Fix abc error message (RustPython#4987)

* Print correct error message for abc instantiation

* Update dataclasses abc test

Author: patrickzbhe

Lib/test/test_abc.py

@@ -149,8 +149,6 @@ def foo(): return 4
             self.assertEqual(D.foo(), 4)
             self.assertEqual(D().foo(), 4)
 
-        # TODO: RUSTPYTHON
-        @unittest.expectedFailure
         def test_object_new_with_one_abstractmethod(self):
             class C(metaclass=abc_ABCMeta):
                 @abc.abstractmethod
@@ -159,8 +157,6 @@ def method_one(self):
             msg = r"class C with abstract method method_one"
             self.assertRaisesRegex(TypeError, msg, C)
 
-        # TODO: RUSTPYTHON
-        @unittest.expectedFailure
         def test_object_new_with_many_abstractmethods(self):
             class C(metaclass=abc_ABCMeta):
                 @abc.abstractmethod
@@ -525,8 +521,6 @@ def foo(self):
             self.assertEqual(A.__abstractmethods__, set())
             A()
 
-        # TODO: RUSTPYTHON
-        @unittest.expectedFailure
         def test_update_new_abstractmethods(self):
             class A(metaclass=abc_ABCMeta):
                 @abc.abstractmethod
@@ -543,8 +537,6 @@ def updated_foo(self):
             msg = "class A with abstract methods bar, foo"
             self.assertRaisesRegex(TypeError, msg, A)
 
-        # TODO: RUSTPYTHON
-        @unittest.expectedFailure
         def test_update_implementation(self):
             class A(metaclass=abc_ABCMeta):
                 @abc.abstractmethod
@@ -596,8 +588,6 @@ def updated_foo(self):
             A()
             self.assertFalse(hasattr(A, '__abstractmethods__'))
 
-        # TODO: RUSTPYTHON
-        @unittest.expectedFailure
         def test_update_del_implementation(self):
             class A(metaclass=abc_ABCMeta):
                 @abc.abstractmethod
@@ -617,8 +607,6 @@ def foo(self):
             msg = "class B with abstract method foo"
             self.assertRaisesRegex(TypeError, msg, B)
 
-        # TODO: RUSTPYTHON
-        @unittest.expectedFailure
         def test_update_layered_implementation(self):
             class A(metaclass=abc_ABCMeta):
                 @abc.abstractmethod

Lib/test/test_dataclasses.py

@@ -3676,8 +3676,6 @@ class Date(Ordered):
         self.assertFalse(inspect.isabstract(Date))
         self.assertGreater(Date(2020,12,25), Date(2020,8,31))
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
     def test_maintain_abc(self):
         class A(abc.ABC):
             @abc.abstractmethod

extra_tests/snippets/stdlib_abc.py

@@ -13,9 +13,8 @@ def __subclasshook__(cls, subclass):
         return NotImplemented
 
 
-# TODO raise an error if there are in any abstract methods not fulfilled
-# with assert_raises(TypeError):
-#     CustomInterface()
+with assert_raises(TypeError):
+    CustomInterface()
 
 
 class Concrete:

vm/src/builtins/object.rs

@@ -6,6 +6,7 @@ use crate::{
     types::{Constructor, PyComparisonOp},
     AsObject, Context, Py, PyObject, PyObjectRef, PyPayload, PyResult, VirtualMachine,
 };
+use itertools::Itertools;
 
 /// object()
 /// --
@@ -38,10 +39,29 @@ impl Constructor for PyBaseObject {
         // Ensure that all abstract methods are implemented before instantiating instance.
         if let Some(abs_methods) = cls.get_attr(identifier!(vm, __abstractmethods__)) {
             if let Some(unimplemented_abstract_method_count) = abs_methods.length_opt(vm) {
-                if unimplemented_abstract_method_count? > 0 {
-                    return Err(
-                        vm.new_type_error("You must implement the abstract methods".to_owned())
-                    );
+                let methods: Vec<PyStrRef> = abs_methods.try_to_value(vm)?;
+                let methods: String =
+                    Itertools::intersperse(methods.iter().map(|name| name.as_str()), ", ")
+                        .collect();
+
+                let unimplemented_abstract_method_count = unimplemented_abstract_method_count?;
+                let name = cls.name().to_string();
+
+                match unimplemented_abstract_method_count {
+                    0 => {}
+                    1 => {
+                        return Err(vm.new_type_error(format!(
+                            "Can't instantiate abstract class {} with abstract method {}",
+                            name, methods
+                        )));
+                    }
+                    2.. => {
+                        return Err(vm.new_type_error(format!(
+                            "Can't instantiate abstract class {} with abstract methods {}",
+                            name, methods
+                        )));
+                    }
+                    _ => unreachable!("unimplemented_abstract_method_count is always positive"),
                 }
             }
         }