Now, the translation code:

return ''.join([gencode.get(sequence[3*i:3*i+3],'X') for i in range(len(sequence)//3)])


Okay, so the sep.join(string_list) part just takes whatever strings are in the list string_list and sticks them together with the string sep in between each. That list, in this case, is of each of the translated codons.

The codon list comes from the list comprehension, which loops through each of the calculated number of codons len(sequence)//3. At each iteration, it indexes sequence to pull out a 3-letter subsequence. That codon is then used as a key to the gencode dictionary to find the corresponding amino acid.

I hope that clarified things a little. The trick is really the list comprehension, which is a pretty awesome thing that Python can do that isn’t available in other languages.

Cheers, R