f = open('./todo.txt', 'r')
newF = open('./todo-run.txt', 'a')
lines = f.readlines()
cLine = lines[int(index) - 1]

for line in lines:
  if line != cLine:
    newF.write(line)
f.close()
newF.close()
os.remove('./todo.txt')
shutil.move('./todo-run.txt', './todo.txt')

sed -i '18 d' filename

f = open(filepath, 'r')
lines = [line.rstrip('n') for line in f if not <CONDITION>]
f.close()
f.open('filepath, 'w')
f.write('n'.join(lines))
f.close()

def cutfile(file, startcut, endcut):
  file.seek(endcut)
  dataafter=file.read()
  file.seek(startcut)
  file.write(dataafter)
  file.truncate()

f = open('todo.txt', 'r+')
line_index = 0
prev_line_head = 0
remove_line_index = 3
move_lines = False
while True:
  line_head = f.tell()
  line = f.readline()

  if line == '': #EOF
    f.seek(prev_line_head)
    f.truncate()
    break

  if move_lines:
    f.seek(prev_line_head)
    f.write(line)
    f.flush()
    line_head = f.tell()
    line = f.readline() # read past the line we already read to start this iteration
  elif line_index == remove_line_index:
    move_lines = True
  prev_line_head = line_head
  line_index += 1
f.close()