python how to write to specific line in existing txt file

Question

Im writing a script which reads input file, takes the value and needs to write in the specific place (line) in output template, I'm absolute rookie, can't do it. It either writes in the first line of the output or in the last one.

opened file as 'r+'

used file.write(xyz) command

Confudes about how to explain to python to write to specific line, eg. line 17 (which is blank line in output template)

edit:

with open (MCNP_input, 'r+') as M:
           line_to_replace = 17
           lines = M.readlines()
           if len(lines) > int (line_to_replace):
               lines[line_to_replace] = shape_sphere + radius + '
'
               M.writelines(lines)

Answer 1

You can read the file and then, write into some specific line.

line_to_replace = 17 #the line we want to remplace
my_file = 'myfile.txt'

with open(my_file, 'r') as file:
    lines = file.readlines()
#now we have an array of lines. If we want to edit the line 17...

if len(lines) > int(line_to_replace):
    lines[line_to_replace] = 'The text you want here'

with open(my_file, 'w') as file:
    file.writelines( lines )