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.