error_already_set::what() is now constructed lazily

include/pybind11/detail/class.h

@@ -455,6 +455,8 @@ extern "C" inline void pybind11_object_dealloc(PyObject *self) {
 #endif
 }
 
+std::string error_string();
+
 /** Create the type which can be used as a common base for all classes.  This is
     needed in order to satisfy Python's requirements for multiple inheritance.
     Return value: New reference. */
@@ -490,7 +492,7 @@ inline PyObject *make_object_base_type(PyTypeObject *metaclass) {
     type->tp_weaklistoffset = offsetof(instance, weakrefs);
 
     if (PyType_Ready(type) < 0) {
-        pybind11_fail("PyType_Ready failed in make_object_base_type():" + error_string());
+        pybind11_fail("PyType_Ready failed in make_object_base_type(): " + error_string());
     }
 
     setattr((PyObject *) type, "__module__", str("pybind11_builtins"));
@@ -707,7 +709,7 @@ inline PyObject *make_new_python_type(const type_record &rec) {
     }
 
     if (PyType_Ready(type) < 0) {
-        pybind11_fail(std::string(rec.name) + ": PyType_Ready failed (" + error_string() + ")!");
+        pybind11_fail(std::string(rec.name) + ": PyType_Ready failed: " + error_string());
     }
 
     assert(!rec.dynamic_attr || PyType_HasFeature(type, Py_TPFLAGS_HAVE_GC));

include/pybind11/detail/type_caster_base.h

@@ -470,73 +470,6 @@ PYBIND11_NOINLINE bool isinstance_generic(handle obj, const std::type_info &tp)
     return isinstance(obj, type);
 }
 
-PYBIND11_NOINLINE std::string error_string(const char *called) {
-    error_scope scope; // Fetch error state (will be restored when this function returns).
-    if (scope.type == nullptr) {
-        if (called == nullptr) {
-            called = "pybind11::detail::error_string()";
-        }
-        pybind11_fail("Internal error: " + std::string(called)
-                      + " called while Python error indicator not set.");
-    }
-
-    PyErr_NormalizeException(&scope.type, &scope.value, &scope.trace);
-    if (scope.trace != nullptr) {
-        PyException_SetTraceback(scope.value, scope.trace);
-    }
-
-    std::string errorString;
-    if (scope.type) {
-        errorString += handle(scope.type).attr("__name__").cast<std::string>();
-        errorString += ": ";
-    }
-    if (scope.value) {
-        errorString += (std::string) str(scope.value);
-    }
-
-#if !defined(PYPY_VERSION)
-    if (scope.trace) {
-        auto *trace = (PyTracebackObject *) scope.trace;
-
-        /* Get the deepest trace possible */
-        while (trace->tb_next) {
-            trace = trace->tb_next;
-        }
-
-        PyFrameObject *frame = trace->tb_frame;
-        Py_XINCREF(frame);
-        errorString += "\n\nAt:\n";
-        while (frame) {
-#    if PY_VERSION_HEX >= 0x030900B1
-            PyCodeObject *f_code = PyFrame_GetCode(frame);
-#    else
-            PyCodeObject *f_code = frame->f_code;
-            Py_INCREF(f_code);
-#    endif
-            int lineno = PyFrame_GetLineNumber(frame);
-            errorString += "  ";
-            errorString += handle(f_code->co_filename).cast<std::string>();
-            errorString += '(';
-            errorString += std::to_string(lineno);
-            errorString += "): ";
-            errorString += handle(f_code->co_name).cast<std::string>();
-            errorString += '\n';
-            Py_DECREF(f_code);
-#    if PY_VERSION_HEX >= 0x030900B1
-            auto *b_frame = PyFrame_GetBack(frame);
-#    else
-            auto *b_frame = frame->f_back;
-            Py_XINCREF(b_frame);
-#    endif
-            Py_DECREF(frame);
-            frame = b_frame;
-        }
-    }
-#endif
-
-    return errorString;
-}
-
 PYBIND11_NOINLINE handle get_object_handle(const void *ptr, const detail::type_info *type) {
     auto &instances = get_internals().registered_instances;
     auto range = instances.equal_range(ptr);

include/pybind11/pybind11.h

@@ -2625,17 +2625,21 @@ void print(Args &&...args) {
     detail::print(c.args(), c.kwargs());
 }
 
-error_already_set::~error_already_set() {
-    if (m_type) {
-        gil_scoped_acquire gil;
-        error_scope scope;
-        m_type.release().dec_ref();
-        m_value.release().dec_ref();
-        m_trace.release().dec_ref();
-    }
+inline void
+error_already_set::m_fetched_error_deleter(detail::error_fetch_and_normalize *raw_ptr) {
+    gil_scoped_acquire gil;
+    error_scope scope;
+    delete raw_ptr;
+}
+
+inline const char *error_already_set::what() const noexcept {
+    gil_scoped_acquire gil;
+    error_scope scope;
+    return m_fetched_error->error_string().c_str();
 }
 
 PYBIND11_NAMESPACE_BEGIN(detail)
+
 inline function
 get_type_override(const void *this_ptr, const type_info *this_type, const char *name) {
     handle self = get_object_handle(this_ptr, this_type);