[core] cpp tests: Allow customizing code generation during tests (#14681)

Co-authored-by: J. Nick Koston <nick@home-assistant.io>
Co-authored-by: J. Nick Koston <nick@koston.org>

esphome/core/__init__.py

@@ -615,10 +615,6 @@ class EsphomeCore:
         self.address_cache: AddressCache | None = None
         # Cached config hash (computed lazily)
         self._config_hash: int | None = None
-        # True if compiling for C++ unit tests
-        self.cpp_testing = False
-        # Allowlist of components whose to_code should run during C++ testing
-        self.cpp_testing_codegen: set[str] = set()
 
     def reset(self):
         from esphome.pins import PIN_SCHEMA_REGISTRY
@@ -648,8 +644,6 @@ class EsphomeCore:
         self.current_component = None
         self.address_cache = None
         self._config_hash = None
-        self.cpp_testing = False
-        self.cpp_testing_codegen = set()
         PIN_SCHEMA_REGISTRY.reset()
 
     @contextmanager

esphome/loader.py

@@ -71,11 +71,6 @@ class ComponentManifest:
 
     @property
     def to_code(self) -> Callable[[Any], None] | None:
-        if CORE.cpp_testing:
-            # During C++ testing, only run to_code for allowlisted components
-            name = self.module.__package__.rsplit(".", 1)[-1]
-            if name not in CORE.cpp_testing_codegen:
-                return None
         return getattr(self.module, "to_code", None)
 
     @property
@@ -243,3 +238,13 @@ def get_platform(domain: str, platform: str) -> ComponentManifest | None:
 _COMPONENT_CACHE: dict[str, ComponentManifest] = {}
 CORE_COMPONENTS_PATH = (Path(__file__).parent / "components").resolve()
 _COMPONENT_CACHE["esphome"] = ComponentManifest(esphome.core.config)
+
+
+def _replace_component_manifest(domain: str, manifest: ComponentManifest) -> None:
+    """Replace the cached manifest for a component.
+
+    This is an intentionally-supported hook for the C++ test infrastructure
+    to install ``ComponentManifestOverride`` wrappers.  Normal application
+    code should never call this.
+    """
+    _COMPONENT_CACHE[domain] = manifest

script/build_helpers.py

@@ -2,21 +2,29 @@
 
 from __future__ import annotations
 
+from collections.abc import Callable
 import hashlib
+import importlib.util
 import os
 from pathlib import Path
 import subprocess
 import sys
 
-from helpers import get_all_dependencies
+from helpers import get_all_dependencies, root_path as _root_path
 import yaml
 
+# Ensure the repo root is on sys.path so that ``tests.testing_helpers`` and
+# override ``__init__.py`` modules can ``from tests.testing_helpers import ...``.
+if _root_path not in sys.path:
+    sys.path.insert(0, _root_path)
+
 from esphome.__main__ import command_compile, parse_args
 from esphome.config import validate_config
 from esphome.const import CONF_PLATFORM
 from esphome.core import CORE
-from esphome.loader import get_component
+from esphome.loader import get_component, get_platform
 from esphome.platformio_api import get_idedata
+from tests.testing_helpers import ComponentManifestOverride, set_testing_manifest
 
 # This must coincide with the version in /platformio.ini
 PLATFORMIO_GOOGLE_TEST_LIB = "google/googletest@^1.15.2"
@@ -32,19 +40,6 @@ ESPHOME_KEY = "esphome"
 HOST_KEY = "host"
 LOGGER_KEY = "logger"
 
-# Base config keys that are always present and must not be fully overridden
-# by component benchmark.yaml files. esphome: allows sub-key merging.
-BASE_CONFIG_KEYS = frozenset({ESPHOME_KEY, HOST_KEY, LOGGER_KEY})
-
-# Shared build flag — enables timezone code paths for testing/benchmarking.
-USE_TIME_TIMEZONE_FLAG = "-DUSE_TIME_TIMEZONE"
-
-# Components whose to_code should always run during C++ test/benchmark builds.
-# These are the minimal infrastructure components needed for host compilation.
-# Note: "core" is the esphome core config module (esphome/core/config.py),
-# which registers under package name "core" not "esphome".
-BASE_CODEGEN_COMPONENTS = {"core", "host", "logger"}
-
 # Exit codes
 EXIT_OK = 0
 EXIT_SKIPPED = 1
@@ -110,6 +105,8 @@ def get_platform_components(components: list[str], tests_dir: Path) -> list[str]
             if not domain_dir.is_dir():
                 continue
             domain = domain_dir.name
+            if domain.startswith("__"):
+                continue
             domain_module = get_component(domain)
             if domain_module is None or not domain_module.is_platform_component:
                 raise ValueError(
@@ -130,7 +127,8 @@ def load_component_yaml_configs(components: list[str], tests_dir: Path) -> dict:
 
     The ``esphome:`` key is special: its sub-keys are merged into the
     existing esphome config (e.g. to add ``areas:`` or ``devices:``).
-    Other base config keys (``host:``, ``logger:``) are not overridable.
+    Keys already present in the base config (e.g. ``host:``, ``logger:``)
+    are protected by ``setdefault`` in the caller.
 
