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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 thumbnail_url(source, alias):
 """ Return the thumbnail url for a source file using an aliased set of thumbnail options. If no matching alias is found, returns an empty string. Example usage:: <img src="{{ person.photo|thumbnail_url:'small' }}" alt=""> """

    try:
        thumb = get_thumbnailer(source)[alias]
    except Exception:
        return ''
    return thumb.url