Better error messages when opening invalid signatures file

gambit/sigs/base.py

@@ -397,6 +397,22 @@ def __getitem__(self, index):
 		return self.signatures[index]
 
 
+class SignaturesFileError(Exception):
+	"""Indicates an error attempting to open a signatures file."""
+
+	message: str
+	filename: str
+	format: str
+
+	def __init__(self, message: str, filename: Optional[FilePath], format: Optional[str]):
+		self.message = message
+		self.filename = str(filename)
+		self.format = format
+
+	def __str__(self):
+		return self.message
+
+
 def load_signatures(path: FilePath, **kw) -> AbstractSignatureArray:
 	"""Load signatures from file.
 

gambit/sigs/hdf5.py

@@ -7,7 +7,7 @@
 import h5py as h5
 
 from .base import SignatureArray, ConcatenatedSignatureArray, AbstractSignatureArray, SignaturesMeta,\
-	ReferenceSignatures
+	ReferenceSignatures, SignaturesFileError
 from gambit.kmers import KmerSpec
 from gambit._cython.metric import BOUNDS_DTYPE
 from gambit.util.io import FilePath
@@ -93,11 +93,11 @@ def __init__(self, group: h5.Group):
 		self.group = group
 
 		if FMT_VERSION_ATTR not in group.attrs:
-			raise RuntimeError('HDF5 group does not contain a signature set')
+			raise SignaturesFileError('HDF5 group does not contain a signature set', None, 'hdf5')
 
 		self.format_version = group.attrs[FMT_VERSION_ATTR]
 		if self.format_version != CURRENT_FMT_VERSION:
-			raise ValueError(f'Unrecognized format version: {self.format_version}')
+			raise ValueError(f'Unrecognized format version: {self.format_version}', None, 'hdf5')
 
 		self.kmerspec = KmerSpec(group.attrs['kmerspec_k'], group.attrs['kmerspec_prefix'])
 		self.meta = read_metadata(group)
@@ -229,13 +229,33 @@ def load_signatures_hdf5(path: FilePath, **kw) -> HDF5Signatures:
 	\\**kw
 		Additional keyword arguments to :func:`h5py.File`.
 	"""
-	return HDF5Signatures(h5.File(path, **kw))
+	exc = SignaturesFileError(f'{path} does not appear to be a GAMBIT signtures file.', path, 'hdf5')
 
+	# Check for HDF5 magic number
+	# The errors raised by the h5py library are a bit cryptic, so make one with a better message if
+	# not a valid HDF5 file.
+	# This also raises the standard errors if file cannot be read.
+	with open(path, 'rb') as f:
+		header = f.read(8)
+	if header != b'\x89HDF\r\n\x1a\n':
+		raise exc
 
-def dump_signatures_hdf5(path: FilePath,
-                         signatures: AbstractSignatureArray,
-                         **kw,
-                         ):
+	h5file = h5.File(path, **kw)
+
+	if FMT_VERSION_ATTR not in h5file.attrs:
+		raise exc
+
+	try:
+		return HDF5Signatures(h5file)
+
+	except SignaturesFileError as exc:
+		# Make sure errors in opening are annotated with the correct file name
+		exc.message = f'Error opening signatures file {path}: {exc.message}'
+		exc.filename = str(path)
+		raise
+
+
+def dump_signatures_hdf5(path: FilePath, signatures: AbstractSignatureArray, **kw):
 	"""Write k-mer signatures and associated metadata to an HDF5 file.
 
 	Parameters

tests/sigs/test_sigs_hdf5.py

@@ -6,6 +6,7 @@
 
 from gambit.sigs.hdf5 import read_metadata, write_metadata, load_signatures_hdf5, dump_signatures_hdf5
 from gambit.sigs import SignaturesMeta, SignatureList, AnnotatedSignatures
+from gambit.sigs.base import SignaturesFileError
 from gambit.sigs.test import AbstractSignatureArrayTests
 from gambit.kmers import KmerSpec
 from gambit.test import make_signatures
@@ -51,6 +52,35 @@ def dump_load(sigs, path, **kw):
 	return load_signatures_hdf5(f)
 
 
+def test_open_not_hdf5(tmp_path):
+	"""Test opening an invalid file."""
+
+	# Not an HDF5 file
+	file = tmp_path / 'not-hdf5.gs'
+	with open(file, 'w') as f:
+		f.write('foo')
+
+	with pytest.raises(SignaturesFileError) as einfo:
+		load_signatures_hdf5(file)
+
+	assert einfo.value.filename == str(file)
+	assert einfo.value.format == 'hdf5'
+
+
+def test_open_invalid(tmp_path):
+	"""Test opening an invalid HDF5 file."""
+
+	file = tmp_path / 'invalid.gs'
+	with h5.File(file, 'w') as f:
+		pass  # Empty
+
+	with pytest.raises(SignaturesFileError) as einfo:
+		load_signatures_hdf5(file)
+
+	assert einfo.value.filename == str(file)
+	assert einfo.value.format == 'hdf5'
+
+
 class TestHDF5Signatures:
 
 	@pytest.fixture(scope='class')