Merge pull request #101 from modern-python/refactor/ref-6-frozen-setattr

refactor: drop frozen=True from instruments; sentinel for FastAPIConfig.application

Author: lesnik512

docs/superpowers/plans/2026-06-01-pr13-frozen-setattr.md

@@ -6,7 +6,7 @@
 
 - **REF-6**: Drop `frozen=True` from the instrument hierarchy. Python's dataclass rules force a cascade: dropping `frozen=True` from `LoggingInstrument` requires dropping it from `BaseInstrument`, which requires dropping it from every other instrument subclass. 23 dataclass declarations across 12 files lose `frozen=True`. In `LoggingInstrument` and `OpenTelemetryInstrument`, the four `object.__setattr__(self, "_x", value)` workarounds for cached runtime state become plain `self._x = value` assignments. **Configs stay frozen** — only instruments lose `frozen=True`.
 
-- **LOW-4**: `FastAPIConfig.application` currently uses `default=None` + `# ty: ignore[invalid-assignment]` because the field is typed `fastapi.FastAPI` (non-Optional). Replace with a typed-sentinel pattern: `_UNSET_FASTAPI_APP: typing.Final = typing.cast("fastapi.FastAPI", object())`. The `__post_init__` checks `self.application is _UNSET_FASTAPI_APP` instead of `not self.application`. Drops the `# ty: ignore`. FastAPIConfig stays frozen (so the existing `object.__setattr__(self, "application", ...)` in `__post_init__` remains — only the default-and-sentinel-check changes).
+- **LOW-4**: `FastAPIConfig.application` currently uses `default=None` + `# ty: ignore[invalid-assignment]` because the field is typed `fastapi.FastAPI` (non-Optional). Replace with a proper sentinel-type pattern: introduce `UnsetType` + `UNSET` in `lite_bootstrap/types.py` (a sentinel class with a singleton instance), type the field as `fastapi.FastAPI | UnsetType`, default to `UNSET`, and replace the truthiness check in `__post_init__` with `isinstance(self.application, UnsetType)`. Add a `_narrow_app(config)` helper at module scope that asserts the type and returns the narrowed value; FastAPI framework instruments call `_narrow_app(self.bootstrap_config)` instead of `self.bootstrap_config.application` directly. Drops the `# ty: ignore`. FastAPIConfig stays frozen — `object.__setattr__(self, "application", ...)` in `__post_init__` remains because the freeze bypass is the only way to mutate a frozen field after construction; a code comment documents the rationale.
 
 **Architecture:** Largest mechanical refactor in the deferred-refactors sequence. The cascade is purely mechanical — every change is `frozen=True` → (delete). The setattr replacements and LOW-4 sentinel are the only meaningful diffs.
 
@@ -48,16 +48,20 @@ The sequencing spec's PR13 section said "drop `frozen=True` from `LoggingInstrum
 - `lite_bootstrap/instruments/swagger_instrument.py` — `SwaggerInstrument` loses `frozen=True`.
 
 **Bootstrapper modules (3 files):**
-- `lite_bootstrap/bootstrappers/fastapi_bootstrapper.py` — 5 framework instruments lose `frozen=True` + LOW-4 sentinel pattern on `FastAPIConfig.application`.
+- `lite_bootstrap/bootstrappers/fastapi_bootstrapper.py` — 5 framework instruments lose `frozen=True` + LOW-4 sentinel-type pattern on `FastAPIConfig.application` + `_narrow_app` helper + every FastAPI instrument bootstrap calls `_narrow_app(self.bootstrap_config)` for the app reference.
 - `lite_bootstrap/bootstrappers/litestar_bootstrapper.py` — 6 framework instruments lose `frozen=True`.
 - `lite_bootstrap/bootstrappers/faststream_bootstrapper.py` — 4 framework instruments lose `frozen=True`.
 
