Fix __qualname__ AttributeError in fiddle config.py

If a partial function generated by `functools.partial` is passed into
fiddle AND certain exceptions are raised, accessing `__qualname__`
will trigger another exception, prevent the users from seeing the
real error messages.

Type type `TypeOrCallableProducingT = Union[Callable[..., T], Type[T]]`
does not guarantee that `__qualname__` will exist.

- Added a regression test case in third_party/py/fiddle/_src/config_test.py that uses functools.partial to trigger the bug.
- Replace all `self.__fn_or_cls__.__qualname__` with the existing `self._fn_or_cls_name_repr()` which already solved the problem.

PiperOrigin-RevId: 893119387

Author: miaout17

fiddle/_src/config.py

@@ -348,22 +348,22 @@ def __getattr__(self, name: str):
     ) 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} '
+          + f'{self._fn_or_cls_name_repr()}.{name} '
           + 'since it uses a default_factory.'
       )
     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} "
-        f"(Buildable='{self.__fn_or_cls__.__qualname__}', missing "
+        f"(Buildable='{self._fn_or_cls_name_repr()}', missing "
         f"parameter='{name}')."
     )
     # TODO(b/219988937): Implement an edit distance function and display valid
     # attributes that are close to `name`.
     if hasattr(self.__fn_or_cls__, name):
       msg += (
           f' Note: {self.__fn_or_cls__.__module__}.'
-          f'{self.__fn_or_cls__.__qualname__} has an attribute/method with '
+          f'{self._fn_or_cls_name_repr()} has an attribute/method with '
           'this name, so this could be caused by using a '
           f'fdl.{self.__class__.__qualname__} in '
           'place of the actual function or class being configured. Did you '

fiddle/_src/config_test.py

@@ -17,6 +17,7 @@
 
 import copy
 import dataclasses
+import functools
 import pickle
 import sys
 import threading
@@ -851,6 +852,18 @@ def test_nonexistent_attribute_error(self):
     with self.assertRaisesRegex(AttributeError, expected_msg):
       getattr(class_config, 'nonexistent_arg')
 
+  def test_partial_nonexistent_attribute_error(self):
+    # This test serves as a regression test that `functools.partial`
+    # wasn't well handled in error reporting code.
+    partial_fn = functools.partial(basic_fn, arg1=1)
+    cfg = fdl.Config(partial_fn)
+    # The expected error should indicate the missing parameter, not crash trying
+    # to access __qualname__ on the partial object.
+    with self.assertRaisesRegex(
+        AttributeError, r"No parameter 'nonexistent_arg' has been set"
+    ):
+      getattr(cfg, 'nonexistent_arg')
+
   def test_nonexistent_parameter_error(self):
     class_config = fdl.Config(SampleClass)
     expected_msg = (r"No parameter named 'nonexistent_arg' exists for "