how to add lines to existing file using python

Open the file for 'append' rather than 'write'.

with open('file.txt', 'a') as file:
    file.write('input')

If you want to append to the file, open it with 'a'. If you want to seek through the file to find the place where you should insert the line, use 'r+'. (docs)

Use 'a', 'a' means append. Anything written to a file opened with 'a' attribute is written at the end of the file.

with open('file.txt', 'a') as file:
    file.write('input')