open file for random write without truncating?

Question

In python, there are a few flags you can supply when opening a file for operation. I am a bit baffled at finding a combination that allow me to do random write without truncating. The behavior I am looking for is equivalent to C: create it if it doesn't exist, otherwise, open for write (not truncating)

open(filename, O_WRONLY|O_CREAT)

Python's document is confusing (to me): "w" will truncate the file first, "+" is supposed to mean updating, but "w+" will truncate it anyway. Is there anyway to achieve this without resorting to the low-level os.open() interface?

Note: the "a" or "a+" doesn't work either (please correct if I am doing something wrong here)

cat test.txt
eee

with open("test.txt", "a+") as f:
  f.seek(0)
  f.write("a")
cat test.txt
eeea

Is that so the append mode insist on writing to the end?

Answer 1

You can do it with os.open:

import os
f = os.fdopen(os.open(filename, os.O_RDWR | os.O_CREAT), 'rb+')

Now you can read, write in the middle of the file, seek, and so on. And it creates the file. Tested on Python 2 and 3.