test: follow up FileSystemSeedReader coverage cleanup

packages/data-designer-engine/src/data_designer/engine/testing/__init__.py

@@ -3,6 +3,7 @@
 
 from __future__ import annotations
 
+from data_designer.engine.testing.seed_readers import LineFanoutDirectorySeedReader
 from data_designer.engine.testing.stubs import (
     StubChoice,
     StubHuggingFaceSeedReader,
@@ -15,6 +16,7 @@
 from data_designer.engine.testing.utils import assert_valid_plugin
 
 __all__ = [
+    LineFanoutDirectorySeedReader.__name__,
     "StubChoice",
     "StubHuggingFaceSeedReader",
     "StubMCPFacade",

packages/data-designer-engine/src/data_designer/engine/testing/seed_readers.py

@@ -0,0 +1,54 @@
+# SPDX-FileCopyrightText: Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+# SPDX-License-Identifier: Apache-2.0
+
+from __future__ import annotations
+
+from pathlib import Path
+from typing import Any
+
+import data_designer.lazy_heavy_imports as lazy
+from data_designer.config.seed_source import DirectorySeedSource
+from data_designer.engine.resources.seed_reader import FileSystemSeedReader, SeedReaderFileSystemContext
+
+
+class LineFanoutDirectorySeedReader(FileSystemSeedReader[DirectorySeedSource]):
+    def __init__(self, *, include_file_name: bool = False) -> None:
+        self.include_file_name = include_file_name
+        self.hydrated_relative_paths: list[str] = []
+        self.output_columns = ["relative_path", "line_index", "line"]
+        if include_file_name:
+            self.output_columns.insert(1, "file_name")
+
+    def build_manifest(self, *, context: SeedReaderFileSystemContext) -> lazy.pd.DataFrame | list[dict[str, str]]:
+        matched_paths = self.get_matching_relative_paths(
+            context=context,
+            file_pattern=self.source.file_pattern,
+            recursive=self.source.recursive,
+        )
+        return [
+            {
+                "relative_path": relative_path,
+                **({"file_name": Path(relative_path).name} if self.include_file_name else {}),
+            }
+            for relative_path in matched_paths
+        ]
+
+    def hydrate_row(
+        self,
+        *,
+        manifest_row: dict[str, Any],
+        context: SeedReaderFileSystemContext,
+    ) -> list[dict[str, Any]]:
+        relative_path = str(manifest_row["relative_path"])
+        self.hydrated_relative_paths.append(relative_path)
+        with context.fs.open(relative_path, "r", encoding="utf-8") as handle:
+            lines = handle.read().splitlines()
+        return [
+            {
+                "relative_path": relative_path,
+                **({"file_name": str(manifest_row["file_name"])} if self.include_file_name else {}),
+                "line_index": line_index,
+                "line": line,
+            }
+            for line_index, line in enumerate(lines)
+        ]

packages/data-designer-engine/tests/engine/resources/test_seed_reader.py

@@ -3,6 +3,7 @@
 
 from __future__ import annotations
 
+from collections.abc import Callable
 from pathlib import Path
 from typing import Any
 
@@ -23,6 +24,7 @@
     SeedReaderRegistry,
 )
 from data_designer.engine.secret_resolver import PlaintextResolver
+from data_designer.engine.testing.seed_readers import LineFanoutDirectorySeedReader
 
 
 class TrackingFileContentsSeedReader(FileContentsSeedReader):
@@ -134,50 +136,10 @@ def hydrate_row(
         }
 
