Better error message when inheriting from a type without a constructor

Author: virtuald

include/pybind11/detail/class.h

@@ -171,8 +171,10 @@ extern "C" inline PyObject *pybind11_meta_call(PyObject *type, PyObject *args, P
     // Ensure that the base __init__ function(s) were called
     for (const auto &vh : values_and_holders(instance)) {
         if (!vh.holder_constructed()) {
-            PyErr_Format(PyExc_TypeError, "%.200s.__init__() must be called when overriding __init__",
-                         vh.type->type->tp_name);
+            auto message = (vh.type->type->tp_init == get_internals().default_object_init()) ?
+                           "%.200s has no __init__ and cannot be used as a base class from Python" :
+                           "%.200s.__init__() must be called when overriding __init__";
+            PyErr_Format(PyExc_TypeError, message, vh.type->type->tp_name);
             Py_DECREF(self);
             return nullptr;
         }
@@ -302,6 +304,8 @@ extern "C" inline PyObject *pybind11_object_new(PyTypeObject *type, PyObject *,
 /// An `__init__` function constructs the C++ object. Users should provide at least one
 /// of these using `py::init` or directly with `.def(__init__, ...)`. Otherwise, the
 /// following default function will be used which simply throws an exception.
+///
+/// Use `get_internals().default_object_init()` instead of using this function directly
 extern "C" inline int pybind11_object_init(PyObject *self, PyObject *, PyObject *) {
     PyTypeObject *type = Py_TYPE(self);
     std::string msg;
@@ -620,7 +624,7 @@ inline PyObject* make_new_python_type(const type_record &rec) {
         type->tp_bases = bases.release().ptr();
 
     /* Don't inherit base __init__ */
-    type->tp_init = pybind11_object_init;
+    type->tp_init = internals.default_object_init();
 
     /* Supported protocols */
     type->tp_as_number = &heap_type->as_number;

include/pybind11/detail/internals.h

@@ -121,6 +121,11 @@ struct internals {
         PYBIND11_TLS_FREE(tstate);
     }
 #endif
+
+    inline initproc default_object_init() {
+        auto heap_type = (PyHeapTypeObject*)instance_base;
+        return heap_type->ht_type.tp_init;
+    }
 };
 
 /// Additional type information which does not fit into the PyTypeObject.

tests/test_class.py

@@ -128,6 +128,17 @@ def __init__(self):
                 "Hamster.__init__() must be called when overriding __init__"]  # PyPy
     assert msg(exc_info.value) in expected
 
+    # Base doesn't have __init__
+    class ChChimera(m.Chimera):
+        def __init__(self):
+            pass
+
+    with pytest.raises(TypeError) as exc_info:
+        ChChimera()
+    expected = ["m.class_.Chimera has no __init__ and cannot be used as a base class from Python",
+                "Chimera has no __init__ and cannot be used as a base class from Python"]  # PyPy
+    assert msg(exc_info.value) in expected
+
 
 def test_automatic_upcasting():
     assert type(m.return_class_1()).__name__ == "DerivedClass1"