inspect: Validate that __signature__ is None or an instance of Signature.

Closes #21801.

Lib/inspect.py

@@ -1912,6 +1912,10 @@ def _signature_internal(obj, follow_wrapper_chains=True, skip_bound_arg=True):
         pass
     else:
         if sig is not None:
+            if not isinstance(sig, Signature):
+                raise TypeError(
+                    'unexpected object {!r} in __signature__ '
+                    'attribute'.format(sig))
             return sig
 
     try:

Lib/test/test_inspect.py

@@ -3048,6 +3048,13 @@ class TestMain(unittest.TestCase):
         self.assertEqual(lines[:-1], inspect.getsource(module).splitlines())
         self.assertEqual(err, b'')
 
+    def test_custom_getattr(self):
+        def foo():
+            pass
+        foo.__signature__ = 42
+        with self.assertRaises(TypeError):
+            inspect.signature(foo)
+
     @unittest.skipIf(ThreadPoolExecutor is None,
             'threads required to test __qualname__ for source files')
     def test_qualname_source(self):

Misc/NEWS

@@ -105,6 +105,8 @@ Library
 - Issue #21538: The plistlib module now supports loading of binary plist files
   when reference or offset size is not a power of two.
 
+- Issue #21801: Validate that __signature__ is None or an instance of Signature.
+
 Build
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