Circular Import Fragility & PEP 8 E402 Violation:

Moving _logger and the module-level constants (_VALID_API_VERSIONS, _DEFAULT_VERSION, etc.) above from google.ads.googleads import config, oauth2, util and from google.ads.googleads.interceptors import ... splits top-level imports in violation of PEP 8 E402 (module level import not at top of file) and Google Python Style.

More importantly, this creates an implicit circular import dependency (client -> interceptors -> exception_interceptor -> errors -> client) that relies on exact statement ordering in client.py. If an automated formatter (ruff, isort, flake8) regroups the imports at the top of client.py, or if google.ads.googleads.errors is ever imported/reloaded before client.py executes line 37, importing fails at runtime with an ImportError.

Suggestion: Revert the changes in client.py to keep all imports grouped at the top of the module, and import _DEFAULT_VERSION lazily inside _get_caller_version() in errors.py (only on the fallback path).

Done.

Optimization & Robustness in _get_caller_version():

1. Break top-level circular import: Importing _DEFAULT_VERSION lazily on the fallback return avoids the top-level circular import with client.py.
2. Skip internal frames & fast-reject non-matching modules: inspect.currentframe() starts at _get_caller_version itself (in google.ads.googleads.errors), and its immediate caller is always in errors.py. When falling back to _DEFAULT_VERSION, the loop currently executes _VERSION_REGEX.match on every stack frame all the way up the call stack (e.g., 20–40+ test/application frames). Starting at frame.f_back and checking module_name.startswith("google.ads.googleads.v") before matching avoids unnecessary regex overhead.
3. Support point-release and root version module names: r"^google\.ads\.googleads\.(v\d+)\." will not match point versions (e.g., v26_1) or a top-level google.ads.googleads.v26 module.

Done.

Guard Empty-String version & Redundant _get_caller_version() Resolution:

1. Using if not version: (both here and at line 150 in raise_formatted_ads_exception) is safer than if version is None:, as an empty string "" would otherwise bypass the check and fail with ModuleNotFoundError: No module named 'google.ads.googleads..errors.types.errors'.
2. Note that raise_formatted_ads_exception already resolves version = _get_caller_version() when version is omitted; because _get_caller_version() walks the stack past all google.ads.googleads.errors frames, delegating raise_formatted_ads_exception(response, version=version) directly produces the exact same result without duplicating the resolution logic (if test_raise_formatted_for_status_default_version asserts mock_raise.assert_called_once_with(self.response, None) or tests end-to-end).

Done.

Test Performance Optimization & Edge-Case Coverage:

1. Avoid loading the entire v25.services package tree in unit tests: Importing google.ads.googleads.v25.services.services.you_tube_video_upload_service.transports.resumable_upload executes google/ads/googleads/v25/__init__.py and google/ads/googleads/v25/services/__init__.py (1,502 lines), which eagerly imports every service, resource, enum, and protobuf descriptor in v25 on Python < 3.15. In addition, calling upload._process_initiate_response couples this test to a private method on a generated transport class.
   You can test stack-frame version inference in <1ms without importing v25.services by executing a small helper with a versioned __name__ in its globals (e.g., exec("def call(r): errors.raise_formatted_for_status(r)", {"__name__": f"google.ads.googleads.{older_version}.services.test", "errors": errors})), or by loading resumable_upload.py without triggering v25/services/__init__.py.
2. Additional edge cases: Consider adding a unit test verifying that _get_caller_version() falls back to self.version when inspect.currentframe() returns None (with mock.patch("inspect.currentframe", return_value=None): ...), and a case verifying raise_formatted_ads_exception(self.response) directly infers a non-default caller version.

Done.