Fix pickle/copy support for the missing singleton (#2029)

CHANGES.rst

@@ -24,6 +24,8 @@ Unreleased
     call. :issue:`2021`
 -   Fix dunder protocol (`copy`/`pickle`/etc) interaction with ``Undefined``
     objects. :issue:`2025`
+-   Fix `copy`/`pickle` support for the internal ``missing`` object.
+    :issue:`2027`
 
 
 Version 3.1.4

src/jinja2/utils.py

@@ -18,8 +18,17 @@ if t.TYPE_CHECKING:
 
 F = t.TypeVar("F", bound=t.Callable[..., t.Any])
 
-# special singleton representing missing values for the runtime
-missing: t.Any = type("MissingType", (), {"__repr__": lambda x: "missing"})()
+
+class _MissingType:
+    def __repr__(self) -> str:
+        return "missing"
+
+    def __reduce__(self) -> str:
+        return "missing"
+
+
+missing: t.Any = _MissingType()
+"""Special singleton representing missing values for the runtime."""
 
 internal_code: t.MutableSet[CodeType] = set()
 

tests/test_utils.py

@@ -1,3 +1,4 @@
+import copy
 import pickle
 import random
 from collections import deque
@@ -183,3 +184,14 @@ def test_consume():
     consume(x)
     with pytest.raises(StopIteration):
         next(x)
+
+
+@pytest.mark.parametrize("protocol", range(pickle.HIGHEST_PROTOCOL + 1))
+def test_pickle_missing(protocol: int) -> None:
+    """Test that missing can be pickled while remaining a singleton."""
+    assert pickle.loads(pickle.dumps(missing, protocol)) is missing
+
+
+def test_copy_missing() -> None:
+    """Test that missing can be copied while remaining a singleton."""
+    assert copy.copy(missing) is missing