[Python] Calling SHGetKnownFolderPath from Python?

I've written this minimal reproducible example to calculate the Desktop folder on Windows "the hard way" (using SHGetKnownFolderPath), but I seem to end up with a Success error code while the output buffer only yields b'C' when dereferenced via the .result property of c_char_p. What am I doing wrong?

My code does this:

1. Converts the desired GUID into the cursed _GUID struct format according to Microsoft's specification
2. Allocates result_ptr = c_char_p() which is initially a NULL pointer but will be overwritten with the pointer to the result
3. Calls SHGetKnownFolderPath with the desired GUID struct, no flags, on the current user, passing our result_ptr by reference so its value can be overwritten
4. If SHGetKnownFolderPath indicated success, dereferences result_ptr using .value

I'm getting a result which is only a single char long, but I thought that c_char_p is supposed to be the pointer to the start of a null-terminated string.

Is Windows writing a bogus string into my pointer, am I reading its value out wrongly, or have I made some other error in building my function?

import contextlib
import ctypes
import ctypes.wintypes
import functools
import os
import pathlib
import types
import uuid

try:
wintypes_GUID = ctypes.wintypes.GUID
except AttributeError:
class wintypes_GUID(ctypes.Structure):
# https://learn.microsoft.com/en-us/windows/win32/api/guiddef/ns-guiddef-guid
# https://github.com/enthought/comtypes/blob/1.3.1/comtypes/GUID.py
_fields_ = [
('Data1', ctypes.c_ulong),
('Data2', ctypes.c_ushort),
('Data3', ctypes.c_ushort),
('Data4', ctypes.c_ubyte * 8)
]

@classmethod
def _from_uuid(cls, u):
u = uuid.UUID(u)
u_str = f'{{{u!s}}}'
result = wintypes_GUID()
errno = ctypes.oledll.ole32.CLSIDFromString(u_str, ctypes.byref(result))
if errno == 0:
return result
else:
raise RuntimeError(f'CLSIDFromString returned error code {errno}')

DESKTOP_UUID = 'B4BFCC3A-DB2C-424C-B029-7FE99A87C641'


def get_known_folder(uuid):
# FIXME this doesn't work, seemingly returning just b'C' no matter what
result_ptr = ctypes.c_char_p()
with _freeing(ctypes.oledll.ole32.CoTaskMemFree, result_ptr):
errno = ctypes.windll.shell32.SHGetKnownFolderPath(
ctypes.pointer(wintypes_GUID._from_uuid(uuid)),
0,
None,
ctypes.byref(result_ptr)
)
if errno == 0:
result = result_ptr.value
if len(result) < 2:
import warnings
warnings.warn(f'result_ptr.value == {result!r}')
return pathlib.Path(os.fsdecode(result))
else:
raise RuntimeError(f'Shell32.SHGetKnownFolderPath returned error code {errno}')


@contextlib.contextmanager
def _freeing(freefunc, obj):
try:
yield obj
finally:
freefunc(obj)


assert get_known_folder(DESKTOP_UUID) ==\
pathlib.Path('~/Desktop').expanduser(),\
f'Result: {get_known_folder(DESKTOP_UUID)!r}; expcected: {pathlib.Path("~/Desktop").expanduser()!r}'

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