Avoid attr("__repr__") in initialize_generic (#2317)

If the default argument value is a class, and not an instance of a
class, `a.value.attr("__repr__")` raises a `ValueError`. Switching to
`repr(a.value)` makes this use case work.

Fixes #2028

include/pybind11/pybind11.h

@@ -228,7 +228,7 @@ class cpp_function : public function {
  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__");

tests/test_kwargs_and_defaults.cpp

@@ -123,4 +123,9 @@ TEST_SUBMODULE(kwargs_and_defaults, m) {
  py::class_<KWClass>(m, "KWClass")
  .def("foo0", &KWClass::foo)
  .def("foo1", &KWClass::foo, "x"_a, "y"_a);
+
+ // Make sure a class (not an instance) can be used as a default argument.
+ // The return value doesn't matter, only that the module is importable.
+ m.def("class_default_argument", [](py::object a) { return py::repr(a); },
+ "a"_a = py::module::import("decimal").attr("Decimal"));
 }

tests/test_kwargs_and_defaults.py

@@ -191,3 +191,5 @@ def test_args_refcount():
  # tuple without having to inc_ref the individual elements, but here we can't, hence the extra
  # refs.
  assert m.mixed_args_refcount(myval, myval, myval) == (exp3 + 3, exp3 + 3, exp3 + 3)
+
+ assert m.class_default_argument() == "<class 'decimal.Decimal'>"