     Args:
         components: List of component directory names
@@ -139,11 +137,6 @@ def load_component_yaml_configs(components: list[str], tests_dir: Path) -> dict:
     Returns:
         Merged dict of component configs to add to the base config
     """
-    # Note: components are processed in sorted order. For conflicting keys
-    # (e.g. two benchmark.yaml files both declaring sensor:), the first
-    # component alphabetically wins via setdefault(). This is fine for now
-    # with a single benchmark component (api) but would need a real merge
-    # strategy if multiple components declare overlapping configs.
     merged: dict = {}
     for component in components:
         yaml_path = tests_dir / component / BENCHMARK_YAML_FILENAME
@@ -153,9 +146,6 @@ def load_component_yaml_configs(components: list[str], tests_dir: Path) -> dict:
             component_config = yaml.safe_load(f)
         if component_config and isinstance(component_config, dict):
             for key, value in component_config.items():
-                if key in BASE_CONFIG_KEYS - {ESPHOME_KEY}:
-                    # host: and logger: are not overridable
-                    continue
                 if key == ESPHOME_KEY and isinstance(value, dict):
                     # Merge esphome sub-keys rather than replacing
                     esphome_extra = merged.setdefault(ESPHOME_KEY, {})
@@ -198,11 +188,78 @@ def create_host_config(
     }
 
 
+def _wrap_manifest(
+    comp_name: str,
+) -> ComponentManifestOverride | None:
+    """Wrap a component manifest in a ComponentManifestOverride with to_code suppressed.
+
+    If the manifest is already wrapped or not found, returns None.
+    Otherwise returns the newly created override after installing it.
+    """
+    if "." in comp_name:
+        domain, component = comp_name.split(".", maxsplit=1)
+        manifest = get_platform(domain, component)
+        cache_key = f"{component}.{domain}"
+    else:
+        manifest = get_component(comp_name)
+        cache_key = comp_name
+
+    if manifest is None or isinstance(manifest, ComponentManifestOverride):
+        return None
+
+    override = ComponentManifestOverride(manifest)
+    override.to_code = None  # suppress by default
+    set_testing_manifest(cache_key, override)
+    return override
+
+
+def load_test_manifest_overrides(
+    components: list[str],
+    tests_dir: Path,
+) -> None:
+    """Apply per-component manifest overrides from test ``__init__.py`` files.
+
+    For every component, wraps its manifest and suppresses ``to_code``.
+    If the component's test directory contains an ``__init__.py`` that
+    defines ``override_manifest(manifest)``, it is called to customise
+    the override (e.g. ``manifest.enable_codegen()``).
+    """
+    for comp_name in components:
+        override = _wrap_manifest(comp_name)
+        if override is None:
+            continue
+
+        if "." in comp_name:
+            domain, component = comp_name.split(".", maxsplit=1)
+            cache_key = f"{component}.{domain}"
+            test_init = tests_dir / component / domain / "__init__.py"
+        else:
+            cache_key = comp_name
+            test_init = tests_dir / comp_name / "__init__.py"
+
+        if not test_init.is_file():
+            continue
+        spec = importlib.util.spec_from_file_location(
+            f"_test_manifest_override.{cache_key}", test_init
+        )
+        if spec is None or spec.loader is None:
+            continue
+        mod = importlib.util.module_from_spec(spec)
+        spec.loader.exec_module(mod)
+        override_fn = getattr(mod, "override_manifest", None)
+        if override_fn is not None:
+            override_fn(override)
+
+
+# Type alias for manifest override loaders
+ManifestOverrideLoader = Callable[[list[str]], None]
+
+
 def compile_and_get_binary(
     config: dict,
     components: list[str],
-    codegen_components: set[str],
     tests_dir: Path,
+    manifest_override_loader: ManifestOverrideLoader,
     label: str = "build",
 ) -> tuple[int, str | None]:
     """Compile an ESPHome configuration and return the binary path.
@@ -210,8 +267,8 @@ def compile_and_get_binary(
     Args:
         config: ESPHome configuration dict (already created via create_host_config)
         components: List of components to include in the build
-        codegen_components: Set of component names whose to_code should run
         tests_dir: Base directory for test files (used as config_path base)
+        manifest_override_loader: Callback to apply manifest overrides for components
         label: Label for log messages (e.g. "unit tests", "benchmarks")
 
     Returns:
@@ -238,18 +295,21 @@ def compile_and_get_binary(
         else:
             config.setdefault(key, value)
 
+    # Apply manifest overrides before dependency resolution so that any
+    # dependency additions made by override_manifest() are picked up.
+    manifest_override_loader(components)
+
     # Obtain possible dependencies BEFORE validate_config, because
     # get_all_dependencies calls CORE.reset() which clears build_path.
-    # Always include 'time' because USE_TIME_TIMEZONE is defined as a build flag,
-    # which causes core/time.h to include components/time/posix_tz.h.
     components_with_dependencies: list[str] = sorted(
-        get_all_dependencies(set(components) | {"time"}, cpp_testing=True)
+        get_all_dependencies(set(components))
     )
 
