New submission from Ranger Harke:
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 #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.
----------
components: ctypes
messages: 296245
nosy: rharke
priority: normal
severity: normal
status: open
title: Automatic upcast does not occur for custom classes
type: behavior
versions: Python 2.7, Python 3.6
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue30692>
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