A function written with ctypes called is_append_mode_win32 which, given a file object, can determine, on Windows, if it was opened in append mode (regardless of what its mode string says). In UNIX this would be equivalent to fcntl(fd, F_GETFL) & O_APPEND. These are the hoops we have to go through to get the same functionality on Windows.

win32_is_append_mode.py

from ctypes import (cdll, c_size_t, c_void_p, c_int, c_char, Structure,
                    POINTER, cast)
from ctypes.util import find_msvcrt


msvcrt = cdll.LoadLibrary(find_msvcrt())


# Constants
IOINFO_L2E = 5
IOINFO_ARRAY_ELTS = 1 << IOINFO_L2E
IOINFO_ARRAYS = 64
FAPPEND = 0x20
_NO_CONSOLE_FILENO = -2


# Types
intptr_t = POINTER(c_int)

class my_ioinfo(Structure):
    _fields_ = [('osfhnd', intptr_t),
                ('osfile', c_char)]

# Functions
_msize = msvcrt._msize
_msize.argtypes = (c_void_p,)
_msize.restype = c_size_t

# Variables
# Since we don't know how large the ioinfo struct is just treat the __pioinfo
# array as an array of byte pointers
__pioinfo = cast(msvcrt.__pioinfo, POINTER(POINTER(c_char)))


_sizeof_ioinfo = None
def is_append_mode_win32(fileobj):
    global _sizeof_ioinfo
    if _sizeof_ioinfo is None:
        if __pioinfo[0] is not None:
            _sizeof_ioinfo = _msize(__pioinfo[0]) / IOINFO_ARRAY_ELTS
        #else:
        #    this shouldn't happen but...
    fd = fileobj.fileno()
    if fd != _NO_CONSOLE_FILENO:
        idx1 = fd >> IOINFO_L2E # The index into the __pioinfo array
        # The n-th ioinfo pointer in __pioinfo[idx1]
        idx2 = fd & ((1 << IOINFO_L2E) - 1)
        if 0 <= idx1 < IOINFO_ARRAYS and __pioinfo[idx1] is not None:
            # Doing pointer arithmetic in ctypes is irritating
            pio = c_void_p(cast(__pioinfo[idx1], c_void_p).value +
                           idx2 * _sizeof_ioinfo)
            ioinfo = cast(pio, POINTER(my_ioinfo)).contents
            return bool(ord(ioinfo.osfile) & FAPPEND)
    return False