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?

Answer 2

You can set the current position of the file object using file.seek(). To jump to the beginning of a file, use

f.seek(0, os.SEEK_SET)

To jump to a file's end, use

f.seek(0, os.SEEK_END)

In your case, to check if a file contains something, and then maybe append append to the file, I'd do something like this:

import os

with open("file.txt", "r+") as f:
    line_found = any("foo" in line for line in f)
    if not line_found:
        f.seek(0, os.SEEK_END)
        f.write("yay, a new line!\n")