fix inheritance problem with mixin

- a mixin should not inherit from module then it has Parameters
- Parameters in mixins must be complete, not just overrides
- check precedence of read_<param> or handler

Change-Id: I72d9355a1982770d1a99d9552a20330103c97edb

secop_psi/historywriter.py

@@ -43,9 +43,17 @@ def make_cvt_list(dt, tail=''):
     else:
         return []   # ArrayType, BlobType and TextType are ignored: too much data, probably not used
     result = []
+    print('START', dt)
     for subkey, elmtype in items:
+        print('MAKE_CVT_LIST', subkey, elmtype)
         for fun,  tail_, opts in make_cvt_list(elmtype, '%s.%s' % (tail, subkey)):
-            result.append((lambda v, k=subkey, f=fun: f(v[k]), tail_, opts))
+            def conv(value, key=subkey, func=fun):
+                try:
+                    return value[key]
+                except KeyError:  # can not use value.get() because value might be a list
+                    return None
+            result.append((conv, tail_, opts))
+    print('END', result)
     return result