How to write to .txt files in Python 3

Open the file in append mode and write a new line (including a \n line separator):

with open(filename, 'a') as out:
    out.write(var + '\n')

This adds the line at the end of the file after all the other contents.

Just to be complete on this question:

You can also use the print function.

with open(filename, 'a') as f:
    print(var, file=f)

The print function will automatically end each print with a newline (unless given an alternative ending in the call, for example print(var, file=f, end='') for no newlines).