feat: copy mutable args (#529)

Co-authored-by: guacs <126393040+guacs@users.noreply.github.com>

polyfactory/factories/base.py

@@ -30,6 +30,7 @@
     ClassVar,
     Collection,
     Generic,
+    Hashable,
     Iterable,
     Mapping,
     Sequence,
@@ -289,7 +290,7 @@ def _get_async_persistence(cls) -> AsyncPersistenceProtocol[T]:
         )
 
     @classmethod
-    def _handle_factory_field(
+    def _handle_factory_field(  # noqa: PLR0911
         cls,
         field_value: Any,
         build_context: BuildContext,
@@ -319,7 +320,10 @@ def _handle_factory_field(
         if isinstance(field_value, Fixture):
             return field_value.to_value()
 
-        return field_value() if callable(field_value) else field_value
+        if callable(field_value):
+            return field_value()
+
+        return field_value if isinstance(field_value, Hashable) else copy.deepcopy(field_value)
 
     @classmethod
     def _handle_factory_field_coverage(

tests/test_factory_fields.py

@@ -1,13 +1,15 @@
 import random
+from dataclasses import dataclass
 from datetime import datetime, timedelta
-from typing import Any, Optional, Union
+from typing import Any, ClassVar, List, Optional, Union
 
 import pytest
 
 from pydantic import BaseModel
 
 from polyfactory.decorators import post_generated
 from polyfactory.exceptions import ConfigurationException, MissingBuildKwargException
+from polyfactory.factories.dataclass_factory import DataclassFactory
 from polyfactory.factories.pydantic_factory import ModelFactory
 from polyfactory.fields import Ignore, PostGenerated, Require, Use
 
@@ -202,3 +204,18 @@ class NoFieldModel(BaseModel):
         match="unknown_field is declared on the factory NoFieldModelFactory but it is not part of the model NoFieldModel",
     ):
         ModelFactory.create_factory(NoFieldModel, bases=None, __check_model__=True, unknown_field=factory_field)
+
+
+def test_mutable_defaults() -> None:
+    @dataclass
+    class A:
+        a: List[str]
+
+    class AFactory(DataclassFactory[A]):
+        a: ClassVar[List[str]] = []
+
+    AFactory.build().a.append("a")
+    assert AFactory.build().a == []
+
+    next(iter(AFactory.coverage())).a.append("value")
+    assert next(iter(AFactory.coverage())).a == []