Not a public change.

fiddle/_src/absl_flags/sample_test_binary.py

@@ -35,6 +35,10 @@ def base_experiment() -> fdl.Config:
   return fake_encoder_decoder.fixture.as_buildable()
 
 
+def base_experiment_with_bias() -> fdl.Config:
+  return fake_encoder_decoder.fixture_with_bias.as_buildable()
+
+
 def set_dtypes(config, dtype: str):
   def traverse(value, state):
     if state.current_path and state.current_path[-1] == daglish.Attr("dtype"):

fiddle/_src/experimental/auto_config.py

@@ -552,6 +552,28 @@ def _make_partial(partial_cls, buildable_or_callable, *args, **kwargs):
     return partial_cls(buildable_or_callable, *args, **kwargs)
 
 
+def _override_values_recursively(
+    base: config.Buildable, overrides: config.Buildable
+) -> None:
+  """Recursively replaces fields in base with values from overrides."""
+  for field_name in dir(overrides):
+    field = getattr(overrides, field_name, config.NO_VALUE)
+    if field != config.NO_VALUE:
+      if isinstance(field, config.Buildable):
+        _override_values_recursively(getattr(base, field_name), field)
+      else:
+        setattr(base, field_name, field)
+
+
+def override_values(
+    base: config.Buildable, overrides: config.Buildable
+) -> config.Buildable:
+  """Returns a copy of base with any values present in overrides overridden."""
+  base = base.__deepcopy__(memo={})
+  _override_values_recursively(base, overrides)
+  return base
+
+
 def exempt(fn_or_cls: Callable[..., Any]) -> Callable[..., Any]:
   """Wrap a callable so that it's exempted from auto_config.
 
@@ -599,6 +621,7 @@ def auto_config(
     experimental_exemption_policy: Optional[auto_config_policy.Policy] = None,
     experimental_config_types: ConfigTypes = ConfigTypes(),
     experimental_result_must_contain_buildable: bool = True,
+    base_config: Optional[AutoConfig] = None,
 ) -> Any:  # TODO(b/272377821): More precise return type.
   """Rewrites the given function to make it generate a ``Config``.
 
@@ -693,6 +716,9 @@ def build_model():
     experimental_result_must_contain_buildable: If true, then raise an error if
       `fn.as_buildable` returns a result that does not contain any `Buildable`
       values -- e.g., if it returns an empty dict.
+    base_config: Ff given, would be used as a default values. This allows to
+      have common settings defined once while defing multiple slightly different
+      configurations.
 
   Returns:
     A wrapped version of ``fn``, but with an additional ``as_buildable``
@@ -857,13 +883,25 @@ def make_auto_config(fn):
     auto_config_fn.__defaults__ = fn.__defaults__
     auto_config_fn.__kwdefaults__ = fn.__kwdefaults__
 
+    if base_config is not None:
+
+      @functools.wraps(auto_config_fn)
+      def auto_config_fn_with_base(*args, **kwargs):
+        return override_values(
+            base_config.as_buildable(*args, **kwargs),
+            auto_config_fn(*args, **kwargs),  # pylint: disable=not-callable
+        )
+
+    else:
+      auto_config_fn_with_base = auto_config_fn
+
     # Finally we wrap the rewritten function to perform additional error
     # checking and enforce that the output contains a `fdl.Buildable`.
     if experimental_result_must_contain_buildable:
 
-      @functools.wraps(auto_config_fn)
+      @functools.wraps(auto_config_fn_with_base)
       def as_buildable(*args, **kwargs):
-        output = auto_config_fn(*args, **kwargs)  # pylint: disable=not-callable
+        output = auto_config_fn_with_base(*args, **kwargs)  # pylint: disable=not-callable
         if not _contains_buildable(output):
           raise TypeError(
               f'The `auto_config` rewritten version of `{fn.__qualname__}` '
@@ -875,7 +913,7 @@ def as_buildable(*args, **kwargs):
         return output
 
     else:
-      as_buildable = auto_config_fn
+      as_buildable = auto_config_fn_with_base
 
     if method_type:
       fn = method_type(fn)

fiddle/_src/testing/example/fake_encoder_decoder.py

@@ -99,3 +99,16 @@ def fixture():
                                                    bias_init),
           mlp=Mlp(dtype, False, ["num_heads", "head_dim", "embed"]),
       ))
+
+
+@auto_config.auto_config(base_config=fixture)
+def fixture_with_bias():
+  # pylint: disable=no-value-for-parameter
+  # pytype: disable=missing-parameter
+  return FakeEncoderDecoder(
+      encoder=FakeEncoder(
+          mlp=Mlp(use_bias=True),
+      ),
+  )
+  # pytype: enable=missing-parameter
+  # pylint: enable=no-value-for-parameter