[substitutions] Improve error messages with include stack trace (#15874)

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

esphome/components/packages/__init__.py

@@ -42,6 +42,11 @@ DOMAIN = CONF_PACKAGES
 # Guard against infinite include chains (e.g. A includes B includes A).
 MAX_INCLUDE_DEPTH = 20
 
+PackageCallback = Callable[
+    [dict | str | yaml_util.IncludeFile, ContextVars | None, yaml_util.DocumentPath],
+    dict,
+]
+
 
 def is_remote_package(package_config: dict) -> bool:
     """Returns True if the package_config is a remote package definition."""
@@ -281,8 +286,9 @@ def _process_remote_package(config: dict, skip_update: bool = False) -> dict:
 
 def _walk_package_dict(
     packages: dict,
-    callback: Callable[[dict, ContextVars | None], dict],
+    callback: PackageCallback,
     context: ContextVars | None,
+    path: yaml_util.DocumentPath,
 ) -> cv.Invalid | None:
     """Iterate a packages dict in reverse priority order, invoking callback on each entry.
 
@@ -291,7 +297,9 @@ def _walk_package_dict(
     for package_name, package_config in reversed(packages.items()):
         with cv.prepend_path(package_name):
             try:
-                packages[package_name] = callback(package_config, context)
+                packages[package_name] = callback(
+                    package_config, context, path + [package_name]
+                )
             except cv.Invalid as err:
                 return err
     return None
@@ -299,20 +307,22 @@ def _walk_package_dict(
 
 def _walk_package_list(
     packages: list,
-    callback: Callable[[dict, ContextVars | None], dict],
+    callback: PackageCallback,
     context: ContextVars | None,
+    path: yaml_util.DocumentPath,
 ) -> None:
     """Iterate a packages list in reverse priority order, invoking callback on each entry."""
     for idx in reversed(range(len(packages))):
         with cv.prepend_path(idx):
-            packages[idx] = callback(packages[idx], context)
+            packages[idx] = callback(packages[idx], context, path + [idx])
 
 
 def _walk_packages(
     config: dict,
-    callback: Callable[[dict, ContextVars | None], dict],
+    callback: PackageCallback,
     context: ContextVars | None = None,
     validate_deprecated: bool = True,
+    path: yaml_util.DocumentPath | None = None,
 ) -> dict:
     """Walks the packages structure in priority order, invoking ``callback`` on each package definition found.
 
@@ -323,19 +333,24 @@ def _walk_packages(
     if CONF_PACKAGES not in config:
         return config
     packages = config[CONF_PACKAGES]
+    packages_path = (path or []) + [CONF_PACKAGES]
 
     with cv.prepend_path(CONF_PACKAGES):
         if isinstance(packages, yaml_util.IncludeFile):
             # If the packages key is an IncludeFile, resolve it first before processing.
-            packages, _ = resolve_include(packages, [], context, strict_undefined=False)
+            packages = resolve_include(
+                packages, packages_path, context, strict_undefined=False
+            )
         if not isinstance(packages, (dict, list)):
             raise cv.Invalid(
                 f"Packages must be a key to value mapping or list, got {type(packages)} instead"
             )
 
         if not isinstance(packages, dict):
-            _walk_package_list(packages, callback, context)
+            _walk_package_list(packages, callback, context, packages_path)
-        elif (result := _walk_package_dict(packages, callback, context)) is not None:
+        elif (
+            result := _walk_package_dict(packages, callback, context, packages_path)
+        ) is not None:
             if not validate_deprecated or any(
                 is_package_definition(v) for v in packages.values()
             ):
@@ -344,14 +359,18 @@ def _walk_packages(
             # This block can be removed once the single-package
             # deprecation period (2026.7.0) is over.
             config[CONF_PACKAGES] = [packages]
-            return _walk_packages(deprecate_single_package(config), callback, context)
+            return _walk_packages(
+                deprecate_single_package(config), callback, context, path=path
+            )
 
     config[CONF_PACKAGES] = packages
     return config
 
 
 def _substitute_package_definition(
-    package_config: dict | str, context_vars: ContextVars | None
+    package_config: dict | str,
+    context_vars: ContextVars | None,
+    path: yaml_util.DocumentPath | None = None,
 ) -> dict | str:
     """Substitute variables in a package definition string or remote package dict.
 