 
-class FanoutDirectorySeedReader(FileSystemSeedReader[DirectorySeedSource]):
-    output_columns = ["relative_path", "file_name", "line_index", "line"]
-
-    def __init__(self) -> None:
-        self.hydrated_relative_paths: list[str] = []
-
-    def build_manifest(self, *, context: SeedReaderFileSystemContext) -> lazy.pd.DataFrame | list[dict[str, str]]:
-        matched_paths = self.get_matching_relative_paths(
-            context=context,
-            file_pattern=self.source.file_pattern,
-            recursive=self.source.recursive,
-        )
-        return [
-            {
-                "relative_path": relative_path,
-                "file_name": Path(relative_path).name,
-            }
-            for relative_path in matched_paths
-        ]
-
-    def hydrate_row(
-        self,
-        *,
-        manifest_row: dict[str, str],
-        context: SeedReaderFileSystemContext,
-    ) -> list[dict[str, Any]]:
-        relative_path = manifest_row["relative_path"]
-        self.hydrated_relative_paths.append(relative_path)
-        with context.fs.open(relative_path, "r", encoding="utf-8") as handle:
-            lines = handle.read().splitlines()
-        return [
-            {
-                "relative_path": relative_path,
-                "file_name": manifest_row["file_name"],
-                "line_index": line_index,
-                "line": line,
-            }
-            for line_index, line in enumerate(lines)
-        ]
-
-
-class InvalidHydrationReturnSeedReader(FileSystemSeedReader[DirectorySeedSource]):
-    def __init__(self, hydrated_return: Any) -> None:
+class ConfigurableHydrationDirectorySeedReader(FileSystemSeedReader[DirectorySeedSource]):
+    def __init__(self, *, hydrated_return: Any, output_columns: list[str] | None = None) -> None:
         self._hydrated_return = hydrated_return
+        self.output_columns = output_columns
 
     def build_manifest(self, *, context: SeedReaderFileSystemContext) -> lazy.pd.DataFrame | list[dict[str, str]]:
         matched_paths = self.get_matching_relative_paths(
@@ -190,36 +152,13 @@ def build_manifest(self, *, context: SeedReaderFileSystemContext) -> lazy.pd.Dat
     def hydrate_row(
         self,
         *,
-        manifest_row: dict[str, str],
+        manifest_row: dict[str, Any],
         context: SeedReaderFileSystemContext,
     ) -> Any:
         del manifest_row, context
         return self._hydrated_return
 
 
-class SchemaMismatchFanoutSeedReader(FileSystemSeedReader[DirectorySeedSource]):
-    def __init__(self, *, output_columns: list[str], hydrated_rows: list[dict[str, str]]) -> None:
-        self.output_columns = output_columns
-        self._hydrated_rows = hydrated_rows
-
-    def build_manifest(self, *, context: SeedReaderFileSystemContext) -> lazy.pd.DataFrame | list[dict[str, str]]:
-        matched_paths = self.get_matching_relative_paths(
-            context=context,
-            file_pattern=self.source.file_pattern,
-            recursive=self.source.recursive,
-        )
-        return [{"relative_path": relative_path} for relative_path in matched_paths]
-
-    def hydrate_row(
-        self,
-        *,
-        manifest_row: dict[str, str],
-        context: SeedReaderFileSystemContext,
-    ) -> list[dict[str, str]]:
-        del manifest_row, context
-        return self._hydrated_rows
-
-
 class ContextCountingDirectorySeedReader(FileSystemSeedReader[DirectorySeedSource]):
     def __init__(self) -> None:
         self.filesystem_context_calls = 0
@@ -237,6 +176,16 @@ def build_manifest(self, *, context: SeedReaderFileSystemContext) -> lazy.pd.Dat
         return [{"relative_path": relative_path} for relative_path in matched_paths]
 
 
+@pytest.fixture
+def write_alpha_beta_text_files(tmp_path: Path) -> Callable[[str, str], Path]:
+    def _write_alpha_beta_text_files(alpha_contents: str, beta_contents: str) -> Path:
+        (tmp_path / "alpha.txt").write_text(alpha_contents, encoding="utf-8")
+        (tmp_path / "beta.txt").write_text(beta_contents, encoding="utf-8")
+        return tmp_path
+
+    return _write_alpha_beta_text_files
+
+
 def test_one_reader_per_seed_type():
     local_1 = LocalFileSeedReader()
     local_2 = LocalFileSeedReader()
@@ -308,7 +257,7 @@ def test_plugin_style_filesystem_seed_reader_can_fan_out_rows(tmp_path: Path) ->
     (tmp_path / "alpha.txt").write_text("alpha-0\nalpha-1", encoding="utf-8")
     (tmp_path / "beta.txt").write_text("beta-0", encoding="utf-8")
 
