Given the input dictionary, return the field value.

def get_value(self, dictionary):
        """
        Given the input dictionary, return the field value.
        """
        # We override the default field access in order to support
        # lists in HTML forms.
        if html.is_html_input(dictionary):
            return html.parse_html_list(dictionary, prefix=self.field_name)
        return dictionary.get(self.field_name, empty)