Add __bool__ for MetaType

Author: jhonabreul

src/embed_tests/ClassManagerTests.cs

@@ -1085,6 +1085,33 @@ def is_enum_value_defined():
             }
         }
 
+        [Test]
+        public void TruthinessCanBeCheckedForTypes()
+        {
+            using (Py.GIL())
+            {
+                var module = PyModule.FromString("TruthinessCanBeCheckedForTypes", $@"
+from clr import AddReference
+AddReference(""Python.EmbeddingTest"")
+
+from Python.EmbeddingTest import *
+
+def throw_if_falsy():
+    if not ClassManagerTests:
+        raise Exception(""ClassManagerTests is falsy"")
+
+def throw_if_not_truthy():
+    if ClassManagerTests:
+        return
+    raise Exception(""ClassManagerTests is not truthy"")
+");
+
+                // Types are always truthy
+                Assert.DoesNotThrow(() => module.InvokeMethod("throw_if_falsy"));
+                Assert.DoesNotThrow(() => module.InvokeMethod("throw_if_not_truthy"));
+            }
+        }
+
         private static TestCaseData[] IDictionaryContainsTestCases =>
         [
             new(typeof(TestDictionary<string, string>)),

src/runtime/Native/ITypeOffsets.cs

@@ -30,6 +30,7 @@ interface ITypeOffsets
         int nb_invert { get; }
         int nb_inplace_add { get; }
         int nb_inplace_subtract { get; }
+        int nb_bool { get; }
         int ob_size { get; }
         int ob_type { get; }
         int qualname { get; }

src/runtime/Native/TypeOffset.cs

@@ -37,6 +37,7 @@ static partial class TypeOffset
         internal static int nb_invert { get; private set; }
         internal static int nb_inplace_add { get; private set; }
         internal static int nb_inplace_subtract { get; private set; }
+        internal static int nb_bool { get; private set; }
         internal static int ob_size { get; private set; }
         internal static int ob_type { get; private set; }
         internal static int qualname { get; private set; }

src/runtime/Types/MetaType.cs

@@ -386,6 +386,15 @@ public static int mp_length(BorrowedReference tp)
             return Enum.GetValues(type).Length;
         }
 
+        /// <summary>
+        /// Implements __bool__ for types, so that Python uses this instead of __len__ as default.
+        /// For types, this is always "true"
+        /// </summary>
+        public static int nb_bool(BorrowedReference tp)
+        {
+            return 1;
+        }
+
         /// <summary>
         /// Implements __contains__ for Enum types.
         /// </summary>