mention type name in "no constructor defined" message (#4481)

fixes #4470

newsfragments/4481.changed.md

@@ -0,0 +1 @@
+Mention the type name in the exception message when trying to instantiate a class with no constructor defined.

pytests/tests/test_pyclasses.py

@@ -65,24 +65,26 @@ def test_new_classmethod():
         _ = AssertingSubClass(expected_type=str)
 
 
-class ClassWithoutConstructorPy:
+class ClassWithoutConstructor:
     def __new__(cls):
-        raise TypeError("No constructor defined")
+        raise TypeError("No constructor defined for ClassWithoutConstructor")
 
 
 @pytest.mark.parametrize(
-    "cls", [pyclasses.ClassWithoutConstructor, ClassWithoutConstructorPy]
+    "cls", [pyclasses.ClassWithoutConstructor, ClassWithoutConstructor]
 )
 def test_no_constructor_defined_propagates_cause(cls: Type):
     original_error = ValueError("Original message")
     with pytest.raises(Exception) as exc_info:
         try:
             raise original_error
         except Exception:
-            cls()  # should raise TypeError("No constructor defined")
+            cls()  # should raise TypeError("No constructor defined for ...")
 
     assert exc_info.type is TypeError
-    assert exc_info.value.args == ("No constructor defined",)
+    assert exc_info.value.args == (
+        "No constructor defined for ClassWithoutConstructor",
+    )
     assert exc_info.value.__context__ is original_error
 
 

src/pyclass/create_type_object.rs

@@ -524,14 +524,19 @@ fn bpo_45315_workaround(py: Python<'_>, class_name: CString) {
 
 /// Default new implementation
 unsafe extern "C" fn no_constructor_defined(
-    _subtype: *mut ffi::PyTypeObject,
+    subtype: *mut ffi::PyTypeObject,
     _args: *mut ffi::PyObject,
     _kwds: *mut ffi::PyObject,
 ) -> *mut ffi::PyObject {
-    trampoline(|_| {
-        Err(crate::exceptions::PyTypeError::new_err(
-            "No constructor defined",
-        ))
+    trampoline(|py| {
+        let tpobj = PyType::from_borrowed_type_ptr(py, subtype);
+        let name = tpobj
+            .name()
+            .map_or_else(|_| "<unknown>".into(), |name| name.to_string());
+        Err(crate::exceptions::PyTypeError::new_err(format!(
+            "No constructor defined for {}",
+            name
+        )))
     })
 }