originPath='file.txt'
savePath='newfile.txt'
# ÿ¸ô¶àÉÙÐб£´æÒ»´Î
l=2000
with open(originPath) as reader, open(savePath, 'w') as writer:
    # ´ÓµÚ0ÐпªÊ¼
    for index, line in enumerate(reader):
        if index % l == 0:
            writer.write(line)
               
    writer.close()