Fix mock_electrodes ignoring n_electrodes when sizing the electrodes table (#2214)

Co-authored-by: Ryan Ly <310197+rly@users.noreply.github.com>

Author: h-mayorquin

CHANGELOG.md

@@ -1,5 +1,11 @@
 # PyNWB Changelog
 
+## PyNWB 4.0.1 (Unreleased)
+
+### Fixed
+- Fixed `mock_electrodes` (and `mock_ElectricalSeries`) sizing the auto-created `ElectrodesTable` to a fixed 5 rows while the `DynamicTableRegion` followed `n_electrodes`, which raised an `IndexError` under HDMF 4.x+ for any data with more than 5 channels. The table is now sized to `n_electrodes`. @h-mayorquin [#2214](https://github.com/NeurodataWithoutBorders/pynwb/pull/2214)
+
+
 ## PyNWB 4.0.0 (June 29, 2026)
 
 ### Removed

src/pynwb/testing/mock/ecephys.py

@@ -65,7 +65,7 @@ def mock_electrodes(
         n_electrodes: int = 5, table: Optional[DynamicTable] = None, nwbfile: Optional[NWBFile] = None
 ) -> DynamicTableRegion:
 
-    table = table or mock_ElectrodesTable(n_rows=5, nwbfile=nwbfile)
+    table = table or mock_ElectrodesTable(n_rows=n_electrodes, nwbfile=nwbfile)
     return DynamicTableRegion(
         name="electrodes",
         data=list(range(n_electrodes)),

tests/unit/test_mock.py

@@ -1,3 +1,5 @@
+import numpy as np
+
 from pynwb import NWBHDF5IO
 
 from pynwb.testing.mock.file import mock_Subject, mock_NWBFile
@@ -33,6 +35,7 @@
 from pynwb.testing.mock.ecephys import (
     mock_ElectrodeGroup,
     mock_ElectrodesTable,
+    mock_electrodes,
     mock_ElectricalSeries,
     mock_SpikeEventSeries,
     mock_Units,
@@ -103,6 +106,20 @@ def test_mock_TimeSeries_w_no_time():
     assert ts.rate == 10.0
 
 
+def test_mock_electrodes_sizes_table_to_n_electrodes():
+    """The auto-created table must have exactly n_electrodes rows so the region of n_electrodes is in range."""
+    region = mock_electrodes(n_electrodes=128)
+    assert len(region.data) == 128
+    assert len(region.table) == 128
+
+
+def test_mock_ElectricalSeries_more_than_five_channels():
+    """mock_ElectricalSeries must support data with more than the default 5 channels."""
+    electrical_series = mock_ElectricalSeries(data=np.ones((10, 128)))
+    assert electrical_series.data.shape[1] == 128
+    assert len(electrical_series.electrodes.table) == 128
+
+
 @pytest.mark.parametrize("mock_function", mock_functions)
 def test_mock_write(mock_function, tmp_path):
     if mock_function is mock_NWBFile: