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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 get_instance(uri):
 """Return an instance of Page."""

    global _instances
    try:
        instance = _instances[uri]
    except KeyError:
        instance = Page(
            uri,
            client.get_instance()
        )
        _instances[uri] = instance

    return instance