Concatenating string to list iteratively

So I’m trying to concatenate a string to a list, but for some reason it only works for the very last values:

labels_and_sents = []
for l in labels:
    for s in sents:
        sl = [l] + s
    
 labels_and_sents.append(sl)

Input:

labels = ['1','2','3']
sents = [['hi hello'], ['i was there'], ['this is a sent']]

Output:

[['3', 'this is a sent']]

What i need is:

[['1', 'hi hello'],['2', 'i was there'],['3', 'this is a sent']]

Answer

For me this looks like task for map, I would do

labels = ['1','2','3']
sents = [['hi hello'], ['i was there'], ['this is a sent']]
output = list(map(lambda x,y:[x]+y,labels,sents))
print(output)

Output:

[['1', 'hi hello'], ['2', 'i was there'], ['3', 'this is a sent']]

Explanation: for every pair of corresponding elements from labels and sents I pack first one into list and then concatenate with latter. This solution assumes len(labels)==len(sents) does hold True.