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class MailboxManager:    cache = {}    parent = None        def __init__(self, parent):        self.parent = parent    def get_contents(self, mbox):        if (not self.cache.has_key(mbox)):            values = []            self.parent.server.select(mbox)            typ, data = self.parent.server.search(None, 'ALL')            for num in data[0].split():                data = self.parent.server.fetch(num, '(BODY[HEADER.FIELDS (FROM)])')                string = data[1][0][1].strip()                values.append(string)            self.cache[mbox] = values        return self.cache[mbox]