[Python] Why does os.fdopen() ignore "mode" argument?

This piece of code when run on Python 2.7.16 and 3.8.3 produces different results:

import tempfile
import os

fd, lockfile = tempfile.mkstemp()
flags = os.O_RDWR | os.O_CREAT
mode = 'w+b'

fd = os.open(lockfile, flags)
fileobj = os.fdopen(fd, mode)

print(fileobj.mode)

os.remove(lockfile)


In 2.7 it prints w+b as expected but in 3.8 it prints rb+. Why wouldn't it be respecting the mode parameter in this way?

I've tried manually creating a file to discount tempfile differences but still get the same result.

I can't see anything obvious in the docs:

• https://docs.python.org/3.8/library/os.html#os.fdopen
• https://docs.python.org/2.7/library/os.html#os.fdopen

Running on MacOS 10.14.6

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