Tribonacci Sequence

def tribonacci_sequence(start_sequence, length):
    
#if length is zero make the start_sequence = null and return start_sequence
    if length==0: start_sequence=[]; return start_sequence
    
#if length is one and return first element of start_sequence or 0th index
    elif length==1: return start_sequence[:1]
    
#if length is two make return first two element of start_sequence or 0th or 1st index
    elif length==2: return start_sequence[:2]
    
#if length is greater than 2 then 
    elif length>2:
        
#loop until length starting from 2
        for i in range(2,length):
        
#sum the last three consecutive elements and append in the list start_sequence
            start_sequence.append(start_sequence[i-2]+start_sequence[i-1]+start_sequence[i])
    
#in case of any other length apart from whole numbers
    else: return "invalid length"
    
#return the array of start_sequence
    return start_sequence[:length]