@@ -369,12 +388,12 @@ def _substitute_package_definition(
         errors: ErrList = []
         package_config = substitute(
             item=package_config,
-            path=[],
+            path=path or [],
             parent_context=context_vars or ContextVars(),
             strict_undefined=False,
             errors=errors,
         )
-        raise_first_undefined(errors, package_config, "package definition")
+        raise_first_undefined(errors, "package definition")
     return package_config
 
 
@@ -432,6 +451,7 @@ class _PackageProcessor:
         self,
         package_config: dict | str | yaml_util.IncludeFile,
         context_vars: ContextVars | None,
+        path: yaml_util.DocumentPath,
     ) -> dict:
         """Resolve a package definition to a concrete ``dict`` and fetch remote packages.
 
@@ -454,15 +474,15 @@ class _PackageProcessor:
         """
         for _ in range(MAX_INCLUDE_DEPTH):
             if isinstance(package_config, yaml_util.IncludeFile):
-                package_config, _ = resolve_include(
+                package_config = resolve_include(
                     package_config,
-                    [],
+                    path,
                     context_vars or ContextVars(),
                     strict_undefined=False,
                 )
 
             package_config = _substitute_package_definition(
-                package_config, context_vars
+                package_config, context_vars, path
             )
             package_config = PACKAGE_SCHEMA(package_config)
             if isinstance(package_config, dict):
@@ -483,13 +503,16 @@ class _PackageProcessor:
             _update_substitutions_context(self.parent_context, subs)
 
     def process_package(
-        self, package_config: dict | str, context_vars: ContextVars | None
+        self,
+        package_config: dict | str,
+        context_vars: ContextVars | None,
+        path: yaml_util.DocumentPath,
     ) -> dict:
         """Resolve a single package and recurse into any nested packages."""
         from_remote = isinstance(package_config, dict) and is_remote_package(
             package_config
         )
-        package_config = self.resolve_package(package_config, context_vars)
+        package_config = self.resolve_package(package_config, context_vars, path)
         self.collect_substitutions(package_config)
 
         if CONF_PACKAGES not in package_config:
@@ -509,6 +532,7 @@ class _PackageProcessor:
             self.process_package,
             context_vars,
             validate_deprecated=not from_remote,
+            path=path,
         )
 
 
@@ -565,11 +589,13 @@ def merge_packages(config: dict) -> dict:
     merge_list: list[dict] = []
 
     def process_package_callback(
-        package_config: dict, context: ContextVars | None
+        package_config: dict,
+        context: ContextVars | None,
+        path: yaml_util.DocumentPath | None = None,
     ) -> dict:
         """This will be called for each package found in the config."""
         merge_list.append(package_config)
-        return _walk_packages(package_config, process_package_callback)
+        return _walk_packages(package_config, process_package_callback, path=path)
 
     _walk_packages(config, process_package_callback, validate_deprecated=False)
     # Merge all packages into the main config:

esphome/components/substitutions/__init__.py

@@ -11,9 +11,11 @@ from esphome.types import ConfigType
 from esphome.util import OrderedDict
 from esphome.yaml_util import (
     ConfigContext,
+    DocumentPath,
     ESPHomeDataBase,
     ESPLiteralValue,
     IncludeFile,
+    format_path,
     make_data_base,
 )
 
@@ -23,8 +25,8 @@ CODEOWNERS = ["@esphome/core"]
 _LOGGER = logging.getLogger(__name__)
 
 ContextVars = ChainMap[str, Any]
-SubstitutionPath = list[int | str]
+ErrList = list[tuple[UndefinedError, DocumentPath, Any]]
-ErrList = list[tuple[UndefinedError, SubstitutionPath, Any]]
+
 # Module-level instance is safe: context_vars is passed per-call, and context_trace
 # is stack-saved/restored within expand(). Not thread-safe — only use from one thread.
 jinja = Jinja()
@@ -32,16 +34,13 @@ jinja = Jinja()
 
 def raise_first_undefined(
     errors: ErrList,
-    source: Any,
     context_label: str,
 ) -> None:
     """If *errors* is non-empty, raise ``cv.Invalid`` for the first undefined variable.
 
-    The raised error names the missing variable, the path walked into *source*
+    The raised error names the missing variable and its location in the include
-    (for nested dicts, e.g. ``url`` or ``ref``), and the YAML source location
+    stack. Only the first error is surfaced; the user will re-run after fixing it
-    when *source* carries one. Only the first error is surfaced; the user will
+    and any remaining undefined variables will be reported then.
-    re-run after fixing it and any remaining undefined variables will be
-    reported then.
 
