Automatic upcast does not occur for custom classes #74877

Activity

1. rharke commented on Jun 17, 2017

   

   rharkemannequin

   on Jun 17, 2017

   MannequinAuthor

   More actions

   I have noticed that the automatic upcast mechanism which works for
   standard ctypes types does not appear to work for custom types with
   _as_parameter_ defined.

   Consider the following example (written for Mac OS X but just change
   the LoadLibrary call to test on a different OS):

   # -- begin --
   from ctypes import *
   
   libc = cdll.LoadLibrary('/usr/lib/libc.dylib')
   
   class Base(Structure): pass
   class Derived(Base): pass
   class Wrapper(object): pass
   
   printf = libc.printf
   printf.argtypes = [ c_char_p, POINTER(Base) ]
   printf.restype = c_int
   
   derived = Derived()
   derived_ptr = pointer(derived)
   derived_ptr_wrapper = Wrapper()
   derived_ptr_wrapper._as_parameter_ = pointer(derived)
   
   # Case 1 WORKS: Derived* is automatically upcast to Base* and passed to
   #               printf
   printf(b'%p\n', derived_ptr)
   
   # Case 2 WORKS: The Wrapper can be explicitly upcast to a Base* and
   #               passed to printf
   printf(b'%p\n', cast(derived_ptr_wrapper, POINTER(Base)))
   
   # Case 3 FAILS: Automatic upcast does NOT happen with the value in the
   #               Wrapper: 'expected LP_Base instance instead of
   #               LP_Derived'
   printf(b'%p\n', derived_ptr_wrapper)
   # -- end --

   Here we have two types, Base which is a ctypes Structure, and Derived
   which derives from Base. A pointer to Derived can be passed to a
   function that expects a pointer to Base, and it will automatically be
   upcast.

   However, once the Wrapper type is introduced, which simply holds a
   pointer to Derived in its _as_parameter_ attribute, the automatic
   upcast no longer occurs. A manual upcast works, though.

   I believe this may be related to bpo-27803, but that is talking about
   byref() whereas this is related to cast(). Perhaps the internal
   mechanism is the same; certainly it sounds like a similar problem.

   I've tested this in Python 2.7 and 3.6, and the result is the same.

   Not sure if this is a bug or an enhancement request- filing it as
   "behavior" for now since to me, this is something that I expected to
   implicitly work.

   Thanks! - Ranger Harke.
2. rharke

   added

   topic-ctypes

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Jun 17, 2017
3. ezio-melotti

   transferred this issue fromon Apr 10, 2022
4. serhiy-storchaka

   added this to Ctypes issueson Jan 1, 2024
5. ZeroIntensity commented on Oct 19, 2026

   

   ZeroIntensity

   on Oct 19, 2026

   Member

   More actions

   Same root cause as #71990, so #159763 should fix it.
6. ZeroIntensity

   closed this as a duplicateof ctypes automatic byref failing on custom classes attributes #71990 on Oct 19, 2026
7. github-project-automation

   moved this to Done in Ctypes issueson Oct 19, 2026