fix: keep subpackages reachable as attributes of a lazy package

Co-authored-by: George Weale <gweale@google.com>
PiperOrigin-RevId: 964141603

src/google/adk/utils/_lazy.py

@@ -36,12 +36,33 @@ def accessors(
   package: str = module_globals['__name__']
 
   def module_getattr(name: str) -> Any:
-    if name not in members:
+    if name in members:
+      module = importlib.import_module(members[name], package)
+      value = getattr(module, name)
+      module_globals[name] = value
+      return value
+
+    # Protocol probes ask for dunders that a package never resolves lazily,
+    # and a failed import is not cached, so answering them here would repeat
+    # the whole finder walk on every copy, pickle or introspection call.
+    if name.startswith('__') and name.endswith('__'):
       raise AttributeError(f'module {package!r} has no attribute {name!r}')
-    module = importlib.import_module(members[name], package)
-    value = getattr(module, name)
-    module_globals[name] = value
-    return value
+
+    # Importing a subpackage eagerly used to bind it on its parent, so
+    # ``package.subpackage`` resolved without importing it by name first.
+    # Resolve it on demand to keep that working, which the import system
+    # then caches by binding the submodule on this package.
+    submodule = f'{package}.{name}'
+    try:
+      return importlib.import_module(submodule)
+    except ModuleNotFoundError as error:
+      # Anything missing deeper than this name is a real dependency error and
+      # has to keep its own message rather than becoming a typo report.
+      if error.name != submodule:
+        raise
+      raise AttributeError(
+          f'module {package!r} has no attribute {name!r}'
+      ) from None
 
   def module_dir() -> list[str]:
     return sorted(set(module_globals) | set(module_globals.get('__all__', ())))

tests/unittests/test_import_loading.py

@@ -136,6 +136,20 @@ for module_name in {_LAZY_PACKAGES!r}:
   assert result.returncode == 0, result.stderr
 
 
+def test_lazy_packages_resolve_subpackages_as_attributes():
+  """A subpackage stays reachable on its parent, as eager imports left it."""
+  result = run_isolated("""
+import types
+
+import google.adk
+
+for name in ('agents', 'events', 'runners', 'sessions', 'tools'):
+  assert isinstance(getattr(google.adk, name), types.ModuleType), name
+""")
+
+  assert result.returncode == 0, result.stderr
+
+
 def test_lazy_packages_reject_unknown_attributes():
   """The lazy hook raises AttributeError rather than masking typos."""
   result = run_isolated(f"""

tests/unittests/utils/test_lazy.py

@@ -0,0 +1,66 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Tests for the lazy package accessors."""
+
+from __future__ import annotations
+
+import importlib
+import types
+
+from google.adk.utils import _lazy
+import pytest
+
+
+@pytest.fixture(name='package')
+def _package(tmp_path, monkeypatch) -> types.ModuleType:
+  """A package with a plain submodule and one that needs an absent library."""
+  root = tmp_path / 'lazy_fixture_pkg'
+  root.mkdir()
+  (root / '__init__.py').write_text('')
+  (root / 'plain.py').write_text('VALUE = 1\n')
+  (root / 'needs_absent.py').write_text('import absent_dependency\n')
+  (root / 'exports.py').write_text('Exported = object()\n')
+  monkeypatch.syspath_prepend(str(tmp_path))
+  return importlib.import_module('lazy_fixture_pkg')
+
+
+def test_declared_member_resolves_from_its_module(package):
+  getattr_, _ = _lazy.accessors(vars(package), {'Exported': '.exports'})
+
+  exports = importlib.import_module('lazy_fixture_pkg.exports')
+  assert getattr_('Exported') is exports.Exported
+
+
+def test_submodule_resolves_as_an_attribute(package):
+  getattr_, _ = _lazy.accessors(vars(package), {})
+
+  assert getattr_('plain').VALUE == 1
+
+
+def test_unknown_name_raises_attribute_error(package):
+  getattr_, _ = _lazy.accessors(vars(package), {})
+
+  with pytest.raises(AttributeError, match='no attribute'):
+    getattr_('not_a_submodule')
+
+
+def test_absent_dependency_keeps_its_own_error(package):
+  """A library missing inside a submodule must not read as a typo."""
+  getattr_, _ = _lazy.accessors(vars(package), {})
+
+  with pytest.raises(ModuleNotFoundError) as error:
+    getattr_('needs_absent')
+
+  assert error.value.name == 'absent_dependency'