Non-empty list validator in schema

Hi,

I want to validate a non-empty list of integers as part of a
schema.  The validation works provided that a key exists in the data for
the list, but if it doesn't exist the validator return an empty list:


>>> class S(Schema):
>>>
...   a = formencode.ForEach(formencode.validators.Int(),  
not_empty=True)
...


>>> S.to_python({'a':None})
>>>
formencode.api.Invalid: a: Please enter a value


>>> S.to_python({})
>>>
{'a': []}

I've tried setting other parameters, but I can't work out what to do  
about
this.  Any help would be greatly appreciated.

Thanks,

Graham

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