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The specification for seek() says:
 seek( 	offset[, whence])
 Note that if the file is opened for appending (mode
'a' or 'a+'), any seek() operations will be undone at
the next write.
Consider operating on an fdopen()ed file. The Python
source simply calls into the OS-provided fdopen():
http://pxr.openlook.org/pxr/source/Modules/posixmodule.c#3530
However, the POSIX standard
http://www.opengroup.org/onlinepubs/009695399/functions/fdopen.html
says:
"Although not explicitly required by this volume of
IEEE Std 1003.1-2001, a good implementation of append
(a) mode would cause the O_APPEND flag to be set."
Thus, to ensure Python semantics, Python's fdopen()
must perform an fcntl() to ensure O_APPEND is set.
For example, on Solaris, this optional O_APPEND
behaviour is not applied:
http://cvs.opensolaris.org/source/xref/on/usr/src/lib/libc/port/stdio/fdopen.c?r=1.22#97
This has recently caused serious problems with the
Mercurial SCM.

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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()

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patch here:
http://sourceforge.net/tracker/index.php?func=detail&aid=1462227&group_id=5470&atid=305470 

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Applied patch in rev. 43501, 43502.

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Shouldn't the documentation now state this change in
behaviour? i.e. if fdopen() fails, O_APPEND may still have
become set.
This behaviour seems a little odd to me in fact. Can't we set
fcntl after a successful fdopen()[1]? Also wouldn't it be
useful to point to the standards sentence I quote above?
[1] not sure what the ALLOW_THREAD thing does?