@@ -1083,6 +1083,21 @@ def _add_member_(cls, name, member):
         #
         cls._member_map_[name] = member
 
+    @property
+    def __signature__(cls):
+        from inspect import Parameter, Signature
+        if cls._member_names_:
+            return Signature([Parameter('values', Parameter.VAR_POSITIONAL)])
+        else:
+            return Signature([Parameter('new_class_name', Parameter.POSITIONAL_ONLY),
+                              Parameter('names', Parameter.POSITIONAL_OR_KEYWORD),
+                              Parameter('module', Parameter.KEYWORD_ONLY, default=None),
+                              Parameter('qualname', Parameter.KEYWORD_ONLY, default=None),
+                              Parameter('type', Parameter.KEYWORD_ONLY, default=None),
+                              Parameter('start', Parameter.KEYWORD_ONLY, default=1),
+                              Parameter('boundary', Parameter.KEYWORD_ONLY, default=None)])
+
+
 EnumMeta = EnumType         # keep EnumMeta name for backwards compatibility
 
 
@@ -1126,13 +1141,6 @@ class Enum(metaclass=EnumType):
     attributes -- see the documentation for details.
     """
 
-    @classmethod
-    def __signature__(cls):
-        if cls._member_names_:
-            return '(*values)'
-        else:
-            return '(new_class_name, /, names, *, module=None, qualname=None, type=None, start=1, boundary=None)'
-
     def __new__(cls, value):
         # all enum instances are actually created during class construction
         # without calling this method; this method is called by the metaclass'

diff --git a/Lib/inspect.py b/Lib/inspect.py
@@ -2547,18 +2547,10 @@ def _signature_from_callable(obj, *,
         pass
     else:
         if sig is not None:
-            # since __text_signature__ is not writable on classes, __signature__
-            # may contain text (or be a callable that returns text);
-            # if so, convert it
-            o_sig = sig
-            if not isinstance(sig, (Signature, str)) and callable(sig):
-                sig = sig()
-            if isinstance(sig, str):
-                sig = _signature_fromstr(sigcls, obj, sig)
             if not isinstance(sig, Signature):
                 raise TypeError(
                     'unexpected object {!r} in __signature__ '
-                    'attribute'.format(o_sig))
+                    'attribute'.format(sig))
             return sig
 
     try:

diff --git a/Lib/test/test_inspect/test_inspect.py b/Lib/test/test_inspect/test_inspect.py
@@ -4202,38 +4202,6 @@ def foo(): pass
                 self.assertEqual(signature_func(foo), inspect.Signature())
         self.assertEqual(inspect.get_annotations(foo), {})
 
-    def test_signature_as_str(self):
-        self.maxDiff = None
-        class S:
-            __signature__ = '(a, b=2)'
-
-        self.assertEqual(self.signature(S),
-                         ((('a', ..., ..., 'positional_or_keyword'),
-                           ('b', 2, ..., 'positional_or_keyword')),
-                          ...))
-
-    def test_signature_as_callable(self):
-        # __signature__ should be either a staticmethod or a bound classmethod
-        class S:
-            @classmethod
-            def __signature__(cls):
-                return '(a, b=2)'
-
-        self.assertEqual(self.signature(S),
-                         ((('a', ..., ..., 'positional_or_keyword'),
-                           ('b', 2, ..., 'positional_or_keyword')),
-                          ...))
-
-        class S:
-            @staticmethod
-            def __signature__():
-                return '(a, b=2)'
-
-        self.assertEqual(self.signature(S),
-                         ((('a', ..., ..., 'positional_or_keyword'),
-                           ('b', 2, ..., 'positional_or_keyword')),
-                          ...))
-
     def test_signature_on_derived_classes(self):
         # gh-105080: Make sure that signatures are consistent on derived classes
 

diff --git a/Misc/NEWS.d/next/Library/2024-02-27-10-22-15.gh-issue-115937.0cVNur.rst b/Misc/NEWS.d/next/Library/2024-02-27-10-22-15.gh-issue-115937.0cVNur.rst
@@ -0,0 +1,3 @@
+Removed extra preprocessing for the ``__signature__`` attribute: the code
+just check if it's a :class:`inspect.Signature` instance.  Patch by Sergey B
+Kirpichev.