Minor consistency issue with the way source_space["nearest_dist"] is saved

[papadop]

Description of the problem

The code in _source_space.py write the "nearest_dist" item as: write_float_matrix( fid, FIFF.FIFF_MNE_SOURCE_SPACE_NEAREST_DIST, this["nearest_dist"] )

But in all the examples, the data type stored is a list of floats (which is a fiff type different from an array). I guess the python code manages to read them back because it is weakly typed, but it would be better to be consistent and not multiply the data types.

The correction is probably as simple as (I chose the vector version because this is what exists in test files and because this piece of data is indeed a vector): write_float(fid, FIFF.FIFF_MNE_SOURCE_SPACE_NEAREST_DIST, this["nearest_dist"]).

If the array version is preferred, it might be better to add some test files that contain arrays....

Steps to reproduce

Difficult to see without modifying the code. Add some print of type info in _read_tag_header (tag.py untested):
def _read_tag_header(fid, pos):
    """Read only the header of a Tag."""
    fid.seek(pos, 0)
    s = fid.read(16)
    if len(s) != 16:
        where = fid.tell() - len(s)
        extra = f" in file {fid.name}" if hasattr(fid, "name") else ""
        warn(f"Invalid tag with only {len(s)}/16 bytes at position {where}{extra}")
        return None
    # struct.unpack faster than np.frombuffer, saves ~10% of time some places
    kind, type_, size, next_ = struct.unpack(">iIii", s)
    if kind==FIFF_MNE_SOURCE_SPACE_NEAREST_DIST:
        if type_==FIFF.FIFFT_FLOAT and size!=4:
            print("Float Vector")
        if type_==FIFF.FIFFT_MATRIX:
            print("Float Array")
    return Tag(kind, type_, size, next_, pos)

Then use:

from mne.minimum_norm import read_inverse_operator
inv = read_inverse_operator('DATA_PATH/sample_audvis-eeg-oct-6-eeg-inv.fif')
mne.minimum_norm.write_inverse_operator('/tmp/toto-inv.fiff',inv)
inv1 = read_inverse_operator('/tmp/toto-inv.fiff')

Link to data

No response

Expected results

The two read should show the same type.

Float Vector Float Vector

Actual results

Float Vector Float Array

Additional information

Platform Linux-6.6.13-200.fc39.x86_64-x86_64-with-glibc2.38 Python 3.12.1 (main, Dec 18 2023, 00:00:00) [GCC 13.2.1 20231205 (Red Hat 13.2.1-6)] Executable /usr/bin/python CPU (20 cores) Memory 62.5 GB

Core ├☑ mne 1.6.1 (latest release) ├☑ numpy 1.24.4 (OpenBLAS 0.3.21 with 20 threads) ├☑ scipy 1.11.1 ├☑ matplotlib 3.8.2 (backend=QtAgg) ├☑ pooch 1.5.2 └☑ jinja2 3.1.3

Numerical (optional) ├☑ sklearn 1.3.0 ├☑ nibabel 5.1.0 ├☑ nilearn 0.10.3 ├☑ pandas 1.5.3 └☐ unavailable numba, dipy, openmeeg, cupy

Visualization (optional) ├☑ pyvista 0.42.3 (OpenGL 4.6 (Core Profile) Mesa 23.3.5 via Mesa Intel(R) Graphics (ADL GT2)) ├☑ pyvistaqt 0.11.0 ├☑ vtk 9.3.0 ├☑ qtpy 2.4.0 (PyQt5=5.15.12) ├☑ pyqtgraph 0.13.3 └☐ unavailable ipympl, mne-qt-browser, ipywidgets, trame_client, trame_server, trame_vtk, trame_vuetify

Ecosystem (optional) └☐ unavailable mne-bids, mne-nirs, mne-features, mne-connectivity, mne-icalabel, mne-bids-pipeline None