This doesn't directly answer your question (that has been addressed already), but you should check out the
. Among other things in this package, there are modules for parsing sequences in many formats (including FASTA) -- using these, your code will never need to search explicity for ">" to begin a line, and you will be able to think more about answering your questions and less about parsing the files. As a byproduct, your code will be much simpler to read and maintain, and problems (like this one) will be easier to track down and debug.