interpreterbase: Allow safely using mutable default values with typed_kwargs

It's really inconvenient to want a thing that is always a list, but not
be able to provide a default value of a list because of mutation. To
that end the typed_kwargs method now makes a shallow copy of the default
when using a `ContainerTypeInfo` as the type. This mean that using a
default of `[]` is perfectly safe.

1 files changed, 11 insertions, 0 deletions

diff --git a/run_unittests.py b/run_unittests.py
index e278675..a0beb48 100755
--- a/run_unittests.py
+++ b/run_unittests.py
@@ -1574,6 +1574,17 @@ class InternalTests(unittest.TestCase):
 
         _(None, mock.Mock(), [], {'input': 'str'})
 
+    def test_typed_kwarg_container_default_copy(self) -> None:
+        default: T.List[str] = []
+        @typed_kwargs(
+            'testfunc',
+            KwargInfo('input', ContainerTypeInfo(list, str), listify=True, default=default),
+        )
+        def _(obj, node, args: T.Tuple, kwargs: T.Dict[str, T.List[str]]) -> None:
+            self.assertIsNot(kwargs['input'], default)
+
+        _(None, mock.Mock(), [], {})
+
     def test_typed_kwarg_container_pairs(self) -> None:
         @typed_kwargs(
             'testfunc',