BF: Allow save and load of empty trx

trx/tests/test_cli.py

@@ -30,6 +30,27 @@
 fetch_data(get_testing_files_dict(), keys=["DSI.zip", "trx_from_scratch.zip"])
 
 
+def _normalize_dtype_dict(dtype_dict):
+    """Normalize dtype dict to use explicit little-endian byte order.
+
+    On little-endian systems, numpy may use '=' (native) or '<' (explicit)
+    interchangeably. This normalizes all dtypes to '<' for consistent comparison.
+    """
+    normalized = {}
+    for key, value in dtype_dict.items():
+        if isinstance(value, dict):
+            normalized[key] = _normalize_dtype_dict(value)
+        elif isinstance(value, np.dtype):
+            # Normalize to little-endian for multi-byte types
+            if value.byteorder == "=" and value.itemsize > 1:
+                normalized[key] = value.newbyteorder("<")
+            else:
+                normalized[key] = value
+        else:
+            normalized[key] = value
+    return normalized
+
+
 # Tests for standalone CLI commands (trx_* commands)
 class TestStandaloneCommands:
     """Tests for standalone CLI commands."""
@@ -391,7 +412,13 @@ def test_execution_manipulate_trx_datatype(self):
                 "groups": {"g_AF_L": np.dtype("int32"), "g_AF_R": np.dtype("int32")},
             }
 
-            assert DeepDiff(trx.get_dtype_dict(), expected_dtype) == {}
+            assert (
+                DeepDiff(
+                    trx.get_dtype_dict(),
+                    _normalize_dtype_dict(expected_dtype),
+                )
+                == {}
+            )
             trx.close()
 
             generated_dtype = {
@@ -416,5 +443,11 @@ def test_execution_manipulate_trx_datatype(self):
             out_gen_path = os.path.join(tmp_dir, "generated.trx")
             manipulate_trx_datatype(expected_trx, out_gen_path, generated_dtype)
             trx = tmm.load(out_gen_path)
-            assert DeepDiff(trx.get_dtype_dict(), generated_dtype) == {}
+            assert (
+                DeepDiff(
+                    trx.get_dtype_dict(),
+                    _normalize_dtype_dict(generated_dtype),
+                )
+                == {}
+            )
             trx.close()

trx/tests/test_memmap.py

@@ -1,6 +1,8 @@
 # -*- coding: utf-8 -*-
 
 import os
+import tempfile
+import zipfile
 
 from nibabel.streamlines import LazyTractogram
 from nibabel.streamlines.tests.test_tractogram import make_dummy_streamline
@@ -341,7 +343,37 @@ def test_copy_fixed_arrays_from():
 
 
 def test_initialize_empty_trx():
-    pass
+    """Test creating, saving, and loading an empty TRX file."""
+    trx = tmm.TrxFile()
+    assert trx.header["NB_STREAMLINES"] == 0
+    assert trx.header["NB_VERTICES"] == 0
+    assert len(trx.streamlines) == 0
+
+    with tempfile.TemporaryDirectory() as tmp_dir:
+        out_path = os.path.join(tmp_dir, "empty.trx")
+        tmm.save(trx, out_path)
+
+        assert os.path.exists(out_path)
+        file_size = os.path.getsize(out_path)
+        assert file_size < 500  # Should be very small, just header.json in zip
+
+        with zipfile.ZipFile(out_path, "r") as zf:
+            filenames = [info.filename for info in zf.filelist]
+            assert "header.json" in filenames
+            positions_files = [f for f in filenames if f.startswith("positions")]
+            offsets_files = [f for f in filenames if f.startswith("offsets")]
+            assert len(positions_files) == 0
+            assert len(offsets_files) == 0
+
+        loaded_trx = tmm.load(out_path)
+        assert loaded_trx.header["NB_STREAMLINES"] == 0
+        assert loaded_trx.header["NB_VERTICES"] == 0
+        assert len(loaded_trx.streamlines) == 0
+        assert len(loaded_trx.groups) == 0
+        assert len(loaded_trx.data_per_streamline) == 0
+        assert len(loaded_trx.data_per_vertex) == 0
+        assert len(loaded_trx.data_per_group) == 0
+        loaded_trx.close()
 
 
 def test_create_trx_from_pointer():
@@ -421,8 +453,11 @@ def test__ensure_little_endian(dtype, test_value):
     # Result should be little-endian (or native if system is little-endian)
     assert result.dtype.byteorder in ("<", "=", "|")
 
-    # Values should be preserved
-    assert result[0] == test_value
+    # Values should be preserved (use isclose for float types due to precision)
+    if np.issubdtype(dtype, np.floating):
+        assert np.isclose(result[0], test_value, rtol=1e-6)
+    else:
+        assert result[0] == test_value
 
 
 def test__ensure_little_endian_big_endian_input():

trx/trx_file_memmap.py

@@ -935,23 +935,25 @@ def deepcopy(self) -> Type["TrxFile"]:  # noqa: C901
         json.dump(tmp_header, out_json)
         out_json.close()
 
-        positions_filename = _generate_filename_from_data(
-            to_dump, os.path.join(tmp_dir.name, "positions")
-        )
-        _ensure_little_endian(to_dump).tofile(positions_filename)
-
-        if not self._copy_safe:
-            to_dump = _append_last_offsets(
-                self.streamlines.copy()._offsets, self.header["NB_VERTICES"]
+        # Only write positions and offsets if TRX is not empty
+        if tmp_header["NB_STREAMLINES"] > 0 and tmp_header["NB_VERTICES"] > 0:
+            positions_filename = _generate_filename_from_data(
+                to_dump, os.path.join(tmp_dir.name, "positions")
             )
-        else:
-            to_dump = _append_last_offsets(
-                self.streamlines._offsets, self.header["NB_VERTICES"]
+            _ensure_little_endian(to_dump).tofile(positions_filename)
+
+            if not self._copy_safe:
+                to_dump = _append_last_offsets(
+                    self.streamlines.copy()._offsets, self.header["NB_VERTICES"]
+                )
+            else:
+                to_dump = _append_last_offsets(
+                    self.streamlines._offsets, self.header["NB_VERTICES"]
+                )
+            offsets_filename = _generate_filename_from_data(
+                self.streamlines._offsets, os.path.join(tmp_dir.name, "offsets")
             )
-        offsets_filename = _generate_filename_from_data(
-            self.streamlines._offsets, os.path.join(tmp_dir.name, "offsets")
-        )
-        _ensure_little_endian(to_dump).tofile(offsets_filename)
+            _ensure_little_endian(to_dump).tofile(offsets_filename)
 
         if len(self.data_per_vertex.keys()) > 0:
             os.mkdir(os.path.join(tmp_dir.name, "dpv/"))
@@ -1243,9 +1245,13 @@ def _create_trx_from_pointer(  # noqa: C901
         TrxFile
             A TrxFile constructed from the pointer provided.
         """
-        # TODO support empty positions, using optional tag?
         trx = TrxFile()
         trx.header = header
+
+        # Handle empty TRX files early - no positions/offsets to load
+        if header["NB_STREAMLINES"] == 0 and header["NB_VERTICES"] == 0:
+            return trx
+
         positions, offsets = None, None
         for elem_filename in dict_pointer_size.keys():
             if root_zip: