List comprehension with a Boolean who's value changes throughout iterations

I'm attempting to create a list comprehension from a nested for loop with if statements, and I can't figure it out. I scanned the internet for hours but couldn't find a solution.

The loop looks like that:

identical_ids = None

with open(fasta_path_, 'r') as fasta_file:
    for line in fasta_file.readlines():
        if identical_ids is True:
            lengths.append(len(line.strip()))
        for seq_id in seqids:
            if seq_id in line:
                identical_ids = True
                continue
            else:
                identical_ids = False
                continue

My last attempt:

lengths = [len(line.strip()) for line in fasta_file.readlines() if identical_ids for seq_id in seqids if seq_id in line identical_ids := True else identical_ids := False]

Maybe it can't be done.

Thanks.