+    # Apply overrides for any transitively discovered dependencies.
+    manifest_override_loader(components_with_dependencies)
+
     CORE.config_path = tests_dir / "dummy.yaml"
     CORE.dashboard = None
-    CORE.cpp_testing = True
-    CORE.cpp_testing_codegen = codegen_components
 
     # Validate config will expand the above with defaults:
     config = validate_config(config, {})
@@ -305,7 +365,7 @@ def compile_and_get_binary(
 def build_and_run(
     selected_components: list[str],
     tests_dir: Path,
-    codegen_components: set[str],
+    manifest_override_loader: ManifestOverrideLoader,
     config_prefix: str,
     friendly_name: str,
     libraries: str | list[str],
@@ -324,7 +384,7 @@ def build_and_run(
     Args:
         selected_components: Components to include (directory names in tests_dir)
         tests_dir: Directory containing test/benchmark files
-        codegen_components: Components whose to_code should run
+        manifest_override_loader: Callback to apply manifest overrides for components
         config_prefix: Prefix for the config name (e.g. "cpptests", "cppbench")
         friendly_name: Human-readable name for the config
         libraries: PlatformIO library specification(s)
@@ -382,7 +442,7 @@ def build_and_run(
     )
 
     exit_code, program_path = compile_and_get_binary(
-        config, components, codegen_components, tests_dir, label
+        config, components, tests_dir, manifest_override_loader, label
     )
 
     if exit_code != EXIT_OK or program_path is None:

script/cpp_benchmark.py

@@ -2,18 +2,18 @@
 """Build and run C++ benchmarks for ESPHome components using Google Benchmark."""
 
 import argparse
+from functools import partial
 import json
 import os
 from pathlib import Path
 import sys
 
-from helpers import root_path
+from build_helpers import (
-from test_helpers import (
-    BASE_CODEGEN_COMPONENTS,
     PLATFORMIO_GOOGLE_BENCHMARK_LIB,
-    USE_TIME_TIMEZONE_FLAG,
     build_and_run,
+    load_test_manifest_overrides,
 )
+from helpers import root_path
 
 # Path to /tests/benchmarks/components
 BENCHMARKS_DIR: Path = Path(root_path) / "tests" / "benchmarks" / "components"
@@ -21,11 +21,6 @@ BENCHMARKS_DIR: Path = Path(root_path) / "tests" / "benchmarks" / "components"
 # Path to /tests/benchmarks/core (always included, not a component)
 CORE_BENCHMARKS_DIR: Path = Path(root_path) / "tests" / "benchmarks" / "core"
 
-# Additional codegen components beyond the base set.
-# json is needed because its to_code adds the ArduinoJson library
-# (auto-loaded by api, but cpp_testing suppresses to_code unless listed).
-BENCHMARK_CODEGEN_COMPONENTS = BASE_CODEGEN_COMPONENTS | {"json"}
-
 PLATFORMIO_OPTIONS = {
     "build_unflags": [
         "-Os",  # remove default size-opt
@@ -33,7 +28,6 @@ PLATFORMIO_OPTIONS = {
     "build_flags": [
         "-O2",  # optimize for speed (CodSpeed recommends RelWithDebInfo)
         "-g",  # debug symbols for profiling
-        USE_TIME_TIMEZONE_FLAG,
         "-DUSE_BENCHMARK",  # disable WarnIfComponentBlockingGuard in finish()
     ],
     # Use deep+ LDF mode to ensure PlatformIO detects the benchmark
@@ -73,7 +67,9 @@ def run_benchmarks(selected_components: list[str], build_only: bool = False) ->
     return build_and_run(
         selected_components=selected_components,
         tests_dir=BENCHMARKS_DIR,
-        codegen_components=BENCHMARK_CODEGEN_COMPONENTS,
+        manifest_override_loader=partial(
+            load_test_manifest_overrides, tests_dir=BENCHMARKS_DIR
+        ),
         config_prefix="cppbench",
         friendly_name="CPP Benchmarks",
         libraries=benchmark_lib,

script/cpp_unit_test.py

@@ -1,15 +1,15 @@
 #!/usr/bin/env python3
 import argparse
+from functools import partial
 from pathlib import Path
 import sys
 
-from helpers import get_all_components, root_path
+from build_helpers import (
-from test_helpers import (
-    BASE_CODEGEN_COMPONENTS,
     PLATFORMIO_GOOGLE_TEST_LIB,
-    USE_TIME_TIMEZONE_FLAG,
     build_and_run,
+    load_test_manifest_overrides,
 )
+from helpers import get_all_components, root_path
 
