Use another file A if file B does not exist

I think it is a simple task. I tried a lot of things with no success.

I want to use a file A on a network directory and when I'm offline I want to use file B on my ssd.

file_PATH = "y:/file_A.txt"

if os.path.isfile(file_PATH):
    file_to_use = "y:/file_A.txt"
else:
    file_to_use = "d:/file_B.txt"

It doesn't work with this piece of code. When I rename file_A for testing, file_B will not be used.