Improve tests and be more robust

mne_bids/path.py

@@ -1310,11 +1310,12 @@ def get_bids_path_from_fname(fname, check=True, verbose=None):
         suffix, extension = _get_bids_suffix_and_ext(suffix)
     else:
         suffix = None
-        extension = Path(fname).suffix
+        extension = Path(fname).suffix  # already starts with a period
         if extension == '':
             extension = None
-        else:
-            extension = f'.{extension}'  # prepend period
+
+    if extension is not None:
+        assert extension.startswith('.')  # better safe than sorry
 
     datatype = _infer_datatype_from_path(fpath)
 

mne_bids/tests/test_path.py

@@ -1165,3 +1165,9 @@ def test_setting_entities():
 
         setattr(bids_path, entity_name, None)
         assert getattr(bids_path, entity_name) is None
+
+
+def test_deprecation():
+    """Test deprecated behavior."""
+    with pytest.warns(FutureWarning, match='This will raise an exception'):
+        BIDSPath(extension='vhdr')  # no leading period