+**Shared types (1 file):**
+- `lite_bootstrap/types.py` — add `UnsetType` class + `UNSET: typing.Final[UnsetType]` singleton. Reusable sentinel for fields that distinguish "not passed" from "explicitly None".
+
 ---
 
 ## Locked decisions
 
 - **Cascade scope:** Drop `frozen=True` from `BaseInstrument` and ALL 22 instrument subclasses. Configs stay frozen. Confirmed by the user after the constraint surfaced.
-- **LOW-4 pattern:** Typed sentinel via `typing.cast`. Cleaner than the current `# ty: ignore` once the cast obscures the type lie.
+- **LOW-4 pattern:** Proper `UnsetType` sentinel class in `lite_bootstrap/types.py`, used via `isinstance(value, UnsetType)`. Honest to the type checker (no `typing.cast` lie). Adds a `_narrow_app` helper that wraps the assert/return narrowing for callers. Revised from the original plan's `typing.cast("fastapi.FastAPI", object())` pattern — the spec was updated retroactively to match what was built.
+- **`FastAPIConfig` stays frozen:** Confirmed by user. The `object.__setattr__(self, "application", ...)` in `__post_init__` remains; a one-line code comment documents the rationale (frozen for user-facing immutability; bypass needed because `application` is constructed using other config fields).
 - **No new tests.** The full existing test suite verifies behavior preservation; pure-refactor changes should not affect runtime semantics other than enabling future direct mutation (which we don't exercise).
 
 ---

docs/superpowers/specs/2026-06-01-deferred-refactors-sequencing.md

@@ -211,60 +211,42 @@ suite passes against the new layout.
 
 **Scope:**
 
-- REF-6: Drop `frozen=True` from `LoggingInstrument` and `OpenTelemetryInstrument`
-  (the only two instruments that legitimately cache mutable runtime state via
-  `object.__setattr__`). Replace `object.__setattr__(self, "_x", value)` with plain
-  `self._x = value`. The exception-safety from PR3's `try/finally` shape stays
-  intact — the pattern simplifies from:
-
-  ```python
-  if self._tracer_provider is not None:
-      try:
-          self._tracer_provider.shutdown()
-      finally:
-          object.__setattr__(self, "_tracer_provider", None)
-  ```
-
-  to:
-
-  ```python
-  if self._tracer_provider is not None:
-      try:
-          self._tracer_provider.shutdown()
-      finally:
-          self._tracer_provider = None
-  ```
-
-  Configs stay frozen — only the two instruments with cached state lose `frozen`.
-  Apply to both `LoggingInstrument._logger_factory` resets and
-  `OpenTelemetryInstrument._tracer_provider` resets, including in their bootstrap
-  paths.
-
-- LOW-4: Address `FastAPIConfig.application` declared with
-  `default=None` + `# ty: ignore[invalid-assignment]` and patched in `__post_init__`.
-  The minimum-risk cleanup is to use a sentinel instead of `None`:
-
-  ```python
-  _UNSET: typing.Final = typing.cast("fastapi.FastAPI", object())
-  application: "fastapi.FastAPI" = _UNSET
-  ```
-
-  Drop the `# ty: ignore`. The `__post_init__` check changes from `if not
-  self.application:` to `if self.application is _UNSET:`. Litestar's
-  `default_factory=lambda: AppConfig()` pattern doesn't work here because the
-  factory needs `service_name` from the config to set `app.title` etc.
-
-**Files:** 3-4 — `logging_instrument.py`, `opentelemetry_instrument.py`,
-`fastapi_bootstrapper.py`. Possibly `tests/instruments/test_*_instrument.py` if any
-tests rely on instrument immutability (`dataclasses.replace`, `frozen` errors).
+- REF-6: Python's dataclass rules forbid surgical unfreezing (a non-frozen
+  dataclass can't inherit from a frozen one). To drop `frozen=True` from
+  `LoggingInstrument` and `OpenTelemetryInstrument` (the two instruments with
+  `object.__setattr__` workarounds), `BaseInstrument` and all 22 instrument
+  subclasses must also lose `frozen=True`. Configs all keep `frozen=True`.
+  After the cascade, 4 `object.__setattr__(self, "_x", value)` call sites in
+  the two instruments become plain `self._x = value`. The `try/finally`
+  exception safety from PR3 is preserved.
+
+- LOW-4: `FastAPIConfig.application` declared with `default=None` +
+  `# ty: ignore[invalid-assignment]`. Replaced with a proper sentinel-type
+  pattern: introduce `UnsetType` + `UNSET` singleton in
+  `lite_bootstrap/types.py`, type the field as `fastapi.FastAPI | UnsetType`,
+  default to `UNSET`, check via `isinstance(self.application, UnsetType)`. Add
+  a `_narrow_app(config)` helper that asserts the type and returns the
+  narrowed app; every FastAPI framework instrument calls it. Drops the
+  `# ty: ignore`. `FastAPIConfig` stays frozen — the existing
+  `object.__setattr__(self, "application", ...)` in `__post_init__` remains
+  (a code comment documents the rationale). Sibling configs (`LitestarConfig`,
+  `FastStreamConfig`) don't have this need because they use `default_factory`
+  for their app fields.
+
+  **Note:** the originally-planned `typing.cast("fastapi.FastAPI", object())`
+  sentinel was replaced during implementation with a proper `UnsetType` class.
+  This spec has been retroactively updated to match what was built.
+
+**Files:** 13 — 9 instrument modules (`base.py` + 8 base instruments), 3
+bootstrapper modules (`fastapi`, `litestar`, `faststream`), and `types.py`
+(new `UnsetType` + `UNSET` sentinel).
 
