Check if list is not empty

xfel_calibrate/calibrate.py

@@ -478,10 +478,12 @@ def flatten_list(l):
     :return: Same string or string with first and last entry of a list
     """
     if isinstance(l, list):
-        if len(l)>1:
+        if len(l) > 1:
             return '{}-{}'.format(l[0], l[-1])
-        else:
+        elif len(l) == 1:
             return '{}'.format(l[0])
+        else:
+            return '{}'
     else:
         return str(l)