inspect.signature: Make sure that if a callable object has '_patialmethod'

attribute, that attribute is an instance of 'functools.partialmethod'.

Lib/inspect.py

@@ -1651,6 +1651,7 @@ def signature(obj):
     except AttributeError:
         pass
     else:
+        if isinstance(partialmethod, functools.partialmethod):
             # Unbound partialmethod (see functools.partialmethod)
             # This means, that we need to calculate the signature
             # as if it's a regular partial object, but taking into

Lib/test/test_inspect.py

@@ -1983,6 +1983,11 @@ class TestSignatureObject(unittest.TestCase):
                            ('c', 1, ..., 'keyword_only')),
                           'spam'))
 
+    def test_signature_on_fake_partialmethod(self):
+        def foo(a): pass
+        foo._partialmethod = 'spam'
+        self.assertEqual(str(inspect.signature(foo)), '(a)')
+
     def test_signature_on_decorated(self):
         import functools