A simple file wrapper to allow seeking to specific lines at random.

lineseekablefile.py

class LineSeekableFile:
    def __init__(self, seekable):
        self.fin = seekable
        self.line_map = list() # Map from line index -> file position.
        self.line_map.append(0)
        while seekable.readline():
            self.line_map.append(seekable.tell())
    
    def __getitem__(self, index):
        # NOTE: This assumes that you're not reading the file sequentially.  For that, just use 'for line in file'.
        self.fin.seek(self.line_map[index])
        return self.fin.readline()