 # Path to /tests/components
 COMPONENTS_TESTS_DIR: Path = Path(root_path) / "tests" / "components"
@@ -21,7 +21,6 @@ PLATFORMIO_OPTIONS = {
     ],
     "build_flags": [
         "-Og",  # optimize for debug
-        USE_TIME_TIMEZONE_FLAG,
         "-DESPHOME_DEBUG",  # enable debug assertions
         # Enable the address and undefined behavior sanitizers
         "-fsanitize=address",
@@ -39,7 +38,9 @@ def run_tests(selected_components: list[str]) -> int:
     return build_and_run(
         selected_components=selected_components,
         tests_dir=COMPONENTS_TESTS_DIR,
-        codegen_components=BASE_CODEGEN_COMPONENTS,
+        manifest_override_loader=partial(
+            load_test_manifest_overrides, tests_dir=COMPONENTS_TESTS_DIR
+        ),
         config_prefix="cpptests",
         friendly_name="CPP Unit Tests",
         libraries=PLATFORMIO_GOOGLE_TEST_LIB,

script/determine-jobs.py

@@ -388,7 +388,7 @@ BENCHMARKS_COMPONENTS_PATH = "tests/benchmarks/components"
 BENCHMARK_INFRASTRUCTURE_FILES = frozenset(
     {
         "script/cpp_benchmark.py",
-        "script/test_helpers.py",
+        "script/build_helpers.py",
         "script/setup_codspeed_lib.py",
     }
 )
@@ -402,7 +402,7 @@ def should_run_benchmarks(branch: str | None = None) -> bool:
     1. Core C++ files changed (esphome/core/*)
     2. A directly changed component has benchmark files (no dependency expansion)
     3. Benchmark infrastructure changed (tests/benchmarks/*, script/cpp_benchmark.py,
-       script/test_helpers.py, script/setup_codspeed_lib.py)
+       script/build_helpers.py, script/setup_codspeed_lib.py)
 
     Unlike unit tests, benchmarks do NOT expand to dependent components.
     Changing ``sensor`` does not trigger ``api`` benchmarks just because

script/helpers.py

@@ -627,14 +627,12 @@ def get_usable_cpu_count() -> int:
 
 
 def get_all_dependencies(
-    component_names: set[str], cpp_testing: bool = False
+    component_names: set[str],
 ) -> set[str]:
     """Get all dependencies for a set of components.
 
     Args:
         component_names: Set of component names to get dependencies for
-        cpp_testing: If True, set CORE.cpp_testing so AUTO_LOAD callables that
-                     conditionally include testing-only dependencies work correctly
 
     Returns:
         Set of all components including dependencies and auto-loaded components
@@ -652,7 +650,6 @@ def get_all_dependencies(
 
     # Reset CORE to ensure clean state
     CORE.reset()
-    CORE.cpp_testing = cpp_testing
 
     # Set up fake config path for component loading
     root = Path(__file__).parent.parent

tests/benchmarks/components/core/__init__.py

@@ -0,0 +1,7 @@
+from tests.testing_helpers import ComponentManifestOverride
+
+
+def override_manifest(manifest: ComponentManifestOverride) -> None:
+    # core (esphome/core/config.py) must run its to_code during builds
+    # because it bootstraps the fundamental application infrastructure.
+    manifest.enable_codegen()

tests/benchmarks/components/host/__init__.py

@@ -0,0 +1,7 @@
+from tests.testing_helpers import ComponentManifestOverride
+
+
+def override_manifest(manifest: ComponentManifestOverride) -> None:
+    # host must run its to_code during builds because it sets up
+    # the host platform target execution environment.
+    manifest.enable_codegen()

tests/benchmarks/components/json/__init__.py

@@ -0,0 +1,7 @@
+from tests.testing_helpers import ComponentManifestOverride
+
+
+def override_manifest(manifest: ComponentManifestOverride) -> None:
+    # json must run its to_code during benchmark builds because it
+    # adds the ArduinoJson library dependency needed by the API component.
+    manifest.enable_codegen()

tests/benchmarks/components/logger/__init__.py

@@ -0,0 +1,7 @@
+from tests.testing_helpers import ComponentManifestOverride
+
+
+def override_manifest(manifest: ComponentManifestOverride) -> None:
+    # logger must run its to_code during builds because it configures
+    # the logging subsystem used by ESP_LOG* macros.
+    manifest.enable_codegen()

tests/benchmarks/components/time/__init__.py

@@ -0,0 +1,9 @@
+import esphome.codegen as cg
+from tests.testing_helpers import ComponentManifestOverride
+
+
+def override_manifest(manifest: ComponentManifestOverride) -> None:
+    async def to_code(config):
+        cg.add_build_flag("-DUSE_TIME_TIMEZONE")
+
+    manifest.to_code = to_code

tests/components/README.md

@@ -14,11 +14,63 @@ include the relevant `.cpp` and `.h` test files there.
 
 ### Override component code generation for testing
 
-When generating code for testing, ESPHome won't invoke the component's `to_code` function, since most components do not
+During C++ test builds, `to_code` is suppressed for every component by default — most components do not
-need to generate configuration code for testing.
+need to generate configuration code for a unit test binary.
 
