FormatMessage() in python, for system messages.

formatmessagesystem.py

__all__ = (
    FormatMessageSystem,
    LCID_ENGLISH,
    LCID_NEUTRAL,
)

import ctypes
import ctypes.wintypes

LANG_NEUTRAL = 0x00
SUBLANG_NEUTRAL = 0x00
SUBLANG_DEFAULT = 0x01

LANG_ENGLISH = 0x09
SUBLANG_ENGLISH_US = 0x01
def MAKELANGID(primary, sublang):
    return (primary & 0xFF) | (sublang & 0xFF) << 16

LCID_ENGLISH = MAKELANGID(LANG_ENGLISH, SUBLANG_ENGLISH_US)
LCID_DEFAULT = MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT)
LCID_NEUTRAL = MAKELANGID(LANG_NEUTRAL, SUBLANG_NEUTRAL)
assert LCID_NEUTRAL == 0

FORMAT_MESSAGE_ALLOCATE_BUFFER = 0x00000100
FORMAT_MESSAGE_FROM_SYSTEM = 0x00001000
FORMAT_MESSAGE_IGNORE_INSERTS = 0x00000200

# Note: You probably want to use the builtin ctypes.FormatError instead: https://docs.python.org/3/library/ctypes.html#ctypes.FormatError
# CPython impl: https://github.com/python/cpython/blob/13c1c3556f2c12d0be2af890fabfbf44280b845c/Modules/_ctypes/callproc.c#L254
def FormatMessageSystem(id, langid=LCID_ENGLISH):
    sys_flag = FORMAT_MESSAGE_ALLOCATE_BUFFER|FORMAT_MESSAGE_FROM_SYSTEM|FORMAT_MESSAGE_IGNORE_INSERTS
    
    bufptr = ctypes.wintypes.LPWSTR()
    
    chars = ctypes.windll.kernel32.FormatMessageW(sys_flag, None, id, langid, ctypes.byref(bufptr), 0, None)
    if chars == 0:
        chars = ctypes.windll.kernel32.FormatMessageW(sys_flag, None, id, LCID_NEUTRAL, ctypes.byref(bufptr), 0, None)
        if chars == 0:
            #XXX: You probably want to call GetLastError() here
            return "FormatMessageW failed"

    val = bufptr.value[:chars]
    
    ctypes.windll.kernel32.LocalFree(bufptr)

    return val

this is very useful, thanks. amazing how complicated windows makes it to print API error messages.

in case it might be useful to anyone else, it looks like ctypes has this functionality built in:
https://docs.python.org/3/library/ctypes.html#ctypes.FormatError

Huh, good catch, it does, and apparently has since 2.5. CPython source: https://github.com/python/cpython/blob/13c1c3556f2c12d0be2af890fabfbf44280b845c/Modules/_ctypes/callproc.c#L254

I suppose this Python implementation would be useful if you wanted a specific language.