Do not set docstring for function when it's empty

Author: lqf96

include/pybind11/pybind11.h

@@ -439,9 +439,14 @@ class cpp_function : public function {
 
         /* Install docstring */
         auto *func = (PyCFunctionObject *) m_ptr;
-        if (func->m_ml->ml_doc)
+        // Free and reset previous docstring if it exists
+        if (func->m_ml->ml_doc) {
             std::free(const_cast<char *>(func->m_ml->ml_doc));
-        func->m_ml->ml_doc = strdup(signatures.c_str());
+            func->m_ml->ml_doc = nullptr;
+        }
+        // Then install new one if it's non-empty (when at least one option is enabled)
+        if (!signatures.empty())
+            func->m_ml->ml_doc = strdup(signatures.c_str());
 
         if (rec->is_method) {
             m_ptr = PYBIND11_INSTANCE_METHOD_NEW(m_ptr, rec->scope.ptr());
@@ -472,7 +477,9 @@ class cpp_function : public function {
                 arg.value.dec_ref();
             }
             if (rec->def) {
-                std::free(const_cast<char *>(rec->def->ml_doc));
+                if (rec->def->ml_doc) {
+                    std::free(const_cast<char *>(rec->def->ml_doc));
+                }
                 // Python 3.9.0 decref's these in the wrong order; rec->def
                 // If loaded on 3.9.0, let these leak (use Python 3.9.1 at runtime to fix)
                 // See https://github.com/python/cpython/pull/22670

tests/test_docstring_options.cpp

@@ -45,6 +45,14 @@ TEST_SUBMODULE(docstring_options, m) {
 
     m.def("test_function7", [](int, int) {}, py::arg("a"), py::arg("b"), "A custom docstring");
 
+    {
+        py::options options;
+        options.disable_user_defined_docstrings();
+        options.disable_function_signatures();
+
+        m.def("test_function8", []() {});
+    }
+
     {
         py::options options;
         options.disable_user_defined_docstrings();

tests/test_docstring_options.py

@@ -34,6 +34,9 @@ def test_docstring_options():
     assert m.test_function7.__doc__.startswith("test_function7(a: int, b: int) -> None")
     assert m.test_function7.__doc__.endswith("A custom docstring\n")
 
+    # when all options are disabled, no docstring (instead of an empty one) should be generated
+    assert m.test_function8.__doc__ is None
+
     # Suppression of user-defined docstrings for non-function objects
     assert not m.DocstringTestFoo.__doc__
     assert not m.DocstringTestFoo.value_prop.__doc__