Improve and simplify loading of transformation chains

src/scippnexus/__init__.py

@@ -18,8 +18,26 @@
  create_class,
  create_field,
 )
-from .field import Attrs, Field
+from .field import Attrs, DependsOn, Field
 from .file import File
 from ._load import load
 from .nexus_classes import *
-from .nxtransformations import compute_positions, zip_pixel_offsets
+from .nxtransformations import compute_positions, zip_pixel_offsets, TransformationChain
+
+__all__ = [
+ 'Attrs',
+ 'DependsOn',
+ 'Field',
+ 'File',
+ 'Group',
+ 'NXobject',
+ 'NexusStructureError',
+ 'TransformationChain',
+ 'base_definitions',
+ 'compute_positions',
+ 'create_class',
+ 'create_field',
+ 'load',
+ 'typing',
+ 'zip_pixel_offsets',
+]

src/scippnexus/base.py

@@ -409,16 +409,13 @@ def isclass(x):
  # For a time-dependent transformation in NXtransformations, an NXlog may
  # take the place of the `value` field. In this case, we need to read the
  # properties of the NXlog group to make the actual transformation.
- from .nxtransformations import maybe_resolve, maybe_transformation
+ from .nxtransformations import maybe_transformation, parse_depends_on_chain
 
- if (
- isinstance(dg, sc.DataGroup)
- and (depends_on := dg.get('depends_on')) is not None
- ):
- if (resolved := maybe_resolve(self['depends_on'], depends_on)) is not None:
- dg['resolved_depends_on'] = resolved
+ if isinstance(dg, sc.DataGroup) and 'depends_on' in dg:
+ if (chain := parse_depends_on_chain(self, dg['depends_on'])) is not None:
+ dg['depends_on'] = chain
 
- return maybe_transformation(self, value=dg, sel=sel)
+ return maybe_transformation(self, value=dg)
 
  def _warn_fallback(self, e: Exception) -> None:
  msg = (

src/scippnexus/field.py

@@ -24,14 +24,22 @@
  from .base import Group
 
 
-def depends_on_to_relative_path(depends_on: str, parent_path: str) -> str:
- """Replace depends_on paths with relative paths.
+@dataclass
+class DependsOn:
+ """
+ Represents a depends_on reference in a NeXus file.
+
+ The parent (the full path within the NeXus file) is stored, as the value may be
+ relative or absolute, so having the path available after loading is essential.
+ """
+
+ parent: str
+ value: str
 
- After loading we will generally not have the same root so absolute paths
- cannot be resolved after loading."""
- if depends_on.startswith('/'):
- return posixpath.relpath(depends_on, parent_path)
- return depends_on
+ def absolute_path(self) -> str | None:
+ if self.value == '.':
+ return None
+ return posixpath.normpath(posixpath.join(self.parent, self.value))
 
 
 def _is_time(obj):
@@ -161,7 +169,7 @@ def __getitem__(self, select: ScippIndex) -> Any | sc.Variable:
  # If the variable is empty, return early
  if np.prod(shape) == 0:
  variable = self._maybe_datetime(variable)
- return maybe_transformation(self, value=variable, sel=select)
+ return maybe_transformation(self, value=variable)
 
  if self.dtype == sc.DType.string:
  try:
@@ -170,10 +178,8 @@ def __getitem__(self, select: ScippIndex) -> Any | sc.Variable:
  strings = self.dataset.asstr(encoding='latin-1')[index]
  _warn_latin1_decode(self.dataset, strings, str(e))
  variable.values = np.asarray(strings).flatten()
- if self.dataset.name.endswith('depends_on') and variable.ndim == 0:
- variable.value = depends_on_to_relative_path(
- variable.value, self.dataset.parent.name
- )
+ if self.dataset.name.endswith('/depends_on') and variable.ndim == 0:
+ return DependsOn(parent=self.dataset.parent.name, value=variable.value)
  elif variable.values.flags["C_CONTIGUOUS"]:
  # On versions of h5py prior to 3.2, a TypeError occurs in some cases
  # where h5py cannot broadcast data with e.g. shape (20, 1) to a buffer
@@ -199,7 +205,7 @@ def __getitem__(self, select: ScippIndex) -> Any | sc.Variable:
  else:
  return variable.value
  variable = self._maybe_datetime(variable)
- return maybe_transformation(self, value=variable, sel=select)
+ return maybe_transformation(self, value=variable)
 
  def _maybe_datetime(self, variable: sc.Variable) -> sc.Variable:
  if _is_time(variable):