python - Parse value to None in ndb custom property -


i have custom ndb property subclass should parse empty string none. when return none in _validate function, none value ignored , empty string still used.
can somehow cast input values none?

class booleanproperty(ndb.booleanproperty):     def _validate(self, value):         v = unicode(value).lower()         # '' should casted none somehow.         if v == '':             return none         if v in ['1', 't', 'true', 'y', 'yes']:             return true         if v in ['0', 'f', 'false', 'n', 'no']:             return false         raise typeerror('unable parse value %r boolean value.' % value) 

maybe looking ndb.computedproperty?

class yourbool(ndb.model):     my_input = stringproperty()     val = ndb.computedproperty(         lambda self: true if self.my_input in ["1","t","true","y","yes"] else false) 

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