inspect.signature: Support classes without user-defined __init__/__new__ #20308

Lib/inspect.py

@@ -1561,6 +1561,17 @@ def signature(obj):
                 init = _get_user_defined_method(obj, '__init__')
                 if init is not None:
                     sig = signature(init)
+
+        if sig is None:
+            if type in obj.__mro__:
+                # 'obj' is a metaclass without user-defined __init__
+                # or __new__. Return a signature of 'type' builtin.
+                return signature(type)
+            else:
+                # We have a class (not metaclass), but no user-defined
+                # __init__ or __new__ for it
+                return signature(object)
+
     elif not isinstance(obj, _NonUserDefinedCallables):
         # An object with __call__
         # We also check that the 'obj' is not an instance of

Lib/test/test_inspect.py

@@ -2045,6 +2045,20 @@ class TestSignatureObject(unittest.TestCase):
                            ('bar', 2, ..., "keyword_only")),
                           ...))
 
+        # Test classes without user-defined __init__ or __new__
+        class C: pass
+        self.assertEqual(str(inspect.signature(C)), '()')
+        class D(C): pass
+        self.assertEqual(str(inspect.signature(D)), '()')
+
+        # Test meta-classes without user-defined __init__ or __new__
+        class C(type): pass
+        self.assertEqual(str(inspect.signature(C)),
+                         '(object_or_name, bases, dict)')
+        class D(C): pass
+        self.assertEqual(str(inspect.signature(D)),
+                         '(object_or_name, bases, dict)')
+
     def test_signature_on_callable_objects(self):
         class Foo:
             def __call__(self, a):

Misc/NEWS

@@ -13,6 +13,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #20308: inspect.signature now works on classes without user-defined
+  __init__ or __new__ methods.
+
 
 What's New in Python 3.4.0 Beta 3?
 ==================================