Support older serialization protocol

Closes #1643

Author: maybe-hello-world

CHANGELOG.md

@@ -11,6 +11,7 @@ See [0Ver](https://0ver.org/).
 ### Bugfixes
 
 - Fixed HKT `to_str` example in documentation
+- Fixed backward deserialization compatibility for BaseContainer
 
 
 ## 0.20.0

returns/primitives/container.py

@@ -1,5 +1,5 @@
 from abc import ABCMeta
-from typing import Any, TypeVar
+from typing import Any, TypeVar, Union
 
 from typing_extensions import TypedDict
 
@@ -56,11 +56,15 @@ def __getstate__(self) -> _PickleState:
         """That's how this object will be pickled."""
         return {'container_value': self._inner_value}  # type: ignore
 
-    def __setstate__(self, state: _PickleState) -> None:
+    def __setstate__(self, state: Union[_PickleState, Any]) -> None:
         """Loading state from pickled data."""
-        object.__setattr__(  # noqa: WPS609
-            self, '_inner_value', state['container_value'],
-        )
+        if isinstance(state, dict) and 'container_value' in state:
+            object.__setattr__(  # noqa: WPS609
+                self, '_inner_value', state['container_value'],
+            )
+        else:
+            # backward compatibility with 0.19.0 and earlier
+            object.__setattr__(self, '_inner_value', state)  # noqa: WPS609
 
 
 def container_equality(

setup.cfg

@@ -87,6 +87,8 @@ per-file-ignores =
   returns/contrib/hypothesis/*.py: WPS437, WPS609
   # TODO: remove after mypy@0.800
   returns/contrib/mypy/_typeops/visitor.py: S101, WPS232
+  # Allow class attributes literals for slots and setattr:
+  returns/primitives/container.py: WPS226
   # There are multiple assert's in tests:
   tests/*.py: S101, WPS204, WPS218, WPS226, WPS432, WPS436
   # Some examples don't have any docs on purpose:

tests/test_io/test_io_container/test_io_pickle.py

@@ -9,5 +9,5 @@ def test_io_pickle():
 def test_io_pickle_restore():
     """Ensures that object can be restored."""
     container = IO(2)
-    container.__setstate__({'container_value': 1})  # type: ignore  # noqa: WPS609, E501
+    container.__setstate__({'container_value': 1})  # noqa: WPS609, E501
     assert container == IO(1)

tests/test_primitives/test_container/test_base_container/test_pickle_backward_deserialization.py

@@ -0,0 +1,14 @@
+import pickle  # noqa: S403
+
+from returns.primitives.container import BaseContainer
+
+
+def test_pickle_backward_deserialization():
+    """Test that BaseContainer can be deserialized from 0.19.0 and earlier."""
+    # BaseContainer(1) serialized as of 0.19.0
+    serialized_container = (
+        b'\x80\x04\x958\x00\x00\x00\x00\x00\x00\x00\x8c\x1c' +
+        b'returns.primitives.container\x94\x8c\rBaseContainer' +
+        b'\x94\x93\x94)\x81\x94K\x01b.'
+    )
+    assert pickle.loads(serialized_container) == BaseContainer(1)  # noqa: S301