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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 isbns(self, key, value):
 """Populate the ``isbns`` key."""

    def _get_medium(value):
        def _normalize(medium):
            schema = load_schema('hep')
            valid_media = schema['properties']['isbns']['items']['properties']['medium']['enum']

            medium = medium.lower().replace('-', '').replace(' ', '')
            if medium in valid_media:
                return medium
            elif medium == 'ebook':
                return 'online'
            elif medium == 'paperback':
                return 'softcover'

            return ''

        medium = force_single_element(value.get('b', ''))
        normalized_medium = _normalize(medium)

        return normalized_medium

    def _get_isbn(value):
        a_value = force_single_element(value.get('a', ''))
        normalized_a_value = a_value.replace('.', '')
        if normalized_a_value:
            return normalize_isbn(normalized_a_value)

    return {
        'medium': _get_medium(value),
        'value': _get_isbn(value),
    }