Allow an attribute to be set to no value (None, [], ..) before being assigning a value.

Author: rohe

src/oidcmsg/__init__.py

@@ -1,5 +1,5 @@
 __author__ = "Roland Hedberg"
-__version__ = "1.5.4"
+__version__ = "1.6.1"
 
 import os
 from typing import Dict

src/oidcmsg/configure.py

@@ -105,7 +105,7 @@ def __getattr__(self, item, default=None):
             return default
 
     def __setattr__(self, key, value):
-        if key in self:
+        if key in self and self.key:
             raise KeyError('{} has already been set'.format(key))
         super(Base, self).__setitem__(key, value)