gh-118285: Fix signatures of operator.{attrgetter,itemgetter,methodcaller} instances (GH-118316)

* Allow to specify the signature of custom callable instances of extension
  type by the __text_signature__ attribute.
* Specify signatures of operator.attrgetter, operator.itemgetter, and
  operator.methodcaller instances.

Lib/inspect.py

@@ -2692,6 +2692,13 @@ def _signature_from_callable(obj, *,
         # An object with __call__
         call = getattr_static(type(obj), '__call__', None)
         if call is not None:
+            try:
+                text_sig = obj.__text_signature__
+            except AttributeError:
+                pass
+            else:
+                if text_sig:
+                    return _signature_fromstr(sigcls, obj, text_sig)
             call = _descriptor_get(call, obj)
             return _get_signature_of(call)
 

Lib/operator.py

@@ -239,7 +239,7 @@ class attrgetter:
     """
     __slots__ = ('_attrs', '_call')
 
-    def __init__(self, attr, *attrs):
+    def __init__(self, attr, /, *attrs):
         if not attrs:
             if not isinstance(attr, str):
                 raise TypeError('attribute name must be a string')
@@ -257,7 +257,7 @@ class attrgetter:
                 return tuple(getter(obj) for getter in getters)
             self._call = func
 
-    def __call__(self, obj):
+    def __call__(self, obj, /):
         return self._call(obj)
 
     def __repr__(self):
@@ -276,7 +276,7 @@ class itemgetter:
     """
     __slots__ = ('_items', '_call')
 
-    def __init__(self, item, *items):
+    def __init__(self, item, /, *items):
         if not items:
             self._items = (item,)
             def func(obj):
@@ -288,7 +288,7 @@ class itemgetter:
                 return tuple(obj[i] for i in items)
             self._call = func
 
-    def __call__(self, obj):
+    def __call__(self, obj, /):
         return self._call(obj)
 
     def __repr__(self):
@@ -315,7 +315,7 @@ class methodcaller:
         self._args = args
         self._kwargs = kwargs
 
-    def __call__(self, obj):
+    def __call__(self, obj, /):
         return getattr(obj, self._name)(*self._args, **self._kwargs)
 
     def __repr__(self):

Lib/test/test_inspect/test_inspect.py

@@ -4090,6 +4090,28 @@ class TestSignatureObject(unittest.TestCase):
                             ((('a', ..., ..., "positional_or_keyword"),),
                             ...))
 
+    def test_signature_on_callable_objects_with_text_signature_attr(self):
+        class C:
+            __text_signature__ = '(a, /, b, c=True)'
+            def __call__(self, *args, **kwargs):
+                pass
+
+        self.assertEqual(self.signature(C), ((), ...))
+        self.assertEqual(self.signature(C()),
+                         ((('a', ..., ..., "positional_only"),
+                           ('b', ..., ..., "positional_or_keyword"),
+                           ('c', True, ..., "positional_or_keyword"),
+                          ),
+                          ...))
+
+        c = C()
+        c.__text_signature__ = '(x, y)'
+        self.assertEqual(self.signature(c),
+                         ((('x', ..., ..., "positional_or_keyword"),
+                           ('y', ..., ..., "positional_or_keyword"),
+                          ),
+                          ...))
+
     def test_signature_on_wrapper(self):
         class Wrapper:
             def __call__(self, b):

Lib/test/test_operator.py

@@ -1,4 +1,5 @@
 import unittest
+import inspect
 import pickle
 import sys
 from decimal import Decimal
@@ -602,6 +603,28 @@ class OperatorTestCase:
             if dunder:
                 self.assertIs(dunder, orig)
 
+    def test_attrgetter_signature(self):
+        operator = self.module
+        sig = inspect.signature(operator.attrgetter)
+        self.assertEqual(str(sig), '(attr, /, *attrs)')
+        sig = inspect.signature(operator.attrgetter('x', 'z', 'y'))
+        self.assertEqual(str(sig), '(obj, /)')
+
+    def test_itemgetter_signature(self):
+        operator = self.module
+        sig = inspect.signature(operator.itemgetter)
+        self.assertEqual(str(sig), '(item, /, *items)')
+        sig = inspect.signature(operator.itemgetter(2, 3, 5))
+        self.assertEqual(str(sig), '(obj, /)')
+
+    def test_methodcaller_signature(self):
+        operator = self.module
+        sig = inspect.signature(operator.methodcaller)
+        self.assertEqual(str(sig), '(name, /, *args, **kwargs)')
+        sig = inspect.signature(operator.methodcaller('foo', 2, y=3))
+        self.assertEqual(str(sig), '(obj, /)')
+
+
 class PyOperatorTestCase(OperatorTestCase, unittest.TestCase):
     module = py_operator