 **Test impact:** Existing tests should pass unchanged. Watch for any test that relied
-on `FrozenInstanceError` being raised on instrument mutation — none expected, but
-verify.
+on `FrozenInstanceError` being raised on instrument mutation — none expected.
 
-**Risk:** Medium. The `frozen` change is observable to user code that relied on
-`dataclasses.replace` for `LoggingInstrument` / `OpenTelemetryInstrument`. Unlikely
-in practice but worth noting.
+**Risk:** Medium. The cascade is mechanical but missing one entry breaks the build
+(TypeError at import). The `frozen` change is observable to user code that relied on
+`dataclasses.replace` for instruments — unlikely in practice but worth noting.
 
 ---
 

lite_bootstrap/bootstrappers/fastapi_bootstrapper.py

@@ -19,6 +19,7 @@
 from lite_bootstrap.instruments.pyroscope_instrument import PyroscopeConfig, PyroscopeInstrument
 from lite_bootstrap.instruments.sentry_instrument import SentryConfig, SentryInstrument
 from lite_bootstrap.instruments.swagger_instrument import SwaggerConfig, SwaggerInstrument
+from lite_bootstrap.types import UNSET, UnsetType
 
 
 if import_checker.is_fastapi_installed:
@@ -47,7 +48,7 @@ class FastAPIConfig(
     SentryConfig,
     SwaggerConfig,
 ):
-    application: "fastapi.FastAPI" = dataclasses.field(default=None)  # ty: ignore[invalid-assignment]
+    application: "fastapi.FastAPI | UnsetType" = UNSET
     application_kwargs: dict[str, typing.Any] = dataclasses.field(default_factory=dict)
     opentelemetry_excluded_urls: list[str] = dataclasses.field(default_factory=list)
     prometheus_instrumentator_params: dict[str, typing.Any] = dataclasses.field(default_factory=dict)
@@ -59,24 +60,32 @@ def __post_init__(self) -> None:
             msg = "fastapi is not installed"
             raise ConfigurationError(msg)
 
-        if not self.application:
-            object.__setattr__(
-                self, "application", fastapi.FastAPI(docs_url=self.swagger_path, **self.application_kwargs)
-            )
-        elif self.application_kwargs:
-            warnings.warn("application_kwargs must be used without application", stacklevel=2)
+        if isinstance(self.application, UnsetType):
+            application = fastapi.FastAPI(docs_url=self.swagger_path, **self.application_kwargs)
+            # FastAPIConfig stays frozen for user-facing immutability; __post_init__ needs
+            # to set application after construction, so we bypass the freeze here.
+            object.__setattr__(self, "application", application)
+        else:
+            application = self.application
+            if self.application_kwargs:
+                warnings.warn("application_kwargs must be used without application", stacklevel=2)
 
-        self.application.title = self.service_name
-        self.application.debug = self.service_debug
-        self.application.version = self.service_version
+        application.title = self.service_name
+        application.debug = self.service_debug
+        application.version = self.service_version
 
 
-@dataclasses.dataclass(kw_only=True, slots=True, frozen=True)
+def _narrow_app(config: "FastAPIConfig") -> "fastapi.FastAPI":
+    assert not isinstance(config.application, UnsetType)
+    return config.application
+
+
+@dataclasses.dataclass(kw_only=True, slots=True)
 class FastAPICorsInstrument(CorsInstrument):
     bootstrap_config: FastAPIConfig
 
     def bootstrap(self) -> None:
-        self.bootstrap_config.application.add_middleware(
+        _narrow_app(self.bootstrap_config).add_middleware(
             CORSMiddleware,
             allow_origins=self.bootstrap_config.cors_allowed_origins,
             allow_methods=self.bootstrap_config.cors_allowed_methods,
@@ -88,7 +97,7 @@ def bootstrap(self) -> None:
         )
 
