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user_id=17929

freebsd 4.11 shows the same behaviour.

import fcntl
import os

def test_fdopen_append():
    
    def is_append(fd):
        return bool(fcntl.fcntl(fd, fcntl.F_GETFL) &
os.O_APPEND)


    fd = os.open("foo.txt", os.O_RDWR | os.O_CREAT)
    assert fd != -1
    
    print "is_append:", is_append(fd)
    
    f=os.fdopen(fd, 'a')
    print "after fdopen is_append:", is_append(fd)

    assert is_append(fd)

test_fdopen_append()