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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 setKeyForAPI(cls, siteStore, apiName, apiKey):
 """ Set the API key for the named API, overwriting any existing key. @param siteStore: The site store to install the key in. @type siteStore: L{axiom.store.Store} @param apiName: The name of the API. @type apiName: C{unicode} (L{APIKey} constant) @param apiKey: The key for accessing the API. @type apiKey: C{unicode} @rtype: L{APIKey} """

        existingKey = cls.getKeyForAPI(siteStore, apiName)
        if existingKey is None:
            return cls(store=siteStore, apiName=apiName, apiKey=apiKey)
        existingKey.apiKey = apiKey
        return existingKey