 
-@dataclasses.dataclass(kw_only=True, slots=True, frozen=True)
+@dataclasses.dataclass(kw_only=True, slots=True)
 class FastAPIHealthChecksInstrument(HealthChecksInstrument):
     bootstrap_config: FastAPIConfig
 
@@ -105,27 +114,27 @@ async def health_check_handler() -> HealthCheckTypedDict:
         return fastapi_router
 
     def bootstrap(self) -> None:
-        self.bootstrap_config.application.include_router(self.build_fastapi_health_check_router())
+        _narrow_app(self.bootstrap_config).include_router(self.build_fastapi_health_check_router())
 
 
-@dataclasses.dataclass(kw_only=True, frozen=True)
+@dataclasses.dataclass(kw_only=True)
 class FastAPIOpenTelemetryInstrument(OpenTelemetryInstrument):
     bootstrap_config: FastAPIConfig
 
     def bootstrap(self) -> None:
         super().bootstrap()
         FastAPIInstrumentor.instrument_app(
-            app=self.bootstrap_config.application,
+            app=_narrow_app(self.bootstrap_config),
             tracer_provider=get_tracer_provider(),
             excluded_urls=",".join(self._build_excluded_urls()),
         )
 
     def teardown(self) -> None:
-        FastAPIInstrumentor.uninstrument_app(self.bootstrap_config.application)
+        FastAPIInstrumentor.uninstrument_app(_narrow_app(self.bootstrap_config))
         super().teardown()
 
 
-@dataclasses.dataclass(kw_only=True, frozen=True)
+@dataclasses.dataclass(kw_only=True)
 class FastAPIPrometheusInstrument(PrometheusInstrument):
     bootstrap_config: FastAPIConfig
     missing_dependency_message = "prometheus_fastapi_instrumentator is not installed"
@@ -135,32 +144,31 @@ def check_dependencies() -> bool:
         return import_checker.is_prometheus_fastapi_instrumentator_installed
 
     def bootstrap(self) -> None:
+        application = _narrow_app(self.bootstrap_config)
         Instrumentator(**self.bootstrap_config.prometheus_instrumentator_params).instrument(
-            self.bootstrap_config.application,
+            application,
             **self.bootstrap_config.prometheus_instrument_params,
         ).expose(
-            self.bootstrap_config.application,
+            application,
             endpoint=self.bootstrap_config.prometheus_metrics_path,
             include_in_schema=self.bootstrap_config.prometheus_metrics_include_in_schema,
             **self.bootstrap_config.prometheus_expose_params,
         )
 
 
-@dataclasses.dataclass(kw_only=True, frozen=True)
+@dataclasses.dataclass(kw_only=True)
 class FastAPISwaggerInstrument(SwaggerInstrument):
     bootstrap_config: FastAPIConfig
 
     def bootstrap(self) -> None:
-        if self.bootstrap_config.swagger_path != self.bootstrap_config.application.docs_url:
+        application = _narrow_app(self.bootstrap_config)
+        if self.bootstrap_config.swagger_path != application.docs_url:
             warnings.warn(
-                f"swagger_path differs from docs_url, "
-                f"{self.bootstrap_config.application.docs_url} will be used for docs path",
+                f"swagger_path differs from docs_url, {application.docs_url} will be used for docs path",
                 stacklevel=2,
             )
         if self.bootstrap_config.swagger_offline_docs:
-            enable_offline_docs(
-                self.bootstrap_config.application, static_path=self.bootstrap_config.swagger_static_path
-            )
+            enable_offline_docs(application, static_path=self.bootstrap_config.swagger_static_path)
 
 
 class FastAPIBootstrapper(BaseBootstrapper["fastapi.FastAPI"]):
@@ -189,8 +197,9 @@ async def lifespan_manager(self, _: "fastapi.FastAPI") -> typing.AsyncIterator[d
     def __init__(self, bootstrap_config: FastAPIConfig) -> None:
         super().__init__(bootstrap_config)
 
-        old_lifespan_manager = self.bootstrap_config.application.router.lifespan_context
-        self.bootstrap_config.application.router.lifespan_context = _merge_lifespan_context(
+        application = _narrow_app(self.bootstrap_config)
+        old_lifespan_manager = application.router.lifespan_context
+        application.router.lifespan_context = _merge_lifespan_context(
             old_lifespan_manager,
             self.lifespan_manager,
         )
@@ -199,4 +208,4 @@ def is_ready(self) -> bool:
         return import_checker.is_fastapi_installed
 
     def _prepare_application(self) -> "fastapi.FastAPI":
-        return self.bootstrap_config.application
+        return _narrow_app(self.bootstrap_config)

lite_bootstrap/bootstrappers/faststream_bootstrapper.py

@@ -70,7 +70,7 @@ class FastStreamConfig(
     faststream_log_level: int = logging.WARNING
 
