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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.