fix: make pydantic errors (un)pickable

closes pydantic#1616

changes/1616-PrettyWood.md

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

pydantic/errors.py

@@ -91,6 +91,17 @@
 )
 
 
+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 PydanticErrorMixin:
     code: str
     msg_template: str
@@ -101,6 +112,9 @@ def __init__(self, **ctx: Any) -> None:
     def __str__(self) -> str:
         return self.msg_template.format(**self.__dict__)
 
+    def __reduce__(self):  # type: ignore
+        return cls_kwargs, (self.__class__, self.__dict__)
+
 
 class PydanticTypeError(PydanticErrorMixin, TypeError):
     pass

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):