Using list comprehension to overwrite a variable based on condition

I have a list of DNA sequences (seqs), and I’m trying to use a list comprehension to find the length of the longest sequence. Essentially, I’m trying to turn this into a list comprehension:

longest_sequence = 0
for sequence in seqs:
    if len(sequence) > longest_sequence:
        longest_sequence = len(sequence)

This is what I thought the list comprehension would look like:

[(longest_sequence = len(sequence)) for sequence in seqs if len(sequence) > longest_sequence]

What am I doing wrong?

Thanks.

>Solution :

List comprehensions are used to obtain lists. With a list comprehension, each element in some input list/iterable goes through some filter/map/conditional logic to obtain an output list.

In your case, you need a scalar (just one number), not a list.

So you’re not helped that much by using list comprehension. But if you want to use it, you can use a list comprehension to obtain a list of the lengths, then just take the max of that list, as such:

lengths = [len(seq) for seq in seqs]
longest_length = max(lengths)

Keep in mind that both this code and your code will output the maximum length, not the sequence with the maximum length.