generalise findByFieldName: filter included

formfield.py

@@ -20,6 +20,7 @@ The information stored for the fields are:
 import subprocess as cmd
 import os.path
 import fdfgen
+import readformdata
 
 
 class FormField():
@@ -331,19 +332,29 @@ class FormField():
             self[info] = value
 
     @staticmethod
-    def findByFieldName(fieldList, fieldName):
+    def findByFieldName(fieldList, fieldName,
+                        include=(lambda x: (readformdata.NON_FORMFIELD
+                                            not in x))):
         """Return a FormField out of a list given the searched fieldName.
 
         If there are several fields with this fieldName,
         the first one is returned. This should not be the case though.
 
+        Attributes:
+            fieldList ([FormField]): list of formfields to search in
+            fieldName (str): the fieldName looked for
+            include (fct: FormField -> Bool):
+                fct. that returns True if the argument should be looked for
+                Default: only use FormFields that do not have the info
+                NON_FORMFIELD.
+
         Raises:
             KeyError: if no such FieldName exist.
 
         """
         for f in fieldList:
             try:
-                if f["FieldName"] == fieldName:
+                if include(f) and f["FieldName"] == fieldName:
                     return f
             except KeyError:
                 pass  # should not happen but who knows?