How to add 1 in a text file ?

[snippsat]

Use code tag BBcode help,use Ctrl+Shift+v when copy in to preserve indentation.

with open('data.txt') as f,open('new.txt', 'w') as f_out:
    lst = [i.strip() for i in f]
    lst.append('test 4')
    f_out.write('\n'.join(lst))

To append to existing file.

with open('data.txt', 'a') as f:
    f.write('test 4\n')