gh-89263: Add typing.get_overloads (GH-31716)

Based on suggestions by Guido van Rossum, Spencer Brown, and Alex Waygood.

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
Co-authored-by: Guido van Rossum <gvanrossum@gmail.com>
Co-authored-by: Ken Jin <kenjin4096@gmail.com>

Lib/test/test_typing.py

@@ -1,5 +1,6 @@
 import contextlib
 import collections
+from collections import defaultdict
 from functools import lru_cache
 import inspect
 import pickle
@@ -7,9 +8,11 @@ import re
 import sys
 import warnings
 from unittest import TestCase, main, skipUnless, skip
+from unittest.mock import patch
 from copy import copy, deepcopy
 
 from typing import Any, NoReturn, Never, assert_never
+from typing import overload, get_overloads, clear_overloads
 from typing import TypeVar, TypeVarTuple, Unpack, AnyStr
 from typing import T, KT, VT  # Not in __all__.
 from typing import Union, Optional, Literal
@@ -3890,11 +3893,22 @@ class ForwardRefTests(BaseTestCase):
         self.assertEqual("x" | X, Union["x", X])
 
 
+@lru_cache()
+def cached_func(x, y):
+    return 3 * x + y
+
+
+class MethodHolder:
+    @classmethod
+    def clsmethod(cls): ...
+    @staticmethod
+    def stmethod(): ...
+    def method(self): ...
+
+
 class OverloadTests(BaseTestCase):
 
     def test_overload_fails(self):
-        from typing import overload
-
         with self.assertRaises(RuntimeError):
 
             @overload
@@ -3904,8 +3918,6 @@ class OverloadTests(BaseTestCase):
             blah()
 
     def test_overload_succeeds(self):
-        from typing import overload
-
         @overload
         def blah():
             pass
@@ -3915,6 +3927,58 @@ class OverloadTests(BaseTestCase):
 
         blah()
 
+    def set_up_overloads(self):
+        def blah():
+            pass
+
+        overload1 = blah
+        overload(blah)
+
+        def blah():
+            pass
+
+        overload2 = blah
+        overload(blah)
+
+        def blah():
+            pass
+
+        return blah, [overload1, overload2]
+
+    # Make sure we don't clear the global overload registry
+    @patch("typing._overload_registry",
+        defaultdict(lambda: defaultdict(dict)))
+    def test_overload_registry(self):
+        # The registry starts out empty
+        self.assertEqual(typing._overload_registry, {})
+
+        impl, overloads = self.set_up_overloads()
+        self.assertNotEqual(typing._overload_registry, {})
+        self.assertEqual(list(get_overloads(impl)), overloads)
+
+        def some_other_func(): pass
+        overload(some_other_func)
+        other_overload = some_other_func
+        def some_other_func(): pass
+        self.assertEqual(list(get_overloads(some_other_func)), [other_overload])
+
+        # Make sure that after we clear all overloads, the registry is
+        # completely empty.
+        clear_overloads()
+        self.assertEqual(typing._overload_registry, {})
+        self.assertEqual(get_overloads(impl), [])
+
+        # Querying a function with no overloads shouldn't change the registry.
+        def the_only_one(): pass
+        self.assertEqual(get_overloads(the_only_one), [])
+        self.assertEqual(typing._overload_registry, {})
+
+    def test_overload_registry_repeated(self):
+        for _ in range(2):
+            impl, overloads = self.set_up_overloads()
+
+            self.assertEqual(list(get_overloads(impl)), overloads)
+
 
 # Definitions needed for features introduced in Python 3.6