-    reader = FanoutDirectorySeedReader()
+    reader = LineFanoutDirectorySeedReader(include_file_name=True)
     reader.attach(
         DirectorySeedSource(path=str(tmp_path), file_pattern="*.txt"),
         PlaintextResolver(),
@@ -444,13 +393,14 @@ def test_file_contents_seed_reader_hydrates_only_selected_manifest_rows(tmp_path
     assert reader.hydrated_relative_paths == ["beta.txt"]
 
 
-def test_filesystem_seed_reader_fanout_keeps_manifest_based_index_selection(tmp_path: Path) -> None:
-    (tmp_path / "alpha.txt").write_text("alpha-0\nalpha-1", encoding="utf-8")
-    (tmp_path / "beta.txt").write_text("beta-0\nbeta-1", encoding="utf-8")
+def test_filesystem_seed_reader_fanout_keeps_manifest_based_index_selection(
+    write_alpha_beta_text_files: Callable[[str, str], Path],
+) -> None:
+    seed_dir = write_alpha_beta_text_files("alpha-0\nalpha-1", "beta-0\nbeta-1")
 
-    reader = FanoutDirectorySeedReader()
+    reader = LineFanoutDirectorySeedReader(include_file_name=True)
     reader.attach(
-        DirectorySeedSource(path=str(tmp_path), file_pattern="*.txt"),
+        DirectorySeedSource(path=str(seed_dir), file_pattern="*.txt"),
         PlaintextResolver(),
     )
 
@@ -467,14 +417,13 @@ def test_filesystem_seed_reader_fanout_keeps_manifest_based_index_selection(tmp_
 
 
 def test_filesystem_seed_reader_batch_reader_raises_for_selected_manifest_rows_with_empty_fanout(
-    tmp_path: Path,
+    write_alpha_beta_text_files: Callable[[str, str], Path],
 ) -> None:
-    (tmp_path / "alpha.txt").write_text("alpha-0", encoding="utf-8")
-    (tmp_path / "beta.txt").write_text("", encoding="utf-8")
+    seed_dir = write_alpha_beta_text_files("alpha-0", "")
 
-    reader = FanoutDirectorySeedReader()
+    reader = LineFanoutDirectorySeedReader(include_file_name=True)
     reader.attach(
-        DirectorySeedSource(path=str(tmp_path), file_pattern="*.txt"),
+        DirectorySeedSource(path=str(seed_dir), file_pattern="*.txt"),
         PlaintextResolver(),
     )
 
@@ -494,14 +443,13 @@ def test_filesystem_seed_reader_batch_reader_raises_for_selected_manifest_rows_w
 
 
 def test_filesystem_seed_reader_batch_reader_skips_empty_fanout_rows_before_returning_records(
-    tmp_path: Path,
+    write_alpha_beta_text_files: Callable[[str, str], Path],
 ) -> None:
-    (tmp_path / "alpha.txt").write_text("", encoding="utf-8")
-    (tmp_path / "beta.txt").write_text("beta-0\nbeta-1", encoding="utf-8")
+    seed_dir = write_alpha_beta_text_files("", "beta-0\nbeta-1")
 
-    reader = FanoutDirectorySeedReader()
+    reader = LineFanoutDirectorySeedReader(include_file_name=True)
     reader.attach(
-        DirectorySeedSource(path=str(tmp_path), file_pattern="*.txt"),
+        DirectorySeedSource(path=str(seed_dir), file_pattern="*.txt"),
         PlaintextResolver(),
     )
 
@@ -518,14 +466,13 @@ def test_filesystem_seed_reader_batch_reader_skips_empty_fanout_rows_before_retu
 
 
 def test_filesystem_seed_reader_batch_reader_stops_cleanly_after_emitting_records_when_only_empty_fanout_rows_remain(
-    tmp_path: Path,
+    write_alpha_beta_text_files: Callable[[str, str], Path],
 ) -> None:
-    (tmp_path / "alpha.txt").write_text("alpha-0", encoding="utf-8")
-    (tmp_path / "beta.txt").write_text("", encoding="utf-8")
+    seed_dir = write_alpha_beta_text_files("alpha-0", "")
 
