计算较大DNA序列内模糊序列的出现次数

I am trying to search and count the number of a small DNA sequence (R) that occurs within a larger sequence (F), but R has a few characters that could be variable. The easiest way I could think to do this would be to set a ratio for R and count all hits above 80% in F, but the only commands that seem to do this (eg. difflib's SequenceMatcher or get_close_matches) need lists to work. I cant break F into any such lists. Any ideas?I am trying to search and count the number of a