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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_assigned_value(self, name):
 """Get the assigned value of an attribute. Get the underlying value of an attribute. If value has not been set, will not return the default for the field. Args: name: Name of attribute to get. Returns: Value of attribute, None if it has not been set. """

        message_type = type(self)
        try:
            field = message_type.field_by_name(name)
        except KeyError:
            raise AttributeError('Message %s has no field %s' % (
                message_type.__name__, name))
        return self.__tags.get(field.number)