Improvements

Line 8, 19 : replace the []  with pass

Generally, isn't it a bit inefficient ?

You are picking one avi file, and then for _each_ avi file you are iterating through the entire subtitles directory

Assuming n files and n subtitles, you are doing the loops n*n times, whereas you could be doing it n + n times, n for avi and n for .srt

Is that possible ?
