Hello everyone. I'm working on Python script that find ORF for a sequence. It runs well only for short sequencece. But when I tries to input much longer sequence it gives me wrong answer(according to this server: https://www.ncbi.nlm.nih.gov/orffinder/) Can anybody find what's the problem with my code?

my code:

def codons(seq):

stops = ["TTA","TGA","TAG"]    
lst1 = [] #List for the stars codons
lst2 = [] #List for the stop codons
start = 0 #The start position of the sequence.
counter = 0 #Counter for 3 optional orfs.
#initializes the lists for 3 optional orfs.
for i in range (3):
    lst1.append([])
    lst2.append([])
#Add to the lists the positions of the start and stop codons.
while (seq and counter < 3):

    for i in range(start,len(seq),3):
        codon = seq[i:i+3] #The codon is 3 nucleotides.
        #print codon+ "\t"
        if(codon == "ATG"): #If the codon is  a start codon.
            lst1[start].append(i+1) #The position of the start codon.

        if(codon in stops): #if the codon is a stop codon.
            lst2[start].append(i+1) #The position of the stop codon.


    start += 1 #promotes the starting position.
    counter += 1 #promotes the counter
print lst1
print "------------------"
print lst2
return lst1,lst2

thanks