     ``context_label`` is the noun describing where the undefined variable
     appeared (e.g. ``"package definition"``).
@@ -57,26 +56,8 @@ def raise_first_undefined(
             for e, p_path, _ in errors[1:]
         )
         _LOGGER.debug("Additional undefined variables in %s: %s", context_label, extras)
-    # Prefer the location of the offending scalar (e.g. the `url:` value) over
-    # the enclosing package-definition dict so the message points at the exact
-    # line/column that carries the undefined variable.
-    location_node = (
-        err_value
-        if isinstance(err_value, ESPHomeDataBase) and err_value.esp_range is not None
-        else source
-    )
-    location = ""
-    if (
-        isinstance(location_node, ESPHomeDataBase)
-        and location_node.esp_range is not None
-    ):
-        mark = location_node.esp_range.start_mark
-        # DocumentLocation.line/column are 0-based (from the YAML Mark). Render
-        # as 1-based to match config.line_info() and editor line numbering.
-        location = f" (in {mark.document} {mark.line + 1}:{mark.column + 1})"
-    field = f" at '{'->'.join(str(p) for p in err_path)}'" if err_path else ""
     raise cv.Invalid(
-        f"Undefined variable in {context_label}{field}: {err.message}{location}"
+        f"Undefined variable in {context_label}: {err.message}\n{format_path(err_path, err_value)}"
     )
 
 
@@ -145,7 +126,7 @@ def _resolve_var(name: str, context_vars: ContextVars) -> Any:
 
 def _handle_undefined(
     err: UndefinedError,
-    path: SubstitutionPath,
+    path: DocumentPath,
     value: Any,
     strict_undefined: bool,
     errors: ErrList | None,
@@ -163,7 +144,7 @@ def _handle_undefined(
 
 def _expand_substitutions(
     value: str,
-    path: SubstitutionPath,
+    path: DocumentPath,
     context_vars: ContextVars,
     strict_undefined: bool,
     errors: ErrList | None,
@@ -236,7 +217,7 @@ def _expand_substitutions(
                 f"\nEvaluation stack: (most recent evaluation last)"
                 f"\n{err.stack_trace_str()}"
                 f"\nRelevant context:\n{err.context_trace_str()}"
-                f"\nSee {'->'.join(str(x) for x in path)}",
+                f"\n{format_path(path, orig_value)}",
                 path,
             ) from err
         else:
@@ -345,15 +326,13 @@ def push_context(
 
 def resolve_include(
     include: IncludeFile,
-    path: list[int | str],
+    path: DocumentPath,
     context_vars: ContextVars,
     strict_undefined: bool = True,
     errors: ErrList | None = None,
-) -> tuple[Any, str]:
+) -> Any:
     """Resolve an include, substituting the filename if needed.
 
-    Returns the loaded content and the resolved filename.
-
     Note: no path-traversal validation is performed on the resolved filename.
     A substitution that resolves to an absolute path will bypass the parent
     directory (Path.__truediv__ ignores the left operand for absolute paths).
@@ -361,44 +340,44 @@ def resolve_include(
     values (including command-line substitutions), so path restrictions are
     an explicit non-goal here.
     """
-    original = str(include.file)
+    original = include.file
+    original_str = str(original)
     filename = str(
         _expand_substitutions(
-            original, path + ["file"], context_vars, strict_undefined, errors
+            original_str, path + ["file"], context_vars, strict_undefined, errors
         )
     )
-    if filename != original:
+    substituted = filename != original_str
+    if substituted:
         include = IncludeFile(
             include.parent_file, filename, include.vars, include.yaml_loader
         )
     try:
-        return include.load(), filename
+        return include.load()
     except esphome.core.EsphomeError as err:
+        resolved = f" (expanded from '{original}')" if substituted else ""
         raise cv.Invalid(
-            f"Error including file '{filename}': {err}",
+            f"Error including file '{filename}'{resolved}: {err}"
+            f"\n{format_path(path, original)}",
             path + [f"<{filename}>"],
         ) from err
 
 
 def _substitute_include(
     include: IncludeFile,
-    path: list[int | str],
+    path: DocumentPath,
     context_vars: ContextVars,
     strict_undefined: bool,
     errors: ErrList | None,
 ) -> Any:
     """Resolve an include and substitute its content."""
-    content, filename = resolve_include(
+    content = resolve_include(include, path, context_vars, strict_undefined, errors)
-        include, path, context_vars, strict_undefined, errors
+    return substitute(content, path, context_vars, strict_undefined, errors)
-    )
-    return substitute(
-        content, path + [f"<{filename}>"], context_vars, strict_undefined, errors
-    )
 
