ABF: raise a clear error on files with an invalid signature

neo/rawio/axonrawio.py

@@ -490,8 +490,13 @@ def parse_axon_soup(filename):
 
     Returns
     -------
-    dict or None
-        Header dictionary with file metadata, or None if file signature is invalid
+    dict
+        Header dictionary with file metadata.
+
+    Raises
+    ------
+    NeoReadWriteError
+        If the file does not start with a valid ABF signature (b"ABF " or b"ABF2").
     """
     with open(filename, "rb") as fid:
         f = StructFile(fid)
@@ -502,7 +507,14 @@ def parse_axon_soup(filename):
         elif signature == b"ABF2":
             return _parse_abf_v2(f, headerDescriptionV2)
         else:
-            return None
+            # The first 4 bytes are the ABF magic; anything else means the file is not an ABF
+            # file, is corrupt, or is an unsupported variant. Raise here rather than returning
+            # None so the caller gets a clear error instead of a downstream NoneType access.
+            raise NeoReadWriteError(
+                f"Could not parse {filename} as an ABF file: expected the header to start with "
+                f"signature b'ABF ' or b'ABF2', but found {signature}. The file is not an ABF "
+                f"file, is corrupt, or is an unsupported variant."
+            )
 
 
 def _parse_abf_v1(f, header_description):

neo/test/rawiotest/test_axonrawio.py

@@ -2,6 +2,7 @@
 import unittest
 
 from neo.rawio.axonrawio import AxonRawIO, parse_axon_soup
+from neo.core import NeoReadWriteError
 
 from neo.test.rawiotest.common_rawio_test import BaseTestRawIO
 
@@ -29,6 +30,20 @@ def test_read_raw_protocol(self):
 
         reader.read_raw_protocol()
 
+    def test_unparseable_file_raises(self):
+        # A file whose header does not start with a valid ABF signature must raise a clear error
+        # rather than a cryptic NoneType error deep in parsing. The fixture has a zeroed signature.
+        path = self.get_local_path("axon/intracellular_data/files_with_errors/unparseable_header.abf")
+        expected_msg = (
+            f"Could not parse {path} as an ABF file: expected the header to start with signature "
+            f"b'ABF ' or b'ABF2', but found b'\\x00\\x00\\x00\\x00'. The file is not an ABF file, "
+            f"is corrupt, or is an unsupported variant."
+        )
+        reader = AxonRawIO(filename=path)
+        with self.assertRaises(NeoReadWriteError) as cm:
+            reader.parse_header()
+        self.assertEqual(str(cm.exception), expected_msg)
+
     def test_v1_reads_real_acquisition_date(self):
         # ABF1 stores the calendar date in lFileStartDate (a YYYYMMDD-packed integer). Older neo
         # ignored that field and hardcoded 1900-01-01, so the recording date was always wrong for