[issue8161] inconsistency behavior in ctypes.c_char_p dereferencing

[Eryk Sun]

Eryk Sun  added the comment:

This appears to be misreported. The implementation in 3.1.2rc1 looks correct to 
me [1]. Either way, it's out of date.

That said, the emulation of a simple type in an aggregate type is still 
incomplete nowadays. It's not implemented when a c_char or c_wchar array type 
is itself the type of an array. It's also inconsistent with simple types 
because the automatic get-set conversion isn't disabled for subclasses of 
c_char and c_wchar array types. For a subclass, one should have to use the 
`value` attribute.

---
[1] 
https://github.com/python/cpython/blob/v3.1.2rc1/Modules/_ctypes/cfield.c#L110-L137

--
nosy: +eryksun
resolution:  -> out of date
stage: needs patch -> resolved
status: pending -> closed

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[issue8161] inconsistency behavior in ctypes.c_char_p dereferencing

[Irit Katriel]

Irit Katriel  added the comment:

On 3.11 I'm not getting str for the second expression, I get an empty bytes 
object:


>>> import ctypes
>>> class T(ctypes.Structure):
... _fields_ = (
... ('member', ctypes.c_char * 16),
... )
... 
>>> print('%r'%((ctypes.c_char * 16)()[:]))
b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
>>> print('%r'%(T().member[:]))
b''

--
nosy: +iritkatriel
status: open -> pending

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[issue8161] inconsistency behavior in ctypes.c_char_p dereferencing

[Vlad Riscutia]

Vlad Riscutia riscutiav...@gmail.com added the comment:

Looks like this was implemented by design at some point. In cfield.c, we have 
specific code to treat character array fields as strings:

/*  Field descriptors for 'c_char * n' are be scpecial cased to
return a Python string instead of an Array object instance...
*/
if (PyCArrayTypeObject_Check(proto)) {
StgDictObject *adict = PyType_stgdict(proto);
StgDictObject *idict;
if (adict  adict-proto) {
idict = PyType_stgdict(adict-proto);
if (!idict) {
PyErr_SetString(PyExc_TypeError,
has no _stginfo_);
Py_DECREF(self);
return NULL;
}
if (idict-getfunc == _ctypes_get_fielddesc(c)-getfunc) {
struct fielddesc *fd = _ctypes_get_fielddesc(s);
getfunc = fd-getfunc;
setfunc = fd-setfunc;
}
#ifdef CTYPES_UNICODE
if (idict-getfunc == _ctypes_get_fielddesc(u)-getfunc) {
struct fielddesc *fd = _ctypes_get_fielddesc(U);
getfunc = fd-getfunc;
setfunc = fd-setfunc;
}
#endif
}
}

Simple fix would be to just remove this whole section though this might break 
code which relied on string assignment to such fields. For example:

class T(ctypes.Structure):
_fields_ = (
('member', ctypes.c_char * 16),
)

x = T()
x.member = bytes('Spam', 'ascii')

Above works now but will fail if change is made. There is a high chance this 
would break existing code as I imagine people using this due to convenience. An 
alternative would be to keep string setfunc but don't change getfunc, though 
that is also pretty inconsistent as you will be able to set a string but not 
get one back.

If we are willing to take the risk, fix is easy.

--
nosy: +vladris

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[issue8161] inconsistency behavior in ctypes.c_char_p dereferencing

[Mark Lawrence]

Changes by Mark Lawrence breamore...@yahoo.co.uk:


--
stage:  - needs patch
versions: +Python 2.7, Python 3.1

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[issue8161] inconsistency behavior in ctypes.c_char_p dereferencing

[Jackson Yang]

New submission from Jackson Yang jackson.y...@augmentum.com:

# Python 3.1.2rc1 (r312rc1:78742, Mar  7 2010, 07:49:40)
# [MSC v.1500 32 bit (Intel)] on win32
import ctypes

class T(ctypes.Structure):
_fields_ = (
('member', ctypes.c_char * 16),
)

# dereference a c_char_Array variable would return bytes
print('%r'%((ctypes.c_char * 16)()[:]))
# dereference from a c_char_Array member would return str, which is buggy
print('%r'%(T().member[:]))

--
assignee: theller
components: ctypes
messages: 101214
nosy: nullnil, theller
severity: normal
status: open
title: inconsistency behavior in ctypes.c_char_p dereferencing
type: behavior
versions: Python 3.2

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