-If you do need to generate code to for example configure compilation flags or add libraries,
+#### Manifest overrides
-add the component name to the `CPP_TESTING_CODEGEN_COMPONENTS` allowlist in `script/cpp_unit_test.py`.
+
+If your component needs to customise code generation behavior for testing — for example to re-enable
+`to_code`, supply a lightweight stub, add a test-only dependency, or change any other manifest attribute —
+create an `__init__.py` in your component's test directory and define `override_manifest`:
+
+**Top-level component** (`tests/components/<component>/__init__.py`):
+
+```python
+from tests.testing_helpers import ComponentManifestOverride
+
+def override_manifest(manifest: ComponentManifestOverride) -> None:
+    # Re-enable the component's own to_code (needed when the component must
+    # emit C++ setup code that the test binary depends on at link time).
+    manifest.enable_codegen()
+```
+
+Or supply a lightweight stub instead of the real `to_code`:
+
+```python
+from tests.testing_helpers import ComponentManifestOverride
+
+def override_manifest(manifest: ComponentManifestOverride) -> None:
+    async def to_code_testing(config):
+        # Only emit what the C++ tests actually need
+        pass
+
+    manifest.to_code = to_code_testing
+    manifest.dependencies = manifest.dependencies + ["some_test_only_dep"]
+```
+
+**Platform component** (`tests/components/<component>/<domain>/__init__.py`,
+e.g. `tests/components/my_sensor/sensor/__init__.py`):
+
+```python
+from tests.testing_helpers import ComponentManifestOverride
+
+def override_manifest(manifest: ComponentManifestOverride) -> None:
+    manifest.enable_codegen()
+```
+
+`override_manifest` receives a `ComponentManifestOverride` that wraps the real manifest.
+Attribute assignments store an override; reads fall back to the real manifest when no
+override is present.
+
+Key methods:
+
+| Method | Effect |
+|---|---|
+| `manifest.enable_codegen()` | Remove the `to_code` suppression, re-enabling code generation |
+| `manifest.restore()` | Clear **all** overrides, reverting every attribute to the original |
+
+The function is called after `to_code` has already been set to `None`, so calling
+`enable_codegen()` is a deliberate opt-in.
 
 ## Running component unit tests
 

tests/components/core/__init__.py

@@ -0,0 +1,8 @@
+from tests.testing_helpers import ComponentManifestOverride
+
+
+def override_manifest(manifest: ComponentManifestOverride) -> None:
+    # core (esphome/core/config.py) must run its to_code during C++ test builds
+    # because it bootstraps the fundamental application infrastructure that all
+    # components depend on (component registration, event loop, etc.).
+    manifest.enable_codegen()

tests/components/host/__init__.py

@@ -0,0 +1,7 @@
+from tests.testing_helpers import ComponentManifestOverride
+
+
+def override_manifest(manifest: ComponentManifestOverride) -> None:
+    # host must run its to_code during C++ test builds because it sets up the
+    # host platform target, which is the execution environment for all unit tests.
+    manifest.enable_codegen()

tests/components/logger/__init__.py

@@ -0,0 +1,7 @@
+from tests.testing_helpers import ComponentManifestOverride
+
+
+def override_manifest(manifest: ComponentManifestOverride) -> None:
+    # logger must run its to_code during C++ test builds because it configures
+    # the logging subsystem used by ESP_LOG* macros throughout component code.
+    manifest.enable_codegen()

tests/components/time/__init__.py

@@ -0,0 +1,9 @@
+import esphome.codegen as cg
+from tests.testing_helpers import ComponentManifestOverride
+
+
+def override_manifest(manifest: ComponentManifestOverride) -> None:
+    async def to_code(config):
+        cg.add_build_flag("-DUSE_TIME_TIMEZONE")
+
+    manifest.to_code = to_code

tests/script/test_determine_jobs.py

@@ -1846,7 +1846,7 @@ def test_should_run_benchmarks_benchmark_infra_change() -> None:
     """Test benchmarks trigger on benchmark infrastructure changes."""
     for infra_file in [
         "script/cpp_benchmark.py",
-        "script/test_helpers.py",
+        "script/build_helpers.py",
         "script/setup_codspeed_lib.py",
     ]:
         with patch.object(determine_jobs, "changed_files", return_value=[infra_file]):

tests/script/test_helpers.py

@@ -1073,28 +1073,6 @@ def test_get_all_dependencies_platform_component_with_dependencies() -> None:
         assert result == {"sensor.bthome", "sensor"}
 
 
-def test_get_all_dependencies_cpp_testing_flag() -> None:
-    """cpp_testing=True propagates to CORE.cpp_testing during resolution."""
-    from esphome.core import CORE
-
-    with (
-        patch("esphome.loader.get_component") as mock_get_component,
-        patch("esphome.loader.get_platform"),
-    ):
-        observed: list[bool] = []
-
-        def capturing_get_component(name: str):
-            observed.append(CORE.cpp_testing)
-
-        mock_get_component.side_effect = capturing_get_component
-
-        helpers.get_all_dependencies({"some_comp"}, cpp_testing=True)
-
-    assert observed and all(observed), (
-        "CORE.cpp_testing should be True during resolution"
-    )
-
-
 def test_get_components_from_integration_fixtures() -> None:
     """Test extraction of components from fixture YAML files."""
     yaml_content = {

