Description
Issue description
The documentation at https://pybind11.readthedocs.io/en/stable/advanced/cast/custom.html suggests that custom type casters do not need to clear any Python exception that may have been set (the implementation of load basically returning !PyErr_Occurred()). Upon first reading, this seems reasonable as pybind11 may perhaps use that info for better error reporting. However, type casters actually need to clear the exception -- at least to make variant loading work.
I don't know if this should be fixed on the docs side, or on the implementation side.
Reproducible example code
The entire example, except the PYBIND11_MODULE block, is copied verbatim from https://pybind11.readthedocs.io/en/stable/advanced/cast/custom.html.
#include <pybind11/pybind11.h>
#include <pybind11/stl.h>
struct inty { long long_value; };
namespace pybind11 { namespace detail {
template <> struct type_caster<inty> {
public:
/**
* This macro establishes the name 'inty' in
* function signatures and declares a local variable
* 'value' of type inty
*/
PYBIND11_TYPE_CASTER(inty, _("inty"));
/**
* Conversion part 1 (Python->C++): convert a PyObject into a inty
* instance or return false upon failure. The second argument
* indicates whether implicit conversions should be applied.
*/
bool load(handle src, bool) {
/* Extract PyObject from handle */
PyObject *source = src.ptr();
/* Try converting into a Python integer value */
PyObject *tmp = PyNumber_Long(source);
if (!tmp) {
return false;
}
/* Now try to convert into a C++ int */
value.long_value = PyLong_AsLong(tmp);
Py_DECREF(tmp);
/* Ensure return code was OK (to avoid out-of-range errors etc) */
return !(value.long_value == -1 && !PyErr_Occurred());
}
/**
* Conversion part 2 (C++ -> Python): convert an inty instance into
* a Python object. The second and third arguments are used to
* indicate the return value policy and parent object (for
* ``return_value_policy::reference_internal``) and are generally
* ignored by implicit casters.
*/
static handle cast(inty src, return_value_policy /* policy */, handle /* parent */) {
return PyLong_FromLong(src.long_value);
}
};
}}
PYBIND11_MODULE(python_example, m) {
m.def("get_index", [](std::variant<inty, std::string> v) { return v.index(); });
}Calling get_index("a") throws
ValueError: invalid literal for int() with base 10: 'a'
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
File "<string>", line 1, in <module>
SystemError: <built-in method get_index of PyCapsule object at 0x7fda858a08d0> returned a result with an error set
Fixing the implementation of load to always clear set exceptions instead makes get_index("a") correctly return 1.