello,', 'who', 'are', 'you?']
>>> ' '.join(join_punctuation(b))
'Hello, who are you?'