temp merge in mne-tools#1477 to get CIs green

Author: drammock

doc/whats_new.rst

@@ -27,6 +27,7 @@ The following authors contributed for the first time. Thank you so much! 🤩
 The following authors had contributed before. Thank you for sticking around! 🤘
 
 * `Stefan Appelhoff`_
+* `Daniel McCloy`_
 
 
 Detailed list of changes
@@ -61,6 +62,7 @@ Detailed list of changes
 - Updated MEG/iEEG writers to satisfy the stricter checks in the latest BIDS validator releases: BTi/4D run folders now retain their ``.pdf`` suffix (falling back to the legacy naming when an older validator is detected), KIT marker files encode the run via the ``acq`` entity instead of ``run``, datasets lacking iEEG montages receive placeholder ``electrodes.tsv``/``coordsystem.json`` files, and the ``AssociatedEmptyRoom`` entry stores dataset-relative paths  by `Bruno Aristimunha`_ (:gh:`1449`)
 - Made the lock helpers skip reference counting when the optional ``filelock`` dependency is missing, preventing spurious ``AttributeError`` crashes during reads, by `Bruno Aristimunha`_ (:gh:`1469`)
 - Fixed a bug in :func:`mne_bids.read_raw_bids` that caused it to fail when reading BIDS datasets where the acquisition time was specified in local time rather than UTC only in Windows, by `Bruno Aristimunha`_ (:gh:`1452`)
+- Fixed bug in :func:`~mne_bids.write_raw_bids` where incorrect unit was sometimes written into ``channels.tsv`` file when converting data to BrainVision, EDF, or EEGLAB formats, by `Daniel McCloy`_ (:gh:`1477`)
 
 ⚕️ Code health
 ^^^^^^^^^^^^^^

mne_bids/tests/test_write.py

@@ -3433,7 +3433,7 @@ def test_convert_eeg_formats(dir_name, fmt, fname, reader, tmp_path):
     # load channels.tsv; the unit should be Volts
     channels_fname = bids_output_path.copy().update(suffix="channels", extension=".tsv")
     channels_tsv = _from_tsv(channels_fname)
-    assert channels_tsv["units"][0] == "V"
+    assert channels_tsv["units"][0] == "µV"
 
     if fmt == "BrainVision":
         assert Path(raw2.filenames[0]).suffix == ".eeg"

mne_bids/write.py

@@ -136,7 +136,7 @@ def _should_use_bti_pdf_suffix() -> bool:
     return use_pdf_suffix
 
 
-def _channels_tsv(raw, fname, overwrite=False):
+def _channels_tsv(raw, fname, *, convert_fmt, overwrite=False):
     """Create a channels.tsv file and save it.
 
     Parameters
@@ -145,6 +145,10 @@ def _channels_tsv(raw, fname, overwrite=False):
         The data as MNE-Python Raw object.
     fname : str | mne_bids.BIDSPath
         Filename to save the channels.tsv to.
+    convert_fmt : str | None
+        Which format the data are being converted to (determines what gets written in
+        the "unit" column). If ``None`` then we assume no conversion is happening (the
+        raw data files are being copied from the source tree into the BIDS folder tree).
     overwrite : bool
         Whether to overwrite the existing file.
         Defaults to False.
@@ -193,11 +197,32 @@ def _channels_tsv(raw, fname, overwrite=False):
         ch_type.append(map_chs[_channel_type])
         description.append(map_desc[_channel_type])
     low_cutoff, high_cutoff = (raw.info["highpass"], raw.info["lowpass"])
-    if raw._orig_units:
+    # If data are being *converted*, unit is determined by destination format:
+    #   - `eeglabio.raw.export_set` always converts V to µV, cf:
+    # https://github.com/jackz314/eeglabio/blob/3961bb29daf082767ea44e7c7d9da2df10971c37/eeglabio/raw.py#L57
+    #
+    #   - `mne.export._edf_bdf._export_raw_edf_bdf` always converts V to µV, cf:
+    # https://github.com/mne-tools/mne-python/blob/1b921f4af5154bad40202d87428a2583ef896a00/mne/export/_edf_bdf.py#L61-L63
+    #
+    #   - `pybv.write_brainvision` converts V to µV by default (and we don't alter that)
+    # https://github.com/bids-standard/pybv/blob/2832c80ee00d12990a8c79f12c843c0d4ddc825b/pybv/io.py#L40
+    # https://github.com/mne-tools/mne-bids/blob/1e0a96e132fc904ba856d42beaa9ddddb985f1ed/mne_bids/write.py#L1279-L1280
+    if convert_fmt:
+        volt_like = "µV" if convert_fmt in ("BrainVision", "EDF", "EEGLAB") else "V"
+        units = [
+            volt_like
+            if ch_i["unit"] == FIFF.FIFF_UNIT_V
+            else _unit2human.get(ch_i["unit"], "n/a")
+            for ch_i in raw.info["chs"]
+        ]
+    # if raw data is merely copied, check `raw._orig_units`
+    elif raw._orig_units:
         units = [raw._orig_units.get(ch, "n/a") for ch in raw.ch_names]
+    # If `raw._orig_units` is missing, assume SI units
     else:
         units = [_unit2human.get(ch_i["unit"], "n/a") for ch_i in raw.info["chs"]]
-        units = [u if u not in ["NA"] else "n/a" for u in units]
+    # fixup "NA" (from `_unit2human`) → "n/a"
+    units = [u if u not in ["NA"] else "n/a" for u in units]
 
     # Translate from MNE to BIDS unit naming
     for idx, mne_unit in enumerate(units):
@@ -2229,7 +2254,6 @@ def write_raw_bids(
         emptyroom_fname=associated_er_path,
         overwrite=overwrite,
     )
-    _channels_tsv(raw, channels_path.fpath, overwrite)
 
     # create parent directories if needed
     _mkdir_p(os.path.dirname(data_path))
@@ -2288,6 +2312,14 @@ def write_raw_bids(
                 f"for {datatype} datatype."
             )
 
+    # this can't happen until after value of `convert` has been determined
+    _channels_tsv(
+        raw,
+        channels_path.fpath,
+        convert_fmt=format if convert else None,
+        overwrite=overwrite,
+    )
+
     # raise error when trying to copy files (copyfile_*) into same location
     # (src == dest, see https://github.com/mne-tools/mne-bids/issues/867)
     if (