How to use ctypes' errcheck?

Question

The Python Library Reference, Release 3.6.5, paragraph 16.16 ctypes - A foreign function library for Python, gives this example, demonstrating output parameters, in section Function prototypes:

The win32 GetWindowRect function:

WINUSERAPI BOOL WINAPI GetWindowRect(HWND hWnd, LPCRECT lprect);

The ctypes wrapper:

from ctypes             import POINTER, WINFUNCTYPE, windll, WinError
from ctypes.wintypes    import BOOL, HWND, RECT

prototype = WINFUNCTYPE(BOOL, HWND, POINTER(RECT))
paramflags = (1, 'hwnd'), (2, 'lprect')
GetWindowRect = prototype(('GetWindowRect', windll.user32), paramflags)

Now, how do I actually use GetWindowRect?

Trying

from ctypes import byref, cast
r = RECT()
h = cast(65552, HWND)   # _65552_ is the return value of _GetDesktopWindow_ on my system
result = GetWindowRect([h, byref(r)])

returns ctypes.ArgumentError: argument 1: <class 'TypeError'>: wrong type.

Answer 1

[Python.Docs]: ctypes - A foreign function library for Python doesn't provide a complete example.

code00.py:

#!/usr/bin/env python

import ctypes as cts
import sys
from ctypes import wintypes as wts


def errcheck(result, func, args):
    if not result:
        raise cts.WinError()
    rc = args[1]
    return rc.left, rc.top, rc.bottom, rc.right


def test_get_window_rect(desktop_wnd_handle):
    print("\n{:s}\n".format(test_get_window_rect.__name__))
    prototype = cts.WINFUNCTYPE(wts.BOOL, wts.HWND, cts.POINTER(wts.RECT))
    paramflags = (1, "hwnd"), (2, "lprect")
    GetWindowRect = prototype(("GetWindowRect", cts.windll.user32), paramflags)

    print("Without errcheck:\n")
    result = GetWindowRect(cts.windll.user32.GetDesktopWindow())
    print("Left: {:d}, Top: {:d}, Right: {:d}, Bottom: {:d}".format(result.left, result.top, result.right, result.bottom))
    result = GetWindowRect(0)
    print("Left: {:d}, Top: {:d}, Right: {:d}, Bottom: {:d}".format(result.left, result.top, result.right, result.bottom))

    GetWindowRect.errcheck = errcheck

    print("\nWith errcheck:\n")
    result = GetWindowRect(desktop_wnd_handle)
    print("Left: {:d}, Top: {:d}, Bottom: {:d}, Right: {:d}".format(*result))
    result = GetWindowRect(0)
    print("Left: {:d}, Top: {:d}, Bottom: {:d}, Right: {:d}".format(*result))


def test_get_window_rect_old_style(desktop_wnd_handle):
    print("\n{:s}\n".format(test_get_window_rect_old_style.__name__))
    user32_dll = cts.WinDLL("user32")
    get_windows_rect_func = user32_dll.GetWindowRect
    get_windows_rect_func.argtypes = (wts.HWND, cts.POINTER(wts.RECT))
    get_windows_rect_func.restype = wts.BOOL
    rect = wts.RECT()
    result = get_windows_rect_func(desktop_wnd_handle, cts.byref(rect))
    print("Result: {:d}\nLeft: {:d}, Top: {:d}, Right: {:d}, Bottom: {:d}".format(result, rect.left, rect.top, rect.right, rect.bottom))
    rect = wts.RECT()
    result = get_windows_rect_func(0, cts.byref(rect))
    print("Result: {:d}\nLeft: {:d}, Top: {:d}, Right: {:d}, Bottom: {:d}".format(result, rect.left, rect.top, rect.right, rect.bottom))


def main(*argv):
    hwnd = cts.windll.user32.GetDesktopWindow()
    test_get_window_rect_old_style(hwnd)
    test_get_window_rect(hwnd)


if __name__ == "__main__":
    print("Python {:s} {:03d}bit on {:s}\n".format(" ".join(elem.strip() for elem in sys.version.split("\n")),
                                                   64 if sys.maxsize > 0x100000000 else 32, sys.platform))
    rc = main(*sys.argv[1:])
    print("\nDone.\n")
    sys.exit(rc)

Notes:

• Function call happens normally (no argument processing - placing them in a list), and only input arguments specified

• errcheck "magic" illustrated

• Also included the old fashioned way of calling the function

Output:

(py35x64_test) e:\Work\Dev\StackOverflow\q050669907> "e:\Work\Dev\VEnvs\py35x64_test\Scripts\python.exe" ./code00.py
Python 3.5.4 (v3.5.4:3f56838, Aug  8 2017, 02:17:05) [MSC v.1900 64 bit (AMD64)] 064bit on win32


test_get_window_rect_old_style

Result: 1
Left: 0, Top: 0, Right: 1920, Bottom: 1080
Result: 0
Left: 0, Top: 0, Right: 0, Bottom: 0

test_get_window_rect

Without errcheck:

Left: 0, Top: 0, Right: 1920, Bottom: 1080
Left: 0, Top: 0, Right: 0, Bottom: 0

With errcheck:

Left: 0, Top: 0, Bottom: 1080, Right: 1920
Traceback (most recent call last):
  File "code.py", line 58, in <module>
    main()
  File "code.py", line 53, in main
    test_get_window_rect(hwnd)
  File "code.py", line 30, in test_get_window_rect
    result = GetWindowRect(0)
  File "code.py", line 9, in errcheck
    raise cts.WinError()
OSError: [WinError 1400] Invalid window handle.