tests/script/test_test_helpers.py

@@ -0,0 +1,260 @@
+"""Unit tests for script/build_helpers.py manifest override and build helpers."""
+
+import os
+from pathlib import Path
+import sys
+import textwrap
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+# Add the script directory to Python path so we can import build_helpers
+sys.path.insert(
+    0, os.path.abspath(os.path.join(os.path.dirname(__file__), "..", "..", "script"))
+)
+
+import build_helpers  # noqa: E402
+
+from esphome.loader import ComponentManifest  # noqa: E402
+from tests.testing_helpers import ComponentManifestOverride  # noqa: E402
+
+# ---------------------------------------------------------------------------
+# Helpers
+# ---------------------------------------------------------------------------
+
+
+def _make_component_manifest(*, to_code=None, dependencies=None) -> ComponentManifest:
+    mod = MagicMock()
+    mod.to_code = to_code
+    mod.DEPENDENCIES = dependencies or []
+    return ComponentManifest(mod)
+
+
+# ---------------------------------------------------------------------------
+# filter_components_with_files
+# ---------------------------------------------------------------------------
+
+
+def test_filter_keeps_components_with_cpp_files(tmp_path: Path) -> None:
+    comp_dir = tmp_path / "mycomp"
+    comp_dir.mkdir()
+    (comp_dir / "mycomp_test.cpp").write_text("")
+
+    result = build_helpers.filter_components_with_files(["mycomp"], tmp_path)
+
+    assert result == ["mycomp"]
+
+
+def test_filter_keeps_components_with_h_files(tmp_path: Path) -> None:
+    comp_dir = tmp_path / "mycomp"
+    comp_dir.mkdir()
+    (comp_dir / "helpers.h").write_text("")
+
+    result = build_helpers.filter_components_with_files(["mycomp"], tmp_path)
+
+    assert result == ["mycomp"]
+
+
+def test_filter_drops_components_without_test_dir(tmp_path: Path) -> None:
+    result = build_helpers.filter_components_with_files(["nodir"], tmp_path)
+
+    assert result == []
+
+
+def test_filter_drops_components_with_no_cpp_or_h(tmp_path: Path) -> None:
+    comp_dir = tmp_path / "mycomp"
+    comp_dir.mkdir()
+    (comp_dir / "README.md").write_text("")
+
+    result = build_helpers.filter_components_with_files(["mycomp"], tmp_path)
+
+    assert result == []
+
+
+# ---------------------------------------------------------------------------
+# get_platform_components
+# ---------------------------------------------------------------------------
+
+
+def test_get_platform_components_discovers_subdirectory(tmp_path: Path) -> None:
+    (tmp_path / "bthome" / "sensor").mkdir(parents=True)
+
+    sensor_mod = MagicMock()
+    sensor_mod.IS_PLATFORM_COMPONENT = True
+
+    with patch(
+        "build_helpers.get_component", return_value=ComponentManifest(sensor_mod)
+    ):
+        result = build_helpers.get_platform_components(["bthome"], tmp_path)
+
+    assert result == ["sensor.bthome"]
+
+
+def test_get_platform_components_skips_pycache(tmp_path: Path) -> None:
+    (tmp_path / "bthome" / "__pycache__").mkdir(parents=True)
+
+    result = build_helpers.get_platform_components(["bthome"], tmp_path)
+
+    assert result == []
+
+
+def test_get_platform_components_raises_for_invalid_domain(tmp_path: Path) -> None:
+    (tmp_path / "bthome" / "notadomain").mkdir(parents=True)
+
+    with (
+        patch("build_helpers.get_component", return_value=None),
+        pytest.raises(ValueError, match="notadomain"),
+    ):
+        build_helpers.get_platform_components(["bthome"], tmp_path)
+
+
+# ---------------------------------------------------------------------------
+# load_test_manifest_overrides
+# ---------------------------------------------------------------------------
+
+
+def test_load_suppresses_to_code(tmp_path: Path) -> None:
+    """to_code is always set to None before the override is called."""
+
+    async def real_to_code(config):
+        pass
+
+    inner = _make_component_manifest(to_code=real_to_code)
+
+    with (
+        patch("build_helpers.get_component", return_value=inner),
+        patch("build_helpers.set_testing_manifest") as mock_set,
+    ):
+        build_helpers.load_test_manifest_overrides(["mycomp"], tmp_path)
+        installed: ComponentManifestOverride = mock_set.call_args[0][1]
+
+    assert installed.to_code is None
+
+
+def test_load_calls_override_fn(tmp_path: Path) -> None:
