py::cast<T> where T is a subclass of py::object is unchecked

[ezyang]

Issue description

I expect something like py::cast<py::bytes>(x) to check if the py::object x is actually bytes. It does not.

Reproducible example code

#include <pybind11/pybind11.h>
#include <iostream>
namespace py = pybind11;
int main() {
  Py_Initialize();
  auto gil = PyGILState_Ensure();
  PyObject *l = PyFloat_FromDouble(3.14);
  //PyObject *l = PyBytes_FromString("foo");
  auto n = py::cast<py::bytes>(l);
  std::cerr << "No problem yet...\n";
  std::cerr << static_cast<std::string>(n);
  PyGILState_Release(gil);
  return 0;
}

[bstaletic]

For the record, Boost.Python behaves the same. Boost expects you to run n.check() before using n. Pybind instead provides py::isinstance<py::bytes>(n).

[YannickJadoul]

It can be considered as unexpected though. I'd vote for thinking this over, at least. I still get lost on when to use the constructor of such a subclass, or py::cast, or py::isinstance and a conversion method, or ...

[bstaletic]

This has been fixed in 2.6.0.

[YannickJadoul]

Should be #2349 that fixed this.