Add test coverage that default_factory doesn't copy its value

changes/1523-therefromhere.md

@@ -0,0 +1 @@
+Add a test assertion that `default_factory` can return a singleton

tests/test_dataclasses.py

@@ -431,6 +431,23 @@ class User:
     assert fields['aliases'].default == {'John': 'Joey'}
 
 
+def test_default_factory_singleton_field():
+    class MySingleton:
+        pass
+
+    class MyConfig:
+        arbitrary_types_allowed = True
+
+    MY_SINGLETON = MySingleton()
+
+    @pydantic.dataclasses.dataclass(config=MyConfig)
+    class Foo:
+        singleton: MySingleton = dataclasses.field(default_factory=lambda: MY_SINGLETON)
+
+    # Returning a singleton from a default_factory is supported
+    assert Foo().singleton is Foo().singleton
+
+
 def test_schema():
     @pydantic.dataclasses.dataclass
     class User:

tests/test_main.py

@@ -1087,6 +1087,20 @@ class FunctionModel(BaseModel):
     m = FunctionModel()
     assert m.uid is uuid4
 
+    # Returning a singleton from a default_factory is supported
+    class MySingleton:
+        pass
+
+    MY_SINGLETON = MySingleton()
+
+    class SingletonFieldModel(BaseModel):
+        singleton: MySingleton = Field(default_factory=lambda: MY_SINGLETON)
+
+        class Config:
+            arbitrary_types_allowed = True
+
+    assert SingletonFieldModel().singleton is SingletonFieldModel().singleton
+
 
 @pytest.mark.skipif(sys.version_info < (3, 7), reason='field constraints are set but not enforced with python 3.6')
 def test_none_min_max_items():