index file for line numbers of large files

file = "Desktop/coordinates.csv"

index = [None]
with open(file, "rt", encoding="utf8", newline=None) as fd:
    offset = 0
    for line_number, line in enumerate(fd, start=1):
        size = len(line)
        index.append((line_number, offset, size))
        offset += size + 1


# index: line number starting with 1
#        offset, where to seek
#        size, how much to read the single line
# first index is None, so the index fits to line numbering.