Add a line after a specific line

[buran]

No, you need to create one extra file

input_file = 'input_file.txt' # '/path/to/inputfile.txt'
output_file = 'output_file.txt' # '/path/to/outputfile.txt'

with open(input_file) as in_file, open(output_file, 'w') as out_file:
    for line in in_file:
        out_file.write(line)
        if line.strip() == '/##/':
            out_file.write('TEST INSERT OF NEW LINE\n')