Cannot use a python class as a default argument

[chatziko]

Issue description

Using any py::object as a default argument seems to be supported. But in the particular case when the object is a class, it fails at import type.

Reproducible example code

This works fine:

m.def(
    "test1",
    [](py::object a) { py::print(py::str(a)); },
    "a"_a = py::module::import("decimal").attr("__version__")
);

$ python
>>> import my_module;
>>> my_module.test1();
1.70

But this fails (only difference is Decimal in place of __version__)

m.def(
    "test2",
    [](py::object a) { py::print(py::str(a)); },
    "a"_a = py::module::import("decimal").attr("Decimal")
);

$ python
>>> import my_module;
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: TypeError: descriptor '__repr__' of 'decimal.Decimal' object needs an argument

I would expect to simply recieve the Decimal class in a, but it seems like pybind11 is trying to construct a Decimal object, or something like that.

[bstaletic]

This is a workaround:

#include <pybind11/pybind11.h>

namespace py = pybind11;
PYBIND11_MODULE(foo, m) {
	py::arg a_temp("a");
	auto arg = py::arg_v(a_temp, py::module::import("decimal").attr("Decimal"), "Decimal");
	m.def( "test1", [](py::object a) { py::print(py::str(a)); }, arg);
}

This happens because foo.__repr__() is not the same as repr(foo) and pybind is tripping over this here:

        for (auto &a: rec->args) {
            if (a.name)
                a.name = strdup(a.name);
            if (a.descr)
                a.descr = strdup(a.descr);
            else if (a.value)
                a.descr = strdup(a.value.attr("__repr__")().cast<std::string>().c_str());
        }

https://github.com/pybind/pybind11/blob/master/include/pybind11/pybind11.h#L225-L232

[bstaletic]

I don't know if there's a better way to do this, but this patch seems to work.

diff --git a/include/pybind11/pybind11.h b/include/pybind11/pybind11.h
index ee28705..93c6f95 100644
--- a/include/pybind11/pybind11.h
+++ b/include/pybind11/pybind11.h
@@ -228,12 +228,17 @@ protected:
            if (a.descr)
                 a.descr = strdup(a.descr);
            else if (a.value)
-                a.descr = strdup(a.value.attr("__repr__")().cast<std::string>().c_str());
+                a.descr = strdup(repr(a.value).cast<std::string>().c_str());
         }

         rec->is_constructor = !strcmp(rec->name, "__init__") || !strcmp(rec->name, "__setstate__");

EDIT: Update patch.