 
 def substitute(
     item: Any,
-    path: SubstitutionPath,
+    path: DocumentPath,
     parent_context: ContextVars,
     strict_undefined: bool,
     errors: ErrList | None = None,
@@ -451,16 +430,12 @@ def _warn_unresolved_variables(errors: ErrList) -> None:
     for err, path, expression in errors:
         if "password" in path:
             continue
-        location: str = "->".join(str(x) for x in path)
-        if isinstance(expression, ESPHomeDataBase) and expression.esp_range is not None:
-            location += f" in {str(expression.esp_range.start_mark)}"
-
         _LOGGER.warning(
             "The string '%s' looks like an expression,"
-            " but could not resolve all the variables: %s (see %s)",
+            " but could not resolve all the variables: %s\n%s",
             expression,
             err.message,
-            location,
+            format_path(path, expression),
         )
 
 
@@ -479,7 +454,7 @@ def resolve_substitutions_block(
         # Single-shot resolution — matches ``_walk_packages`` for the
         # ``packages: !include`` entry point.  Chained includes (an include that
         # itself loads another ``!include`` at the top level) are not supported.
-        substitutions, _ = resolve_include(
+        substitutions = resolve_include(
             substitutions,
             [],
             ContextVars(command_line_substitutions or {}),

esphome/yaml_util.py

@@ -48,6 +48,8 @@ _SECRET_VALUES = {}
 # Not thread-safe — config processing is single-threaded today.
 _load_listeners: list[Callable[[Path], None]] = []
 
+DocumentPath = list[str | int]
+
 
 @contextmanager
 def track_yaml_loads() -> Generator[list[Path]]:
@@ -679,6 +681,123 @@ def is_secret(value):
         return None
 
 
+def _path_doc(item: Any) -> str | None:
+    """Return the source document name if *item* carries location info."""
+    if isinstance(item, ESPHomeDataBase) and (r := item.esp_range) is not None:
+        return r.start_mark.document
+    return None
+
+
+def _fmt_mark(loc: Any) -> str:
+    """Render a DocumentLocation as a 1-based 'file line:col' string."""
+    return f"{loc.document} {loc.line + 1}:{loc.column + 1}"
+
+
+def _obj_loc(obj: Any) -> str:
+    """Return formatted source location for *obj*, or '' if it has none."""
+    if isinstance(obj, ESPHomeDataBase) and (r := obj.esp_range) is not None:
+        return _fmt_mark(r.start_mark)
+    return ""
+
+
+def _fmt_segment(seg: list) -> str:
+    """Format a path segment, rendering integers as [n] subscripts."""
+    parts: list[str] = []
+    for item in seg:
+        if isinstance(item, int):
+            if parts:
+                parts[-1] = f"{parts[-1]}[{item}]"
+            else:
+                parts.append(f"[{item}]")
+        else:
+            parts.append(str(item))
+    return "->".join(parts)
+
+
+def _split_into_frames(
+    path: DocumentPath,
+) -> list[tuple[list, str]]:
+    """Group *path* into per-file frames at include boundaries.
+
+    A "frame" is the slice of the path that belongs to one source document.
+    Each path item is either:
+
+      * a **located key** — has an ``ESPHomeDataBase`` source mark; this is
+        what tells us which document owns the surrounding keys.
+      * an **integer** — a list subscript; always attaches to the open frame
+        (renders as ``foo[3]`` on the previous name).
+      * an **unlocated string** — a key with no source mark (e.g. constants
+        like ``CONF_PACKAGES``); it describes the parent of the *next* file,
+        so it migrates to the next frame when the document changes.
+
+    Returns a list of ``(items, "file line:col")`` tuples in walk order
+    (outermost frame first).
+    """
+    frames: list[tuple[list, str]] = []
+    open_frame: list = []
+    next_frame_keys: list = []  # unlocated strings buffered for the next frame
+    open_doc: str | None = None
+    open_loc = ""
+
+    for item in path:
+        doc = _path_doc(item)
+        if doc is None:
+            # Ints subscript the open frame's last name; everything else
+            # (strings, or leading ints with no open frame) is buffered for
+            # the next frame.
+            if isinstance(item, int) and open_doc is not None:
+                open_frame.append(item)
+            else:
+                next_frame_keys.append(item)
+            continue
+        if open_doc is not None and doc != open_doc:
+            # Crossed an include boundary: close the open frame.
+            frames.append((open_frame, open_loc))
+            open_frame = []
+        open_frame.extend(next_frame_keys)
+        next_frame_keys.clear()
+        open_frame.append(item)
+        open_doc = doc
+        open_loc = _fmt_mark(item.esp_range.start_mark)
+
+    if open_doc is not None:
+        # Trailing buffered keys belong to the innermost (last) frame.
+        open_frame.extend(next_frame_keys)
+        frames.append((open_frame, open_loc))
+    return frames
+
+
+def format_path(path: DocumentPath, current_obj: Any) -> str:
+    """Build a human-readable include stack from a config path.
+
+    Each YAML key in *path* that carries an ``ESPHomeDataBase`` ``esp_range``
+    reveals which file it came from.  When the source document changes between
+    consecutive such keys, that is an include boundary.  The path is split
+    into per-file frames and formatted innermost-first, e.g.::
+
+        In: packages->roam in common/package/wifi.yaml 26:10
+          Included from packages->net in common/hardware.yaml 44:2
+          Included from packages->device in my_project.yaml 11:2
+
+    The innermost ``In:`` line uses the location from *current_obj* when
+    available (the value that triggered the error) for extra precision.
+    """
+    frames = _split_into_frames(path)
+    obj_loc = _obj_loc(current_obj)
+
+    if not frames:
+        # No source info anywhere in the path: render as a flat path,
+        # using current_obj's location if it happens to have one.
+        suffix = f" in {obj_loc}" if obj_loc else ""
+        return f"In: {_fmt_segment(path)}{suffix}"
+
+    inner_seg, inner_loc = frames[-1]
+    lines = [f"In: {_fmt_segment(inner_seg)} in {obj_loc or inner_loc}"]
+    for seg, loc in reversed(frames[:-1]):
+        lines.append(f"  Included from {_fmt_segment(seg)} in {loc}")
+    return "\n".join(lines)
+
+
 class ESPHomeDumper(yaml.SafeDumper):
     def represent_mapping(self, tag, mapping, flow_style=None):
         value = []

