From 1011ddeb7d9c8fee5b23c5ac590057740c04c902 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Thu, 22 Apr 2021 07:08:34 -0700
Subject: [PATCH] pickle setstate: setattr __dict__ only if not empty, to not
 force use of py::dynamic_attr() unnecessarily.

---
 include/pybind11/detail/init.h | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/include/pybind11/detail/init.h b/include/pybind11/detail/init.h
index 3ef78c1179f..376f1142b10 100644
--- a/include/pybind11/detail/init.h
+++ b/include/pybind11/detail/init.h
@@ -293,6 +293,11 @@ template <typename Class, typename T, typename O,
           enable_if_t<std::is_convertible<O, handle>::value, int> = 0>
 void setstate(value_and_holder &v_h, std::pair<T, O> &&result, bool need_alias) {
     construct<Class>(v_h, std::move(result.first), need_alias);
+    auto d = handle(result.second);
+    if (PyDict_Check(d.ptr()) && PyDict_Size(d.ptr()) == 0) {
+        // Skipping setattr below, to not force use of py::dynamic_attr() for Class unnecessarily.
+        return;
+    }
     setattr((PyObject *) v_h.inst, "__dict__", result.second);
 }