 
-@dataclasses.dataclass(kw_only=True, slots=True, frozen=True)
+@dataclasses.dataclass(kw_only=True, slots=True)
 class FastStreamHealthChecksInstrument(HealthChecksInstrument):
     bootstrap_config: FastStreamConfig
 
@@ -104,7 +104,7 @@ async def _define_health_status(self) -> bool:
         return await self.bootstrap_config.application.broker.ping(timeout=5)
 
 
-@dataclasses.dataclass(kw_only=True, frozen=True)
+@dataclasses.dataclass(kw_only=True)
 class FastStreamLoggingInstrument(LoggingInstrument):
     bootstrap_config: FastStreamConfig
 
@@ -117,7 +117,7 @@ def bootstrap(self) -> None:
             broker.config.logger.params_storage = ManualLoggerStorage(logger)
 
 
-@dataclasses.dataclass(kw_only=True, frozen=True)
+@dataclasses.dataclass(kw_only=True)
 class FastStreamOpenTelemetryInstrument(OpenTelemetryInstrument):
     bootstrap_config: FastStreamConfig
     not_ready_message = OpenTelemetryInstrument.not_ready_message + " or opentelemetry_middleware_cls is empty"
@@ -136,7 +136,7 @@ def _make_collector_registry() -> "prometheus_client.CollectorRegistry":
     return prometheus_client.CollectorRegistry()
 
 
-@dataclasses.dataclass(kw_only=True, frozen=True)
+@dataclasses.dataclass(kw_only=True)
 class FastStreamPrometheusInstrument(PrometheusInstrument):
     bootstrap_config: FastStreamConfig
     collector_registry: "prometheus_client.CollectorRegistry" = dataclasses.field(

lite_bootstrap/bootstrappers/litestar_bootstrapper.py

@@ -116,7 +116,7 @@ class LitestarConfig(
     swagger_extra_params: dict[str, typing.Any] = dataclasses.field(default_factory=dict)
 
 
-@dataclasses.dataclass(kw_only=True, slots=True, frozen=True)
+@dataclasses.dataclass(kw_only=True, slots=True)
 class LitestarCorsInstrument(CorsInstrument):
     bootstrap_config: LitestarConfig
 
@@ -132,7 +132,7 @@ def bootstrap(self) -> None:
         )
 
 
-@dataclasses.dataclass(kw_only=True, slots=True, frozen=True)
+@dataclasses.dataclass(kw_only=True, slots=True)
 class LitestarHealthChecksInstrument(HealthChecksInstrument):
     bootstrap_config: LitestarConfig
 
@@ -152,7 +152,7 @@ def bootstrap(self) -> None:
         self.bootstrap_config.application_config.route_handlers.append(self.build_litestar_health_check_router())
 
 
-@dataclasses.dataclass(kw_only=True, frozen=True)
+@dataclasses.dataclass(kw_only=True)
 class LitestarLoggingInstrument(LoggingInstrument):
     bootstrap_config: LitestarConfig
 
@@ -175,7 +175,7 @@ def bootstrap(self) -> None:
         self._configure_foreign_loggers()
 
 
-@dataclasses.dataclass(kw_only=True, frozen=True)
+@dataclasses.dataclass(kw_only=True)
 class LitestarOpenTelemetryInstrument(OpenTelemetryInstrument):
     bootstrap_config: LitestarConfig
 
@@ -189,7 +189,7 @@ def bootstrap(self) -> None:
         )
 
 
-@dataclasses.dataclass(kw_only=True, frozen=True)
+@dataclasses.dataclass(kw_only=True)
 class LitestarPrometheusInstrument(PrometheusInstrument):
     bootstrap_config: LitestarConfig
     missing_dependency_message = "prometheus_client is not installed"
@@ -213,7 +213,7 @@ class LitestarPrometheusController(PrometheusController):
         self.bootstrap_config.application_config.middleware.append(litestar_prometheus_config.middleware)
 
 
-@dataclasses.dataclass(kw_only=True, frozen=True)
+@dataclasses.dataclass(kw_only=True)
 class LitestarSwaggerInstrument(SwaggerInstrument):
     bootstrap_config: LitestarConfig
     not_ready_message = "swagger_path is empty or not valid"