python how to flatten a list

flat_list = [item for sublist in l for item in sublist]

flat_list = [item for sublist in l for item in sublist]

#which is equivalent to this 
flat_list = []
for sublist in l:
    for item in sublist:
        flat_list.append(item)

flattened = [val for sublist in list_of_lists for val in sublist]

flat_list = []
for sublist in l:
    for item in sublist:
        flat_list.append(item)