Ensures that fdl.Config values can only be built once. This prevents accidental stateful coupling of config values.

fiddle/_src/building.py

@@ -25,6 +25,7 @@
 from fiddle._src import daglish
 from fiddle._src import partial
 from fiddle._src import reraised_exception
+from fiddle._src.mutate_buildable import update_callable
 
 T = TypeVar('T')
 
@@ -44,7 +45,8 @@ def _in_build():
   """A context manager to ensure fdl.build is not called recursively."""
   if _state.in_build:
     raise ValueError(
-        'It is forbidden to call `fdl.build` inside another `fdl.build` call.')
+        'It is forbidden to call `fdl.build` inside another `fdl.build` call.'
+    )
   _state.in_build = True
   try:
     yield
@@ -192,4 +194,11 @@ def _build(value: Any, state: daglish.State) -> Any:
         type(buildable),
     )
 
+  # Poison the buildable to prevent future builds.
+  def _poison(*args, **kwargs):  # pylint: disable=unused-argument
+    raise ValueError('Only call fld.build() once.')
+
+  if isinstance(buildable, config_lib.Buildable):
+    update_callable(buildable, _poison)
+
   return result

fiddle/_src/building_test.py

@@ -14,12 +14,12 @@
 # limitations under the License.
 
 """Tests for history."""
+
 import dataclasses
 import unittest
 import warnings
 from absl import logging
 from absl.testing import absltest
-
 from fiddle._src import building
 from fiddle._src import config
 
@@ -82,6 +82,12 @@ def test_traversable_w_buildable(self):
       building.build(value)
       self.assertEmpty(log_output)
 
+  def test_build_poisoning(self):
+    foo = config.Config(Foo, 1, 2)
+    with self.assertRaises(ValueError):
+      building.build(foo)
+      building.build(foo)
+
 
 if __name__ == '__main__':
   unittest.main()