From 2819ce64a4fcc406822382b9ad70bbe12aec90bf Mon Sep 17 00:00:00 2001
From: Boris Staletic <boris.staletic@gmail.com>
Date: Fri, 24 Jul 2020 18:43:59 +0200
Subject: [PATCH] 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        | 2 +-
 tests/test_kwargs_and_defaults.cpp | 5 +++++
 tests/test_kwargs_and_defaults.py  | 2 ++
 3 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/include/pybind11/pybind11.h b/include/pybind11/pybind11.h
index ee2870524..1a415a405 100644
--- a/include/pybind11/pybind11.h
+++ b/include/pybind11/pybind11.h
@@ -228,7 +228,7 @@ 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__");
diff --git a/tests/test_kwargs_and_defaults.cpp b/tests/test_kwargs_and_defaults.cpp
index 8f095fe4a..64bc2377b 100644
--- a/tests/test_kwargs_and_defaults.cpp
+++ b/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"));
 }
diff --git a/tests/test_kwargs_and_defaults.py b/tests/test_kwargs_and_defaults.py
index 531a9fdae..dad40dbeb 100644
--- a/tests/test_kwargs_and_defaults.py
+++ b/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'>"