refactor: move imports to top level, document import policy in CLAUDE.md

Move json, h5py, get_nwb_extensions, and _SCHEMA_BAREASSET_HAS_DATASTANDARD
imports to module top level in tests.  Keep pynwb_utils import deferred in
bases.py (heavy transitive deps: h5py/pynwb/hdmf/numpy) per existing
convention.  Add import guidance to CLAUDE.md.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: yarikoptic

CLAUDE.md

@@ -31,6 +31,11 @@ This file provides guidance to Claude Code (claude.ai/code) when working with co
 - Prefer specific exceptions over generic ones
 - For CLI, use click library patterns
 - Imports organized: stdlib, third-party, local (alphabetical within groups)
+- **Imports must be at the top of the file** — do NOT place imports inside
+  functions or methods unless there is a concrete reason (circular dependency,
+  or heavy transitive imports like `pynwb`/`h5py`/`nwbinspector` that would
+  slow down module load for unrelated code paths).  When deferring an import
+  for weight, add the comment `# Avoid heavy import by importing within function:`.
 
 ## Documentation
 - Keep docstrings updated when changing function signatures

dandi/files/bases.py

@@ -520,7 +520,7 @@ def get_metadata(
         metadata.path = self.path
         if _SCHEMA_BAREASSET_HAS_DATASTANDARD:
             kwargs: dict[str, Any] = dict(nwb_standard)
-            # Populate NWB extensions (ndx-*) from the h5 specifications group
+            # Avoid heavy import by importing within function:
             from dandi.pynwb_utils import get_nwb_extensions
 
             try:

dandi/tests/test_files.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import json
 from operator import attrgetter
 import os
 from pathlib import Path
@@ -16,6 +17,7 @@
 from ..consts import ZARR_MIME_TYPE, dandiset_metadata_file
 from ..dandiapi import AssetType, RemoteZarrAsset
 from ..exceptions import UnknownAssetError
+from ..files.bases import _SCHEMA_BAREASSET_HAS_DATASTANDARD
 from ..files import (
     BIDSDatasetDescriptionAsset,
     DandisetMetadataFile,
@@ -616,8 +618,6 @@ class TestBIDSDatasetDescriptionDataStandard:
 
     @staticmethod
     def _make_bids_dd(tmp_path: Path, content: dict) -> BIDSDatasetDescriptionAsset:
-        import json
-
         dd_path = tmp_path / "dataset_description.json"
         dd_path.write_text(json.dumps(content))
         return BIDSDatasetDescriptionAsset(
@@ -628,8 +628,6 @@ def _make_bids_dd(tmp_path: Path, content: dict) -> BIDSDatasetDescriptionAsset:
 
     @staticmethod
     def _standard_names(metadata):  # type: ignore[no-untyped-def]
-        from ..files.bases import _SCHEMA_BAREASSET_HAS_DATASTANDARD
-
         if not _SCHEMA_BAREASSET_HAS_DATASTANDARD:
             pytest.skip("dandischema too old, no dataStandard on BareAsset")
         return [s.name for s in (metadata.dataStandard or [])]
@@ -674,8 +672,6 @@ def test_hed_detected_with_list_hedversion(self, tmp_path: Path) -> None:
 
     def test_hed_library_schemas_as_extensions(self, tmp_path: Path) -> None:
         """HED library schemas in list HEDVersion populate extensions."""
-        from ..files.bases import _SCHEMA_BAREASSET_HAS_DATASTANDARD
-
         if not _SCHEMA_BAREASSET_HAS_DATASTANDARD:
             pytest.skip("dandischema too old, no dataStandard on BareAsset")
         asset = self._make_bids_dd(

dandi/tests/test_pynwb_utils.py

@@ -6,10 +6,11 @@
 import re
 from typing import Any, NoReturn
 
+import h5py
 import numpy as np
 from pynwb import NWBHDF5IO, NWBFile, TimeSeries
 
-from ..pynwb_utils import _sanitize_nwb_version, nwb_has_external_links
+from ..pynwb_utils import _sanitize_nwb_version, get_nwb_extensions, nwb_has_external_links
 
 
 def test_pynwb_io(simple1_nwb: Path) -> None:
@@ -107,10 +108,6 @@ def test_nwb_has_external_links(tmp_path):
 
 def test_get_nwb_extensions(tmp_path: Path) -> None:
     """Test extraction of NWB extensions from HDF5 specifications group."""
-    import h5py
-
-    from ..pynwb_utils import get_nwb_extensions
-
     h5path = tmp_path / "test.nwb"
     with h5py.File(h5path, "w") as f:
         specs = f.create_group("specifications")
@@ -133,10 +130,6 @@ def test_get_nwb_extensions(tmp_path: Path) -> None:
 
 def test_get_nwb_extensions_no_specs(tmp_path: Path) -> None:
     """No specifications group returns empty dict."""
-    import h5py
-
-    from ..pynwb_utils import get_nwb_extensions
-
     h5path = tmp_path / "test.nwb"
     with h5py.File(h5path, "w") as f:
         f.attrs["nwb_version"] = "2.7.0"