How to open (read-write) or create a file with truncation allowed?

Question

I want to:

• open a file in read-write mode if it exists;
• create it if it doesn\'t exist;
• be able to truncate it anytime-anywhere.

Answer 1

Well, there are only these modes, and all of them have the "defects" you listed.

Your only option is to wrap open(). Why not something like this? (Python)

def touchopen(filename, *args, **kwargs):
    open(filename, "a").close() # "touch" file
    return open(filename, *args, **kwargs)

it behaves just like open, you could even rebind it to open() if you really wish.

all of open's features are preserved, you can even do:

with touchopen("testfile", "r+") as testfile:
    do_stuff()

You could of course create a contextmanager which opens the file in a+ mode, reads it into memory, and intercepts writes so you handle truncation by magically creating a temporary file in w mode, and renames that tempfile to your original file when you close it, but that would be overkill I guess.