+    """override_manifest() in test_init is called with the ComponentManifestOverride."""
+    comp_dir = tmp_path / "mycomp"
+    comp_dir.mkdir()
+    init_py = comp_dir / "__init__.py"
+    init_py.write_text(
+        textwrap.dedent("""\
+        def override_manifest(manifest):
+            manifest.dependencies = ["injected"]
+    """)
+    )
+
+    inner = _make_component_manifest()
+    override = ComponentManifestOverride(inner)
+    override.to_code = None
+
+    with (
+        patch("build_helpers.get_component", return_value=inner),
+        patch("build_helpers.set_testing_manifest") as mock_set,
+    ):
+        build_helpers.load_test_manifest_overrides(["mycomp"], tmp_path)
+        installed: ComponentManifestOverride = mock_set.call_args[0][1]
+
+    assert installed.dependencies == ["injected"]
+
+
+def test_load_enable_codegen_in_override(tmp_path: Path) -> None:
+    """An override_manifest that calls enable_codegen() restores to_code."""
+
+    async def real_to_code(config):
+        pass
+
+    comp_dir = tmp_path / "mycomp"
+    comp_dir.mkdir()
+    init_py = comp_dir / "__init__.py"
+    init_py.write_text(
+        textwrap.dedent("""\
+        def override_manifest(manifest):
+            manifest.enable_codegen()
+    """)
+    )
+
+    inner = _make_component_manifest(to_code=real_to_code)
+
+    with (
+        patch("build_helpers.get_component", return_value=inner),
+        patch("build_helpers.set_testing_manifest") as mock_set,
+    ):
+        build_helpers.load_test_manifest_overrides(["mycomp"], tmp_path)
+        installed: ComponentManifestOverride = mock_set.call_args[0][1]
+
+    assert installed.to_code is real_to_code
+
+
+def test_load_no_override_file(tmp_path: Path) -> None:
+    """No override file: manifest is wrapped and to_code suppressed, nothing else."""
+    inner = _make_component_manifest()
+
+    with (
+        patch("build_helpers.get_component", return_value=inner),
+        patch("build_helpers.set_testing_manifest") as mock_set,
+    ):
+        build_helpers.load_test_manifest_overrides(["mycomp"], tmp_path)
+
+    mock_set.assert_called_once()
+    key, installed = mock_set.call_args[0]
+    assert key == "mycomp"
+    assert isinstance(installed, ComponentManifestOverride)
+
+
+def test_load_skips_already_wrapped(tmp_path: Path) -> None:
+    """Components already wrapped as ComponentManifestOverride are not double-wrapped."""
+    inner = _make_component_manifest()
+    already_wrapped = ComponentManifestOverride(inner)
+
+    with (
+        patch("build_helpers.get_component", return_value=already_wrapped),
+        patch("build_helpers.set_testing_manifest") as mock_set,
+    ):
+        build_helpers.load_test_manifest_overrides(["mycomp"], tmp_path)
+
+    mock_set.assert_not_called()
+
+
+def test_load_skips_platform_component_already_wrapped(tmp_path: Path) -> None:
+    inner = _make_component_manifest()
+    already_wrapped = ComponentManifestOverride(inner)
+
+    with (
+        patch("build_helpers.get_platform", return_value=already_wrapped),
+        patch("build_helpers.set_testing_manifest") as mock_set,
+    ):
+        build_helpers.load_test_manifest_overrides(["sensor.bthome"], tmp_path)
+
+    mock_set.assert_not_called()
+
+
+def test_load_wraps_top_level_component(tmp_path: Path) -> None:
+    inner = _make_component_manifest()
+
+    with (
+        patch("build_helpers.get_component", return_value=inner),
+        patch("build_helpers.set_testing_manifest") as mock_set,
+    ):
+        build_helpers.load_test_manifest_overrides(["mycomp"], tmp_path)
+
+    mock_set.assert_called_once()
+    key, installed = mock_set.call_args[0]
+    assert key == "mycomp"
+    assert isinstance(installed, ComponentManifestOverride)
+    assert installed.to_code is None
+
+
+def test_load_wraps_platform_component(tmp_path: Path) -> None:
+    inner = _make_component_manifest()
+
+    with (
+        patch("build_helpers.get_platform", return_value=inner),
+        patch("build_helpers.set_testing_manifest") as mock_set,
+    ):
+        build_helpers.load_test_manifest_overrides(["sensor.bthome"], tmp_path)
+
+    mock_set.assert_called_once()
+    key, installed = mock_set.call_args[0]
+    assert key == "bthome.sensor"
+    assert isinstance(installed, ComponentManifestOverride)
+    assert installed.to_code is None

