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)
Here is what the above code is Doing:
1. We create an empty list called flat_list.
2. We iterate over the outer list.
3. For each outer list element, we iterate over the inner list.
4. For each inner list element, we append it to flat_list.
5. We return flat_list.