In a fdl.Config for a dataclass, when accessing a field that uses a default_factory, populate a child Config (if not already present) wrapping the default_factory call, rather than raising a "Can't get default value for dataclass field ... since it uses a default_factory" error.

fiddle/_src/config.py

@@ -23,7 +23,7 @@
 import dataclasses
 import functools
 import types
-from typing import Any, Callable, Collection, Dict, FrozenSet, Generic, Iterable, Mapping, NamedTuple, Optional, Set, Tuple, Type, TypeVar, Union
+from typing import Any, Callable, Collection, Dict, FrozenSet, Generic, Iterable, Mapping, NamedTuple, Optional, Set, Tuple, Type, TypeVar, Union, cast
 
 from fiddle._src import daglish
 from fiddle._src import history
@@ -341,14 +341,42 @@ def __getattr__(self, name: str):
 
     if value is not _UNSET_SENTINEL:
       return value
-    if dataclasses.is_dataclass(
-        self.__fn_or_cls__
-    ) and _field_uses_default_factory(self.__fn_or_cls__, name):
-      raise ValueError(
-          "Can't get default value for dataclass field "
-          + f'{self.__fn_or_cls__.__qualname__}.{name} '
-          + 'since it uses a default_factory.'
-      )
+    if dataclasses.is_dataclass(self.__fn_or_cls__) and (
+        default_factory := _field_default_factory(self.__fn_or_cls__, name)
+    ):
+      if hasattr(default_factory, 'as_buildable'):
+        # The default_factory is a function wrapped by auto_config. In this case
+        # it would appear reasonable to populate a child config by setting:
+        # self.__arguments__[name] = default_factory.as_buildable()
+        # However paxml.tools.fiddle / praxis.pax_fiddle unfortunately implement
+        # some tricky magic of their own relating to auto_config and dataclass
+        # default_factories, which would appear to be broken (or at least
+        # interfered with) by any support for this here. It is likely a fairly
+        # rare use case anyway outside of paxml, so we are choosing not to
+        # support it here.
+        raise ValueError(
+            "We don't currently support exposing a sub-config to build the "
+            f"default value for an auto_config'd default_factory (field {name} "
+            f'of dataclass {self.__fn_or_cls__}.'
+        )
+      elif _is_resolvable(default_factory):
+        self.__arguments__[name] = Config(default_factory)
+      elif isinstance(default_factory, functools.partial) and _is_resolvable(
+          default_factory.func
+      ):
+        self.__arguments__[name] = Config(
+            default_factory.func,
+            *default_factory.args,
+            **default_factory.keywords,
+        )
+      else:
+        raise ValueError(
+            "Can't expose a sub-config to build default value for field "
+            f'{name} of dataclass {self.__fn_or_cls__} since it uses an '
+            'anonymous default_factory.'
+        )
+      return self.__arguments__[name]
+
     if param is not None and param.default is not param.empty:
       return param.default
     msg = f"No parameter '{name}' has been set on {self!r}."
@@ -848,12 +876,27 @@ def __build__(self, /, *args: Any, **kwargs: Any) -> T:
     return self.__fn_or_cls__(tags=self.tags, *args, **kwargs)
 
 
-def _field_uses_default_factory(dataclass_type: Type[Any], field_name: str):
-  """Returns true if <dataclass_type>.<field_name> uses a default_factory."""
+def _field_default_factory(
+    dataclass_type: Type[Any], field_name: str
+) -> Callable[[], Any] | None:
+  """Returns the default_factory of <dataclass_type>.<field_name> if present."""
   for field in dataclasses.fields(dataclass_type):
-    if field.name == field_name:
-      return field.default_factory != dataclasses.MISSING
-  return False
+    if (
+        field.name == field_name
+        and field.default_factory != dataclasses.MISSING
+    ):
+      return cast(Callable[[], Any], field.default_factory)
+  return None
+
+
+def _is_resolvable(value: Any) -> bool:
+  return (
+      hasattr(value, '__module__')
+      and hasattr(value, '__qualname__')
+      and
+      # Rules out anonymous objects like <lambda>,  foo.<locals>.bar etc:
+      '<' not in value.__qualname__
+  )
 
 
 BuildableT = TypeVar('BuildableT', bound=Buildable)

fiddle/_src/config_test.py

@@ -17,6 +17,7 @@
 
 import copy
 import dataclasses
+import functools
 import pickle
 import sys
 import threading
@@ -111,6 +112,13 @@ class DataclassParent:
   child: DataclassChild = dataclasses.field(default_factory=DataclassChild)
 
 
+@dataclasses.dataclass
+class DataclassParentPartialDefaultFactoryChild:
+  child: DataclassChild = dataclasses.field(
+      default_factory=functools.partial(DataclassChild, x=1)
+  )
+
+
 def raise_error():
   raise ValueError('My fancy exception')
 
@@ -1042,21 +1050,40 @@ def test_copy_constructor_with_updates_errors(self):
     with self.assertRaises(ValueError):
       fdl.Partial(cfg1, 5, a='a', b='b')
 
-  def test_dataclass_default_factory(self):
+  def test_dataclass_default_factory_can_read_default(self):
+    cfg = fdl.Config(DataclassParent)
+    child_config = cfg.child
+    self.assertIsInstance(child_config, fdl.Config)
+    self.assertEqual(child_config.__fn_or_cls__, DataclassChild)
+    self.assertEqual(fdl.build(cfg), DataclassParent(child=DataclassChild(x=0)))
+
+  def test_dataclass_partial_default_factory_can_read_default(self):
+    cfg = fdl.Config(DataclassParentPartialDefaultFactoryChild)
+    child_config = cfg.child
+    self.assertIsInstance(child_config, fdl.Config)
+    self.assertEqual(child_config.__fn_or_cls__, DataclassChild)
+    self.assertEqual(child_config.x, 1)
+    self.assertEqual(
+        fdl.build(cfg),
+        DataclassParentPartialDefaultFactoryChild(child=DataclassChild(x=1)),
+    )
 
+  def test_dataclass_default_factory_overriding_child_config(self):
     cfg = fdl.Config(DataclassParent)
+    cfg.child.x = 5
+    self.assertEqual(fdl.build(cfg), DataclassParent(DataclassChild(x=5)))
+
+  def test_dataclass_partial_default_factory_overriding_child_config(self):
+    cfg = fdl.Config(DataclassParentPartialDefaultFactoryChild)
+    cfg.child.x = 5
+    self.assertEqual(
+        fdl.build(cfg),
+        DataclassParentPartialDefaultFactoryChild(DataclassChild(x=5)),
+    )
 
-    with self.subTest('read_default_is_error'):
-      expected_error = (
-          r"Can't get default value for dataclass field DataclassParent\.child "
-          r'since it uses a default_factory\.')
-      with self.assertRaisesRegex(ValueError, expected_error):
-        cfg.child.x = 5
-
-    with self.subTest('read_ok_after_override'):
-      cfg.child = fdl.Config(DataclassChild)  # override default w/ a value
-      cfg.child.x = 5  # now it's ok to configure child.
-      self.assertEqual(fdl.build(cfg), DataclassParent(DataclassChild(5)))
+  def test_dataclass_default_factory_not_used_when_child_config_given(self):
+    cfg = fdl.Config(DataclassParent, child=fdl.Config(DataclassChild, x=1))
+    self.assertEqual(cfg.child.x, 1)
 
   def test_unbound_method(self):
     sample = fdl.Config(SampleClass, 0, 1)