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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 addExtensionArg(self, namespace, key, value):
 """Add an extension argument to this OpenID authentication request. Use caution when adding arguments, because they will be URL-escaped and appended to the redirect URL, which can easily get quite long. @param namespace: The namespace for the extension. For example, the simple registration extension uses the namespace C{sreg}. @type namespace: str @param key: The key within the extension namespace. For example, the nickname field in the simple registration extension's key is C{nickname}. @type key: str @param value: The value to provide to the server for this argument. @type value: str """

        self.message.setArg(namespace, key, value)