tests/component_tests/packages/test_packages.py

@@ -46,7 +46,7 @@ from esphome.const import (
 )
 from esphome.core import CORE
 from esphome.util import OrderedDict
-from esphome.yaml_util import IncludeFile, add_context, load_yaml
+from esphome.yaml_util import DocumentPath, IncludeFile, add_context, load_yaml
 
 # Test strings
 TEST_DEVICE_NAME = "test_device_name"
@@ -1113,7 +1113,7 @@ def test_packages_include_file_resolves_to_list(mock_resolve_include) -> None:
     """When packages: is an IncludeFile that resolves to a list, it is processed correctly."""
     include_file = MagicMock(spec=IncludeFile)
     package_content = {CONF_WIFI: {CONF_SSID: TEST_PACKAGE_WIFI_SSID}}
-    mock_resolve_include.return_value = ([package_content], None)
+    mock_resolve_include.return_value = [package_content]
 
     config = {CONF_PACKAGES: include_file}
     result = do_packages_pass(config)
@@ -1127,7 +1127,7 @@ def test_packages_include_file_resolves_to_dict(mock_resolve_include) -> None:
     """When packages: is an IncludeFile that resolves to a dict, it is processed correctly."""
     include_file = MagicMock(spec=IncludeFile)
     package_content = {CONF_WIFI: {CONF_SSID: TEST_PACKAGE_WIFI_SSID}}
-    mock_resolve_include.return_value = ({"network": package_content}, None)
+    mock_resolve_include.return_value = {"network": package_content}
 
     config = {CONF_PACKAGES: include_file}
     result = do_packages_pass(config)
@@ -1142,7 +1142,7 @@ def test_packages_include_file_resolves_to_invalid_type_raises(
 ) -> None:
     """When packages: is an IncludeFile that resolves to an invalid type, cv.Invalid is raised."""
     include_file = MagicMock(spec=IncludeFile)
-    mock_resolve_include.return_value = ("not_a_dict_or_list", None)
+    mock_resolve_include.return_value = "not_a_dict_or_list"
 
     config = {CONF_PACKAGES: include_file}
     with pytest.raises(
@@ -1215,7 +1215,9 @@ def test_named_dict_with_include_files_no_false_deprecation_warning(
 
