bpo-43224: Implement pickling of TypeVarTuples (#32119)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

2 files changed, 74 insertions, 7 deletions

diff --git a/Lib/test/test_typing.py b/Lib/test/test_typing.py
index d480847..1fd99a0 100644
--- a/Lib/test/test_typing.py
+++ b/Lib/test/test_typing.py
@@ -1,7 +1,7 @@
 import contextlib
 import collections
 from collections import defaultdict
-from functools import lru_cache
+from functools import lru_cache, wraps
 import inspect
 import pickle
 import re
@@ -70,6 +70,18 @@ class BaseTestCase(TestCase):
             f()
 
 
+def all_pickle_protocols(test_func):
+    """Runs `test_func` with various values for `proto` argument."""
+
+    @wraps(test_func)
+    def wrapper(self):
+        for proto in range(pickle.HIGHEST_PROTOCOL + 1):
+            with self.subTest(pickle_proto=proto):
+                test_func(self, proto=proto)
+
+    return wrapper
+
+
 class Employee:
     pass
 
@@ -911,6 +923,48 @@ class TypeVarTupleTests(BaseTestCase):
         self.assertNotEqual(C[Unpack[Ts1]], C[Unpack[Ts2]])
 
 
+class TypeVarTuplePicklingTests(BaseTestCase):
+    # These are slightly awkward tests to run, because TypeVarTuples are only
+    # picklable if defined in the global scope. We therefore need to push
+    # various things defined in these tests into the global scope with `global`
+    # statements at the start of each test.
+
+    @all_pickle_protocols
+    def test_pickling_then_unpickling_results_in_same_identity(self, proto):
+        global Ts1  # See explanation at start of class.
+        Ts1 = TypeVarTuple('Ts1')
+        Ts2 = pickle.loads(pickle.dumps(Ts1, proto))
+        self.assertIs(Ts1, Ts2)
+
+    @all_pickle_protocols
+    def test_pickling_then_unpickling_unpacked_results_in_same_identity(self, proto):
+        global Ts  # See explanation at start of class.
+        Ts = TypeVarTuple('Ts')
+        unpacked1 = Unpack[Ts]
+        unpacked2 = pickle.loads(pickle.dumps(unpacked1, proto))
+        self.assertIs(unpacked1, unpacked2)
+
+    @all_pickle_protocols
+    def test_pickling_then_unpickling_tuple_with_typevartuple_equality(
+            self, proto
+    ):
+        global T, Ts  # See explanation at start of class.
+        T = TypeVar('T')
+        Ts = TypeVarTuple('Ts')
+
+        a1 = Tuple[Unpack[Ts]]
+        a2 = pickle.loads(pickle.dumps(a1, proto))
+        self.assertEqual(a1, a2)
+
+        a1 = Tuple[T, Unpack[Ts]]
+        a2 = pickle.loads(pickle.dumps(a1, proto))
+        self.assertEqual(a1, a2)
+
+        a1 = Tuple[int, Unpack[Ts]]
+        a2 = pickle.loads(pickle.dumps(a1, proto))
+        self.assertEqual(a1, a2)
+
+
 class UnionTests(BaseTestCase):
 
     def test_basics(self):
diff --git a/Lib/typing.py b/Lib/typing.py
index 3e0fbdb..a6f4fa9 100644
--- a/Lib/typing.py
+++ b/Lib/typing.py
@@ -867,6 +867,13 @@ def _is_typevar_like(x: Any) -> bool:
     return isinstance(x, (TypeVar, ParamSpec)) or _is_unpacked_typevartuple(x)
 
 
+class _PickleUsingNameMixin:
+    """Mixin enabling pickling based on self.__name__."""
+
+    def __reduce__(self):
+        return self.__name__
+
+
 class _BoundVarianceMixin:
     """Mixin giving __init__ bound and variance arguments.
 
@@ -903,11 +910,9 @@ class _BoundVarianceMixin:
             prefix = '~'
         return prefix + self.__name__
 
-    def __reduce__(self):
-        return self.__name__
 
-
-class TypeVar(_Final, _Immutable, _BoundVarianceMixin, _root=True):
+class TypeVar(_Final, _Immutable, _BoundVarianceMixin, _PickleUsingNameMixin,
+              _root=True):
     """Type variable.
 
     Usage::
@@ -973,7 +978,7 @@ class TypeVar(_Final, _Immutable, _BoundVarianceMixin, _root=True):
         return arg
 
 
-class TypeVarTuple(_Final, _Immutable, _root=True):
+class TypeVarTuple(_Final, _Immutable, _PickleUsingNameMixin, _root=True):
     """Type variable tuple.
 
     Usage:
@@ -994,11 +999,18 @@ class TypeVarTuple(_Final, _Immutable, _root=True):
       C[()]        # Even this is fine
 
     For more details, see PEP 646.
+
+    Note that only TypeVarTuples defined in global scope can be pickled.
     """
 
     def __init__(self, name):
         self.__name__ = name
 
+        # Used for pickling.
+        def_mod = _caller()
+        if def_mod != 'typing':
+            self.__module__ = def_mod
+
     def __iter__(self):
         yield Unpack[self]
 
@@ -1057,7 +1069,8 @@ class ParamSpecKwargs(_Final, _Immutable, _root=True):
         return self.__origin__ == other.__origin__
 
 
-class ParamSpec(_Final, _Immutable, _BoundVarianceMixin, _root=True):
+class ParamSpec(_Final, _Immutable, _BoundVarianceMixin, _PickleUsingNameMixin,
+                _root=True):
     """Parameter specification variable.
 
     Usage::