Address error when no gating veto argument was given (#3513)

* Address error when no gating veto argument was given

* Fix error when no gating veto argument is given

* add comment on default empty dict assignment

* shorten too long line

Co-authored-by: Thomas Dent <thomas.dent@usc.es>

pycbc/io/hdf.py

@@ -987,8 +987,9 @@ def to_coinc_xml_object(self, file_name):
         ligolw_utils.write_filename(outdoc, file_name)
 
 class ReadByTemplate(object):
+    # default assignment to {} is OK for a variable used only in __init__
     def __init__(self, filename, bank=None, segment_name=None, veto_files=None,
-                 gating_veto_windows=None):
+                 gating_veto_windows={}):
         self.filename = filename
         self.file = h5py.File(filename, 'r')
         self.ifo = tuple(self.file.keys())[0]
@@ -1008,7 +1009,7 @@ def __init__(self, filename, bank=None, segment_name=None, veto_files=None,
             veto_segs = veto.select_segments_by_definer(vfile, ifo=self.ifo,
                                                         segment_name=name)
             self.segs = (self.segs - veto_segs).coalesce()
-        if gating_veto_windows is not None:
+        if self.ifo in gating_veto_windows:
             gating_veto = gating_veto_windows[self.ifo].split(',')
             gveto_before = float(gating_veto[0])
             gveto_after = float(gating_veto[1])