     call_count = 0
 
-    def failing_callback(package_config: dict, context: object) -> dict:
+    def failing_callback(
+        package_config: dict, context: object, path: DocumentPath | None = None
+    ) -> dict:
         nonlocal call_count
         call_count += 1
         if call_count == 1:
@@ -1251,7 +1253,9 @@ def test_validate_deprecated_false_raises_directly(
 
     call_count = 0
 
-    def failing_callback(package_config: dict, context: object) -> dict:
+    def failing_callback(
+        package_config: dict, context: object, path: DocumentPath | None = None
+    ) -> dict:
         nonlocal call_count
         call_count += 1
         if call_count == 1:
@@ -1283,7 +1287,9 @@ def test_error_on_first_declared_package_still_detected() -> None:
 
     call_count = 0
 
-    def fail_on_last(package_config: dict, context: object) -> dict:
+    def fail_on_last(
+        package_config: dict, context: object, path: DocumentPath | None = None
+    ) -> dict:
         nonlocal call_count
         call_count += 1
         # Reverse iteration: third_pkg (1), second_pkg (2), first_pkg (3)
@@ -1312,7 +1318,9 @@ def test_deprecated_single_package_fallback_still_works(
 
     attempt = 0
 
-    def fail_then_succeed(package_config: dict, context: object) -> dict:
+    def fail_then_succeed(
+        package_config: dict, context: object, path: DocumentPath | None = None
+    ) -> dict:
         nonlocal attempt
         attempt += 1
         if attempt == 1:

tests/unit_tests/test_substitutions.py

@@ -659,7 +659,7 @@ def test_resolve_package_max_depth_exceeded(tmp_path: Path) -> None:
         cv.Invalid,
         match=f"Maximum include nesting depth \\({MAX_INCLUDE_DEPTH}\\) exceeded",
     ):
-        processor.resolve_package(package_config, substitutions.ContextVars())
+        processor.resolve_package(package_config, substitutions.ContextVars(), [])
 
 
 def test_include_filename_substitution_undefined_var(tmp_path: Path) -> None:
@@ -690,7 +690,7 @@ def test_raise_first_undefined_logs_extras_at_debug(
         caplog.at_level(logging.DEBUG, logger="esphome.components.substitutions"),
         pytest.raises(cv.Invalid) as exc_info,
     ):
-        substitutions.raise_first_undefined(errors, None, "package definition")
+        substitutions.raise_first_undefined(errors, "package definition")
 
     # First error is surfaced as the cv.Invalid message.
     raised = str(exc_info.value)
@@ -706,7 +706,7 @@ def test_raise_first_undefined_logs_extras_at_debug(
 
 def test_raise_first_undefined_noop_on_empty() -> None:
     """An empty errors list is a no-op — no exception, no log."""
-    substitutions.raise_first_undefined([], None, "package definition")
+    substitutions.raise_first_undefined([], "package definition")
 
 
 def test_do_substitution_pass_included_substitutions_must_be_mapping(
@@ -778,4 +778,43 @@ def test_resolve_package_undefined_var_in_include_filename(tmp_path: Path) -> No
     )
     processor = _PackageProcessor({}, None, False)
     with pytest.raises(cv.Invalid, match="unresolved substitutions"):
-        processor.resolve_package(package_config, substitutions.ContextVars())
+        processor.resolve_package(package_config, substitutions.ContextVars(), [])
+
+
+def test_resolve_include_error_shows_expanded_from_when_substituted(
+    tmp_path: Path,
+) -> None:
+    """When a substituted filename fails to load, the error includes '(expanded from ...)'."""
+    parent = tmp_path / "main.yaml"
+    parent.write_text("")
+
+    def failing_loader(_path: Path) -> None:
+        raise EsphomeError("File not found")
+
+    include = yaml_util.IncludeFile(parent, "${device}.yaml", None, failing_loader)
+    context = substitutions.ContextVars({"device": "my_device"})
+
+    with pytest.raises(cv.Invalid) as exc_info:
+        substitutions.resolve_include(include, [], context)
+
+    msg = str(exc_info.value)
+    assert "my_device.yaml" in msg
+    assert "expanded from '${device}.yaml'" in msg
+
+
+def test_resolve_include_error_no_expanded_from_for_literal_filename(
+    tmp_path: Path,
+) -> None:
+    """When a literal filename fails to load, the error has no 'expanded from' clause."""
+    parent = tmp_path / "main.yaml"
+    parent.write_text("")
+
+    def failing_loader(_path: Path) -> None:
+        raise EsphomeError("File not found")
+
+    include = yaml_util.IncludeFile(parent, "literal.yaml", None, failing_loader)
+
+    with pytest.raises(cv.Invalid) as exc_info:
+        substitutions.resolve_include(include, [], substitutions.ContextVars())
+
+    assert "expanded from" not in str(exc_info.value)

