fix: make pydantic errors (un)pickable

closes pydantic#1616

changes/1616-PrettyWood.md

@@ -0,0 +1 @@
+make *pydantic* errors (un)pickable

pydantic/errors.py

@@ -3,6 +3,7 @@
 from typing import Any, Set, Union
 
 from .typing import AnyType, display_as_type
+from .utils import PickableError
 
 # explicitly state exports to avoid "from .errors import *" also importing Decimal, Path etc.
 __all__ = (
@@ -91,7 +92,7 @@
 )
 
 
-class PydanticErrorMixin:
+class PydanticErrorMixin(PickableError):
     code: str
     msg_template: str
 

pydantic/utils.py

@@ -46,6 +46,7 @@
     'ValueItems',
     'version_info',  # required here to match behaviour in v1.3
     'ClassAttribute',
+    'PickableError',
 )
 
 
@@ -283,6 +284,22 @@ def __repr__(self) -> str:
         return f'{self.__repr_name__()}({self.__repr_str__(", ")})'
 
 
+def cls_kwargs(cls, kwargs):  # type: ignore
+    """
+    For built-in exceptions like ValueError or TypeError, we need to implement
+    __reduce__ to override the default behaviour (instead of __getstate__/__setstate__)
+    By default pickle protocol 2 calls `cls.__new__(cls, *args)`.
+    Since we only use kwargs, we need a little constructor to change that.
+    Note: the callable can't be a lambda as pickle looks in the namespace to find it
+    """
+    return cls(**kwargs)
+
+
+class PickableError:
+    def __reduce__(self):  # type: ignore
+        return cls_kwargs, (self.__class__, self.__dict__)
+
+
 class GetterDict(Representation):
     """
     Hack to make object's smell just enough like dicts for validate_model.

tests/test_errors.py

@@ -1,3 +1,4 @@
+import pickle
 import sys
 from typing import Dict, List, Optional, Union
 from uuid import UUID, uuid4
@@ -6,6 +7,7 @@
 
 from pydantic import UUID1, BaseConfig, BaseModel, PydanticTypeError, ValidationError, conint, errors, validator
 from pydantic.error_wrappers import flatten_errors, get_exc_type
+from pydantic.errors import StrRegexError
 from pydantic.typing import Literal
 
 
@@ -22,6 +24,17 @@ def __init__(self, *, test_ctx: int) -> None:
     assert str(exc_info.value) == 'test message template "test_value"'
 
 
+def test_pydantic_error_pickable():
+    """
+    Pydantic errors should be (un)pickable.
+    (this test does not create a custom local error as we can't pickle local objects)
+    """
+    p = pickle.dumps(StrRegexError(pattern='pika'))
+    error = pickle.loads(p)
+    assert isinstance(error, StrRegexError)
+    assert error.pattern == 'pika'
+
+
 @pytest.mark.skipif(not Literal, reason='typing_extensions not installed')
 def test_interval_validation_error():
     class Foo(BaseModel):