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ctypes: wrong exception message in docs #99266

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kamilturek opened this issue Nov 8, 2022 · 3 comments · Fixed by #99760
Closed

ctypes: wrong exception message in docs #99266

kamilturek opened this issue Nov 8, 2022 · 3 comments · Fixed by #99760
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docs Documentation in the Doc dir

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@kamilturek
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kamilturek commented Nov 8, 2022 •
edited by bedevere-bot
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Documentation

The ctypes docs say that the message of the ArgumentError for the presented example should be as follows:
image

In fact, it looks like this:

>>> strchr = libc.strchr
>>> strchr.restype = c_char_p
>>> strchr.argtypes = [c_char_p, c_char]
>>> 
>>> strchr(b"abcdef", b"def")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ctypes.ArgumentError: argument 2: TypeError: wrong type

I will prepare a PR.
@kamilturek kamilturek added the docs Documentation in the Doc dir label Nov 8, 2022
@kamilturek kamilturek mentioned this issue Nov 8, 2022
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@eryksun
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eryksun commented Nov 9, 2022

Note that the better error message from the c_set() set function gets cleared by PyCSimpleType_from_param() before it looks for an _as_parameter_ attribute on the object. We're left with the less useful message "wrong type". The original exception could be saved and then restored if the object doesn't have an _as_parameter_ attribute.

c_set():

cpython/Modules/_ctypes/cfield.c

Lines 1114 to 1137 in 2eee9d9

static PyObject *
c_set(void *ptr, PyObject *value, Py_ssize_t size)
{
if (PyBytes_Check(value) && PyBytes_GET_SIZE(value) == 1) {
*(char *)ptr = PyBytes_AS_STRING(value)[0];
_RET(value);
}
if (PyByteArray_Check(value) && PyByteArray_GET_SIZE(value) == 1) {
*(char *)ptr = PyByteArray_AS_STRING(value)[0];
_RET(value);
}
if (PyLong_Check(value))
{
long longval = PyLong_AsLong(value);
if (longval < 0 || longval >= 256)
goto error;
*(char *)ptr = (char)longval;
_RET(value);
}
error:
PyErr_Format(PyExc_TypeError,
"one character bytes, bytearray or integer expected");
return NULL;
}

PyCSimpleType_from_param():

cpython/Modules/_ctypes/_ctypes.c

Lines 2208 to 2274 in 2eee9d9

/*
* This is a *class method*.
* Convert a parameter into something that ConvParam can handle.
*/
static PyObject *
PyCSimpleType_from_param(PyObject *type, PyObject *value)
{
StgDictObject *dict;
const char *fmt;
PyCArgObject *parg;
struct fielddesc *fd;
PyObject *as_parameter;
int res;
/* If the value is already an instance of the requested type,
we can use it as is */
res = PyObject_IsInstance(value, type);
if (res == -1)
return NULL;
if (res) {
Py_INCREF(value);
return value;
}
dict = PyType_stgdict(type);
if (!dict) {
PyErr_SetString(PyExc_TypeError,
"abstract class");
return NULL;
}
/* I think we can rely on this being a one-character string */
fmt = PyUnicode_AsUTF8(dict->proto);
assert(fmt);
fd = _ctypes_get_fielddesc(fmt);
assert(fd);
parg = PyCArgObject_new();
if (parg == NULL)
return NULL;
parg->tag = fmt[0];
parg->pffi_type = fd->pffi_type;
parg->obj = fd->setfunc(&parg->value, value, 0);
if (parg->obj)
return (PyObject *)parg;
PyErr_Clear();
Py_DECREF(parg);
if (_PyObject_LookupAttr(value, &_Py_ID(_as_parameter_), &as_parameter) < 0) {
return NULL;
}
if (as_parameter) {
if (_Py_EnterRecursiveCall("while processing _as_parameter_")) {
Py_DECREF(as_parameter);
return NULL;
}
value = PyCSimpleType_from_param(type, as_parameter);
_Py_LeaveRecursiveCall();
Py_DECREF(as_parameter);
return value;
}
PyErr_SetString(PyExc_TypeError,
"wrong type");
return NULL;
}

Actually, I don't see why it doesn't first check for an _as_parameter_ attribute on value, and I don't see why it needs to recursively call PyCSimpleType_from_param().

@kamilturek
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As per the docs, _as_parameter_ can return another object with the _as_parameter_ attribute. I believe that's why the recursion is needed.

The from_param() class method receives the Python object passed to the function call, it should do a typecheck or whatever is needed to make sure this object is acceptable, and then return the object itself, its as_parameter attribute, or whatever you want to pass as the C function argument in this case. Again, the result should be an integer, string, bytes, a ctypes instance, or an object with an as_parameter attribute.

https://docs.python.org/3/library/ctypes.html

@kamilturek kamilturek mentioned this issue Nov 24, 2022
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@kamilturek
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kamilturek commented Nov 24, 2022 •
edited
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@eryksun I followed your suggestion of saving and restoring the exception. I opened a PR.

kumaraditya303 added a commit to kamilturek/cpython that referenced this issue Dec 24, 2022
@kumaraditya303
Merge branch 'main' into pythongh-99266
54ece59
kumaraditya303 added a commit that referenced this issue Jan 21, 2023
@kamilturek @kumaraditya303
gh-99266: ctypes: Preserve more detailed exception in ArgumentError
b4e11a7 
Co-authored-by: Kumar Aditya <[email protected]>
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