bpo-46323: _ctypes.CFuncPtr fails if _argtypes_ is too long (GH-31188)

ctypes.CFUNCTYPE() and ctypes.WINFUNCTYPE() now fail to create the
type if its "_argtypes_" member contains too many arguments.
Previously, the error was only raised when calling a function.

Change also how CFUNCTYPE() and WINFUNCTYPE() handle KeyError to
prevent creating a chain of exceptions if ctypes.CFuncPtr raises an
error.

Lib/ctypes/__init__.py

@@ -92,15 +92,18 @@ def CFUNCTYPE(restype, *argtypes, **kw):
         flags |= _FUNCFLAG_USE_LASTERROR
     if kw:
         raise ValueError("unexpected keyword argument(s) %s" % kw.keys())
+
     try:
         return _c_functype_cache[(restype, argtypes, flags)]
     except KeyError:
-        class CFunctionType(_CFuncPtr):
-            _argtypes_ = argtypes
-            _restype_ = restype
-            _flags_ = flags
-        _c_functype_cache[(restype, argtypes, flags)] = CFunctionType
-        return CFunctionType
+        pass
+
+    class CFunctionType(_CFuncPtr):
+        _argtypes_ = argtypes
+        _restype_ = restype
+        _flags_ = flags
+    _c_functype_cache[(restype, argtypes, flags)] = CFunctionType
+    return CFunctionType
 
 if _os.name == "nt":
     from _ctypes import LoadLibrary as _dlopen
@@ -116,15 +119,18 @@ if _os.name == "nt":
             flags |= _FUNCFLAG_USE_LASTERROR
         if kw:
             raise ValueError("unexpected keyword argument(s) %s" % kw.keys())
+
         try:
             return _win_functype_cache[(restype, argtypes, flags)]
         except KeyError:
-            class WinFunctionType(_CFuncPtr):
-                _argtypes_ = argtypes
-                _restype_ = restype
-                _flags_ = flags
-            _win_functype_cache[(restype, argtypes, flags)] = WinFunctionType
-            return WinFunctionType
+            pass
+
+        class WinFunctionType(_CFuncPtr):
+            _argtypes_ = argtypes
+            _restype_ = restype
+            _flags_ = flags
+        _win_functype_cache[(restype, argtypes, flags)] = WinFunctionType
+        return WinFunctionType
     if WINFUNCTYPE.__doc__:
         WINFUNCTYPE.__doc__ = CFUNCTYPE.__doc__.replace("CFUNCTYPE", "WINFUNCTYPE")
 

Lib/ctypes/test/test_callbacks.py

@@ -294,15 +294,22 @@ class SampleCallbacksTestCase(unittest.TestCase):
             return len(args)
 
         CTYPES_MAX_ARGCOUNT = 1024
+
+        # valid call with nargs <= CTYPES_MAX_ARGCOUNT
         proto = CFUNCTYPE(c_int, *(c_int,) * CTYPES_MAX_ARGCOUNT)
         cb = proto(func)
         args1 = (1,) * CTYPES_MAX_ARGCOUNT
         self.assertEqual(cb(*args1), CTYPES_MAX_ARGCOUNT)
 
+        # invalid call with nargs > CTYPES_MAX_ARGCOUNT
         args2 = (1,) * (CTYPES_MAX_ARGCOUNT + 1)
         with self.assertRaises(ArgumentError):
             cb(*args2)
 
+        # error when creating the type with too many arguments
+        with self.assertRaises(ArgumentError):
+            CFUNCTYPE(c_int, *(c_int,) * (CTYPES_MAX_ARGCOUNT + 1))
+
     def test_convert_result_error(self):
         def func():
             return ("tuple",)