Python: Open a file, search then append, if not exist

Question

I am trying to append a string to a file, if the string doesn\'t exit in the file. However, opening a file with a+ option doesn\'t allow me to do at once, because o

Answer 1

To leave the input file unchanged if needle is on any line or to append the needle at the end of the file if it is missing:

with open("filename", "r+") as file:
    for line in file:
        if needle in line:
           break
    else: # not found, we are at the eof
        file.write(needle) # append missing data

I've tested it and it works on both Python 2 (stdio-based I/O) and Python 3 (POSIX read/write-based I/O).

The code uses obscure else after a loop Python syntax. See Why does python use 'else' after for and while loops?