BUG: Allow pass-through of correct units

mne/io/edf/edf.py

@@ -17,7 +17,7 @@
 
 import numpy as np
 
-from ...utils import verbose, logger, warn
+from ...utils import verbose, logger, warn, _validate_type
 from ..utils import _blk_read_lims, _mult_cal_one
 from ..base import BaseRaw, _get_scaling
 from ..meas_info import _empty_info, _unique_channel_names
@@ -141,17 +141,21 @@ def __init__(self, input_fname, eog=None, misc=None, stim_channel='auto',
                                                preload, include)
         logger.info('Creating raw.info structure...')
 
-        if units is not None and isinstance(units, str):
-            units = {ch_name: units for ch_name in info['ch_names']}
-        elif units is None:
+        _validate_type(units, (str, None, dict), 'units')
+        if units is None:
             units = dict()
+        elif isinstance(units, str):
+            units = {ch_name: units for ch_name in info['ch_names']}
 
         for k, (this_ch, this_unit) in enumerate(orig_units.items()):
-            if this_unit != "" and this_ch in units:
-                raise ValueError(f'Unit for channel {this_ch} is present in '
-                                 'the file. Cannot overwrite it with the '
-                                 'units argument.')
-            if this_unit == "" and this_ch in units:
+            if this_ch not in units:
+                continue
+            if this_unit not in ("", units[this_ch]):
+                raise ValueError(
+                    f'Unit for channel {this_ch} is present in the file as '
+                    f'{repr(this_unit)}, cannot overwrite it with the units '
+                    f'argument {repr(units[this_ch])}.')
+            if this_unit == "":
                 orig_units[this_ch] = units[this_ch]
                 ch_type = edf_info["ch_types"][k]
                 scaling = _get_scaling(ch_type.lower(), orig_units[this_ch])

mne/io/edf/tests/test_edf.py

@@ -80,7 +80,7 @@ def test_orig_units():
 def test_units_params():
     """Test enforcing original channel units."""
     with pytest.raises(ValueError,
-                       match=r"Unit for channel .* is present .* Cannot "
+                       match=r"Unit for channel .* is present .* cannot "
                        "overwrite it"):
         _ = read_raw_edf(edf_path, units='V', preload=True)
 
@@ -601,6 +601,7 @@ def test_ch_types():
     assert raw.ch_names == labels
 
     raw = read_raw_edf(edf_chtypes_path, infer_types=True)
+    data = raw.get_data()
 
     labels = ['Fp1-Ref', 'Fp2-Ref', 'F3-Ref', 'F4-Ref', 'C3-Ref', 'C4-Ref',
               'P3-Ref', 'P4-Ref', 'O1-Ref', 'O2-Ref', 'F7-Ref', 'F8-Ref',
@@ -617,3 +618,9 @@ def test_ch_types():
 
     assert raw.get_channel_types() == types
     assert raw.ch_names == labels
+
+    with pytest.raises(ValueError, match="cannot overwrite"):
+        read_raw_edf(edf_chtypes_path, units='V')
+    raw = read_raw_edf(edf_chtypes_path, units='uV')  # should be okay
+    data_units = raw.get_data()
+    assert_allclose(data, data_units)