bpo-30149: Fix partialmethod without explicit self parameter (python#1308)

Author: corona10

Lib/inspect.py

@@ -2241,11 +2241,16 @@ def _signature_from_callable(obj, *,
                 sigcls=sigcls)
 
             sig = _signature_get_partial(wrapped_sig, partialmethod, (None,))
-
             first_wrapped_param = tuple(wrapped_sig.parameters.values())[0]
-            new_params = (first_wrapped_param,) + tuple(sig.parameters.values())
-
-            return sig.replace(parameters=new_params)
+            if first_wrapped_param.kind is Parameter.VAR_POSITIONAL:
+                # First argument of the wrapped callable is `*args`, as in
+                # `partialmethod(lambda *args)`.
+                return sig
+            else:
+                sig_params = tuple(sig.parameters.values())
+                assert first_wrapped_param is not sig_params[0]
+                new_params = (first_wrapped_param,) + sig_params
+                return sig.replace(parameters=new_params)
 
     if isfunction(obj) or _signature_is_functionlike(obj):
         # If it's a pure Python function, or an object that is duck type

Lib/test/test_inspect.py

@@ -1986,6 +1986,41 @@ def test(a, b:'foo'=10, *args:'bar', spam:'baz', ham=123, **kwargs:int):
                            ('kwargs', ..., int, "var_keyword")),
                           ...))
 
+    def test_signature_without_self(self):
+        def test_args_only(*args):  # NOQA
+            pass
+
+        def test_args_kwargs_only(*args, **kwargs):  # NOQA
+            pass
+
+        class A:
+            @classmethod
+            def test_classmethod(*args):  # NOQA
+                pass
+
+            @staticmethod
+            def test_staticmethod(*args):  # NOQA
+                pass
+
+            f1 = functools.partialmethod((test_classmethod), 1)
+            f2 = functools.partialmethod((test_args_only), 1)
+            f3 = functools.partialmethod((test_staticmethod), 1)
+            f4 = functools.partialmethod((test_args_kwargs_only),1)
+
+        self.assertEqual(self.signature(test_args_only),
+                         ((('args', ..., ..., 'var_positional'),), ...))
+        self.assertEqual(self.signature(test_args_kwargs_only),
+                         ((('args', ..., ..., 'var_positional'),
+                           ('kwargs', ..., ..., 'var_keyword')), ...))
+        self.assertEqual(self.signature(A.f1),
+                         ((('args', ..., ..., 'var_positional'),), ...))
+        self.assertEqual(self.signature(A.f2),
+                         ((('args', ..., ..., 'var_positional'),), ...))
+        self.assertEqual(self.signature(A.f3),
+                         ((('args', ..., ..., 'var_positional'),), ...))
+        self.assertEqual(self.signature(A.f4), 
+                         ((('args', ..., ..., 'var_positional'),
+                            ('kwargs', ..., ..., 'var_keyword')), ...))
     @cpython_only
     @unittest.skipIf(MISSING_C_DOCSTRINGS,
                      "Signature information for builtins requires docstrings")

Misc/NEWS

@@ -323,6 +323,10 @@ Extension Modules
 Library
 -------
 
+- bpo-30149: inspect.signature() now supports callables with
+  variable-argument parameters wrapped with partialmethod.
+  Patch by Dong-hee Na.
+
 - bpo-30301: Fix AttributeError when using SimpleQueue.empty() under
   *spawn* and *forkserver* start methods.