tests/testing_helpers.py

@@ -0,0 +1,63 @@
+from typing import Any
+
+from esphome.loader import ComponentManifest, _replace_component_manifest
+
+
+class ComponentManifestOverride:
+    """Mutable wrapper around ComponentManifest for test-specific attribute overrides.
+
+    When ``tests/components/<name>/__init__.py`` defines::
+
+        def override_manifest(manifest: ComponentManifestOverride) -> None:
+            ...
+
+    the function receives an instance of this class wrapping the real component
+    manifest.  Any attribute assignment stores an override; reads fall back to
+    the underlying ``ComponentManifest`` when no override has been set.
+
+    Example::
+
+        def override_manifest(manifest: ComponentManifestOverride) -> None:
+            async def to_code_testing(config):
+                pass  # lightweight no-op stub for C++ unit tests
+
+            manifest.to_code = to_code_testing
+            manifest.dependencies = manifest.dependencies + ["extra_dep_for_tests"]
+    """
+
+    def __init__(self, wrapped: "ComponentManifest") -> None:
+        object.__setattr__(self, "_wrapped", wrapped)
+        object.__setattr__(self, "_overrides", {})
+
+    def __getattr__(self, name: str) -> Any:
+        overrides: dict[str, Any] = object.__getattribute__(self, "_overrides")
+        if name in overrides:
+            return overrides[name]
+        wrapped: ComponentManifest = object.__getattribute__(self, "_wrapped")
+        return getattr(wrapped, name)
+
+    def __setattr__(self, name: str, value: Any) -> None:
+        overrides: dict[str, Any] = object.__getattribute__(self, "_overrides")
+        overrides[name] = value
+
+    def enable_codegen(self) -> None:
+        """Remove the to_code suppression, re-enabling code generation for this component.
+
+        Call this from ``override_manifest`` when the component needs its real (or a
+        custom stub) ``to_code`` to run during C++ unit test builds.
+        """
+        overrides: dict[str, Any] = object.__getattribute__(self, "_overrides")
+        overrides.pop("to_code", None)
+
+    def restore(self) -> None:
+        """Clear all overrides, reverting to the wrapped manifest's values."""
+        object.__getattribute__(self, "_overrides").clear()
+
+
+def set_testing_manifest(domain: str, manifest: ComponentManifestOverride) -> None:
+    """Install a testing manifest override into the component cache.
+
+    Called from the C++ unit test infrastructure when a component's test
+    directory provides an ``override_manifest`` function.
+    """
+    _replace_component_manifest(domain, manifest)

tests/unit_tests/test_loader.py

@@ -2,7 +2,104 @@
 
 from unittest.mock import MagicMock, patch
 
-from esphome.loader import ComponentManifest
+from esphome.loader import ComponentManifest, _replace_component_manifest, get_component
+from tests.testing_helpers import ComponentManifestOverride
+
+# ---------------------------------------------------------------------------
+# ComponentManifestOverride
+# ---------------------------------------------------------------------------
+
+
+def _make_manifest(*, to_code=None, dependencies=None) -> ComponentManifest:
+    """Return a ComponentManifest backed by a minimal mock module."""
+    mod = MagicMock()
+    mod.to_code = to_code
+    mod.DEPENDENCIES = dependencies or []
+    return ComponentManifest(mod)
+
+
+def test_testing_manifest_delegates_to_wrapped() -> None:
+    """Unoverridden attributes fall through to the wrapped manifest."""
+    inner = _make_manifest(dependencies=["wifi"])
+    tm = ComponentManifestOverride(inner)
+    assert tm.dependencies == ["wifi"]
+
+
+def test_testing_manifest_override_shadows_wrapped() -> None:
+    """An assigned attribute shadows the wrapped value."""
+    inner = _make_manifest(dependencies=["wifi"])
+    tm = ComponentManifestOverride(inner)
+    tm.dependencies = ["ble"]
+    assert tm.dependencies == ["ble"]
+    # Wrapped value unchanged
+    assert inner.dependencies == ["wifi"]
+
+
+def test_testing_manifest_to_code_suppression() -> None:
+    """Setting to_code=None suppresses code generation."""
+
+    async def real_to_code(config):
+        pass
+
+    inner = _make_manifest(to_code=real_to_code)
+    tm = ComponentManifestOverride(inner)
+    tm.to_code = None
+    assert tm.to_code is None
+
+
+def test_testing_manifest_enable_codegen_removes_suppression() -> None:
+    """enable_codegen() removes the to_code override, restoring the original."""
+
+    async def real_to_code(config):
+        pass
+
+    inner = _make_manifest(to_code=real_to_code)
+    tm = ComponentManifestOverride(inner)
+    tm.to_code = None
+    assert tm.to_code is None
+
+    tm.enable_codegen()
+    assert tm.to_code is real_to_code
+
+
+def test_testing_manifest_enable_codegen_preserves_other_overrides() -> None:
+    """enable_codegen() only removes to_code; other overrides survive."""
+    inner = _make_manifest(dependencies=["wifi"])
+    tm = ComponentManifestOverride(inner)
+    tm.to_code = None
+    tm.dependencies = ["ble"]
+
+    tm.enable_codegen()
+
+    assert tm.to_code is inner.to_code
+    assert tm.dependencies == ["ble"]
+
+
+def test_testing_manifest_restore_clears_all_overrides() -> None:
+    """restore() removes every override, reverting all attributes to wrapped values."""
+
+    async def real_to_code(config):
+        pass
+
+    inner = _make_manifest(to_code=real_to_code, dependencies=["wifi"])
+    tm = ComponentManifestOverride(inner)
+    tm.to_code = None
+    tm.dependencies = ["ble"]
+
+    tm.restore()
+
+    assert tm.to_code is real_to_code
+    assert tm.dependencies == ["wifi"]
+
+
+def test_replace_component_manifest_installs_override() -> None:
+    """_replace_component_manifest replaces the cached manifest for a domain."""
+    inner = _make_manifest()
+    override = ComponentManifestOverride(inner)
+
+    _replace_component_manifest("_test_dummy_domain", override)
+
+    assert get_component("_test_dummy_domain") is override
 
 
 def test_component_manifest_resources_with_filter_source_files() -> None: