def validate(self, value, model=None, context=None):
if value not in self.choices:<EOL><INDENT>return Error(self.invalid_choice)<EOL><DEDENT>return Error()<EOL>
Validate
Perform value validation against validation settings and return
error object.

:param value:           str, value to check
:param model:           parent model being validated
:param context:         object or None, validation context
:return:                shiftschema.result.Error