tests/unit_tests/test_yaml_util.py

@@ -9,8 +9,9 @@ from esphome import core, yaml_util
 from esphome.components import substitutions
 from esphome.config_helpers import Extend, Remove
 import esphome.config_validation as cv
-from esphome.core import EsphomeError
+from esphome.core import DocumentLocation, DocumentRange, EsphomeError
 from esphome.util import OrderedDict
+from esphome.yaml_util import ESPHomeDataBase, format_path, make_data_base
 
 
 @pytest.fixture(autouse=True)
@@ -712,3 +713,181 @@ def test_yaml_merge_chain_include_depth_exceeded() -> None:
     yaml_text = "base:\n  <<: !include loop.yaml\n"
     with pytest.raises(EsphomeError, match="Maximum include chain depth"):
         yaml_util.parse_yaml(parent, io.StringIO(yaml_text), self_referencing_loader)
+
+
+def _located(value, doc: str, line: int, col: int):
+    """Return *value* wrapped with a fake ESPHomeDataBase source location."""
+    loc = DocumentLocation(doc, line, col)
+    obj = make_data_base(value)
+    if isinstance(obj, ESPHomeDataBase):
+        obj._esp_range = DocumentRange(loc, loc)
+    return obj
+
+
+def test_format_path_no_location_info_returns_flat_path():
+    """Plain path items with no esp_range produce a simple flat 'In:' line."""
+    result = format_path(["wifi", "ssid"], None)
+    assert result == "In: wifi->ssid"
+
+
+def test_format_path_no_location_info_current_obj_adds_file():
+    """When path has no location but current_obj does, its location is shown."""
+    obj = _located("${var}", "main.yaml", 5, 10)
+    result = format_path(["wifi", "ssid"], obj)
+    assert result == "In: wifi->ssid in main.yaml 6:11"
+
+
+def test_format_path_single_frame_no_include_boundary():
+    """All located keys from the same document → single 'In:' line, no 'Included from'."""
+    path = ["packages", _located("pkg1", "root.yaml", 5, 2)]
+    result = format_path(path, None)
+    assert result.startswith("In: packages->pkg1 in root.yaml 6:3")
+    assert "Included from" not in result
+
+
+def test_format_path_two_frames_shows_included_from():
+    """Keys from two different documents produce 'In:' + one 'Included from' line."""
+    path = [
+        "packages",
+        _located("device", "root.yaml", 10, 2),
+        "packages",
+        _located("inner", "hardware.yaml", 3, 2),
+    ]
+    result = format_path(path, None)
+    assert "In: packages->inner in hardware.yaml 4:3" in result
+    assert "Included from packages->device in root.yaml 11:3" in result
+
+
+def test_format_path_three_frames_full_include_stack():
+    """Three document levels produce two 'Included from' lines in correct order."""
+    path = [
+        "packages",
+        _located("device", "root.yaml", 10, 2),
+        "packages",
+        _located("_wifi_", "hardware.yaml", 43, 2),
+        "packages",
+        _located("_roam_", "wifi.yaml", 25, 2),
+    ]
+    result = format_path(path, None)
+    lines = result.splitlines()
+    assert lines[0].startswith("In: packages->_roam_ in wifi.yaml")
+    assert lines[1].startswith("  Included from packages->_wifi_ in hardware.yaml")
+    assert lines[2].startswith("  Included from packages->device in root.yaml")
+
+
+def test_format_path_current_obj_overrides_innermost_location():
+    """current_obj's esp_range replaces the key's column for the 'In:' line."""
+    path = ["packages", _located("pkg1", "root.yaml", 5, 2)]
+    # Value (the expression) sits at column 10, not column 2 like the key
+    value = _located("${undefined}", "root.yaml", 5, 10)
+    result = format_path(path, value)
+    assert "6:11" in result
+    assert "6:3" not in result
+
+
+def test_format_path_empty_path_with_no_location():
+    """Empty path with no location info returns 'In: '."""
+    result = format_path([], None)
+    assert result == "In: "
+
+
+def test_format_path_integer_path_items_formatted_as_subscript():
+    """Integer indices are rendered as [n] subscripts in the flat fallback."""
+    result = format_path(["packages", 0], None)
