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>

2 files changed, 102 insertions, 4 deletions

diff --git a/Lib/test/test_typing.py b/Lib/test/test_typing.py
index ffd0592..d480847 100644
--- a/Lib/test/test_typing.py
+++ b/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
 
diff --git a/Lib/typing.py b/Lib/typing.py
index b26adc6..3e0fbdb 100644
--- a/Lib/typing.py
+++ b/Lib/typing.py
@@ -21,6 +21,7 @@ At large scale, the structure of the module is following:
 
 from abc import abstractmethod, ABCMeta
 import collections
+from collections import defaultdict
 import collections.abc
 import contextlib
 import functools
@@ -121,9 +122,11 @@ __all__ = [
     'assert_type',
     'assert_never',
     'cast',
+    'clear_overloads',
     'final',
     'get_args',
     'get_origin',
+    'get_overloads',
     'get_type_hints',
     'is_typeddict',
     'LiteralString',
@@ -2450,6 +2453,10 @@ def _overload_dummy(*args, **kwds):
         "by an implementation that is not @overload-ed.")
 
 
+# {module: {qualname: {firstlineno: func}}}
+_overload_registry = defaultdict(functools.partial(defaultdict, dict))
+
+
 def overload(func):
     """Decorator for overloaded functions/methods.
 
@@ -2475,10 +2482,37 @@ def overload(func):
       def utf8(value: str) -> bytes: ...
       def utf8(value):
           # implementation goes here
+
+    The overloads for a function can be retrieved at runtime using the
+    get_overloads() function.
     """
+    # classmethod and staticmethod
+    f = getattr(func, "__func__", func)
+    try:
+        _overload_registry[f.__module__][f.__qualname__][f.__code__.co_firstlineno] = func
+    except AttributeError:
+        # Not a normal function; ignore.
+        pass
     return _overload_dummy
 
 
+def get_overloads(func):
+    """Return all defined overloads for *func* as a sequence."""
+    # classmethod and staticmethod
+    f = getattr(func, "__func__", func)
+    if f.__module__ not in _overload_registry:
+        return []
+    mod_dict = _overload_registry[f.__module__]
+    if f.__qualname__ not in mod_dict:
+        return []
+    return list(mod_dict[f.__qualname__].values())
+
+
+def clear_overloads():
+    """Clear all overloads in the registry."""
+    _overload_registry.clear()
+
+
 def final(f):
     """A decorator to indicate final methods and final classes.