-    reader = FanoutDirectorySeedReader()
+    reader = LineFanoutDirectorySeedReader(include_file_name=True)
     reader.attach(
-        DirectorySeedSource(path=str(tmp_path), file_pattern="*.txt"),
+        DirectorySeedSource(path=str(seed_dir), file_pattern="*.txt"),
         PlaintextResolver(),
     )
 
@@ -545,6 +492,21 @@ def test_filesystem_seed_reader_batch_reader_stops_cleanly_after_emitting_record
     assert reader.hydrated_relative_paths == ["alpha.txt", "beta.txt"]
 
 
+def test_filesystem_seed_reader_full_output_raises_when_all_manifest_rows_fan_out_to_empty(
+    write_alpha_beta_text_files: Callable[[str, str], Path],
+) -> None:
+    seed_dir = write_alpha_beta_text_files("", "")
+
+    reader = LineFanoutDirectorySeedReader(include_file_name=True)
+    reader.attach(
+        DirectorySeedSource(path=str(seed_dir), file_pattern="*.txt"),
+        PlaintextResolver(),
+    )
+
+    with pytest.raises(SeedReaderError, match="Seed source at .* did not produce any rows"):
+        reader.create_duckdb_connection().execute(f"SELECT * FROM '{reader.get_dataset_uri()}'").df()
+
+
 def test_local_file_seed_reader_uses_load_time_runtime_path_when_cwd_changes(
     tmp_path: Path,
     monkeypatch: pytest.MonkeyPatch,
@@ -649,10 +611,11 @@ def test_filesystem_seed_reader_raises_for_undeclared_hydrated_columns(
 @pytest.mark.parametrize(
     ("hydrated_return", "error_pattern"),
     [
+        (None, "Manifest row index 0 returned NoneType"),
         (123, "Manifest row index 0 returned int"),
         (["not-a-record"], "Manifest row index 0 returned an iterable containing str"),
     ],
-    ids=["scalar", "iterable-of-invalid-records"],
+    ids=["none", "scalar", "iterable-of-invalid-records"],
 )
 def test_filesystem_seed_reader_rejects_invalid_hydrate_row_returns(
     tmp_path: Path,
@@ -661,15 +624,15 @@ def test_filesystem_seed_reader_rejects_invalid_hydrate_row_returns(
 ) -> None:
     (tmp_path / "alpha.txt").write_text("alpha", encoding="utf-8")
 
-    reader = InvalidHydrationReturnSeedReader(hydrated_return)
+    reader = ConfigurableHydrationDirectorySeedReader(hydrated_return=hydrated_return)
     reader.attach(DirectorySeedSource(path=str(tmp_path), file_pattern="*.txt"), PlaintextResolver())
 
     with pytest.raises(SeedReaderError, match=error_pattern):
         reader.create_duckdb_connection().execute(f"SELECT * FROM '{reader.get_dataset_uri()}'").df()
 
 
 @pytest.mark.parametrize(
-    ("output_columns", "hydrated_rows", "error_pattern"),
+    ("output_columns", "hydrated_return", "error_pattern"),
     [
         (
             ["relative_path", "content"],
@@ -693,12 +656,15 @@ def test_filesystem_seed_reader_rejects_invalid_hydrate_row_returns(
 def test_filesystem_seed_reader_validates_each_fanout_record_against_output_columns(
     tmp_path: Path,
     output_columns: list[str],
-    hydrated_rows: list[dict[str, str]],
+    hydrated_return: list[dict[str, str]],
     error_pattern: str,
 ) -> None:
     (tmp_path / "alpha.txt").write_text("alpha", encoding="utf-8")
 
-    reader = SchemaMismatchFanoutSeedReader(output_columns=output_columns, hydrated_rows=hydrated_rows)
+    reader = ConfigurableHydrationDirectorySeedReader(
+        output_columns=output_columns,
+        hydrated_return=hydrated_return,
+    )
     reader.attach(DirectorySeedSource(path=str(tmp_path), file_pattern="*.txt"), PlaintextResolver())
 
     with pytest.raises(SeedReaderError, match=error_pattern):