+    assert result == "In: packages[0]"
+
+
+def test_format_path_integer_list_index_attached_to_previous_frame():
+    """A list index between two include boundaries attaches to the outer frame."""
+    path = [
+        "packages",
+        _located("packages", "main.yaml", 5, 0),
+        0,
+        _located("packages", "level1.yaml", 2, 0),
+        0,
+        _located("esphome", "level2.yaml", 0, 0),
+        _located("name", "level2.yaml", 1, 8),
+    ]
+    result = format_path(path, None)
+    lines = result.splitlines()
+    assert lines[0].startswith("In: esphome->name in level2.yaml")
+    assert "packages[0]" in lines[1] and "level1.yaml" in lines[1]
+    assert "packages[0]" in lines[2] and "main.yaml" in lines[2]
+
+
+def test_format_path_trailing_unlocated_string_after_located_key():
+    """Plain string keys after the last located key must still appear in output."""
+    path = [_located("packages", "main.yaml", 5, 0), "sub", "key"]
+    result = format_path(path, None)
+    assert result == "In: packages->sub->key in main.yaml 6:1"
+
+
+def test_format_path_trailing_unlocated_int_attaches_to_current_frame():
+    """Trailing ints attach to the open frame's last key (subscript), strings
+    buffer until end-of-path and then flush behind."""
+    path = [_located("packages", "main.yaml", 5, 0), 0, "sub"]
+    result = format_path(path, None)
+    # Int attaches to 'packages' as [0] subscript; trailing 'sub' is flushed
+    # at end and appears after.
+    assert result == "In: packages[0]->sub in main.yaml 6:1"
+
+
+def test_format_path_only_trailing_unlocated_strings_are_preserved():
+    """Trailing pending items must not be silently dropped after the last frame."""
+    path = [
+        _located("packages", "main.yaml", 5, 0),
+        _located("inner", "hardware.yaml", 3, 0),
+        "tail1",
+        "tail2",
+    ]
+    result = format_path(path, None)
+    lines = result.splitlines()
+    assert lines[0] == "In: inner->tail1->tail2 in hardware.yaml 4:1"
+    assert lines[1] == "  Included from packages in main.yaml 6:1"
+
+
+def test_format_path_leading_int_with_no_current_doc_goes_to_pending():
+    """An int before any located key is buffered and shown in the first frame."""
+    path = [0, _located("name", "main.yaml", 1, 0)]
+    result = format_path(path, None)
+    # Leading ints have no preceding name to subscript onto, so they render
+    # as bare [n] in the formatted segment.
+    assert result == "In: [0]->name in main.yaml 2:1"
+
+
+def test_format_path_only_unlocated_int_returns_flat_fallback():
+    """Path with only an int and no location info renders via the flat fallback."""
+    result = format_path([0], None)
+    assert result == "In: [0]"
+
+
+def test_format_path_current_obj_in_different_doc_than_innermost_frame():
+    """current_obj's location is preferred even when its document differs from the frame's."""
+    path = [_located("packages", "root.yaml", 1, 0)]
+    value = _located("${var}", "other.yaml", 9, 4)
+    result = format_path(path, value)
+    # Innermost line uses current_obj's mark (other.yaml 10:5), not the key's.
+    assert result == "In: packages in other.yaml 10:5"
+
+
+def test_format_path_current_obj_without_location_falls_back_to_key():
+    """An ESPHomeDataBase current_obj with no esp_range falls back to the key's location."""
+
+    class _NoRange(ESPHomeDataBase, str):
+        pass
+
+    obj = _NoRange.__new__(_NoRange, "value")
+    str.__init__(obj)
+    # No _esp_range set on this instance.
+    assert obj.esp_range is None
+
+    path = [_located("packages", "main.yaml", 5, 2)]
+    result = format_path(path, obj)
+    assert result == "In: packages in main.yaml 6:3"
+
+
+def test_format_path_empty_path_with_located_current_obj():
+    """An empty path with a located current_obj still surfaces the location."""
+    obj = _located("${var}", "main.yaml", 0, 0)
+    result = format_path([], obj)
+    assert result == "In:  in main.yaml 1:1"