Get the namespace the given prefix maps to.

        Args:
            prefix (str): The prefix

        Returns:
            str: The namespace, or None if the prefix isn't mapped to
                anything in this set.

def namespace_for_prefix(self, prefix):
        """Get the namespace the given prefix maps to.

        Args:
            prefix (str): The prefix

        Returns:
            str: The namespace, or None if the prefix isn't mapped to
                anything in this set.
        """
        try:
            ni = self.__lookup_prefix(prefix)
        except PrefixNotFoundError:
            return None
        else:
            return ni.uri