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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 list_extensions(request):
 """List all nova extensions, except the ones in the blacklist."""

    blacklist = set(getattr(settings,
                            'OPENSTACK_NOVA_EXTENSIONS_BLACKLIST', []))
    nova_api = _nova.novaclient(request)
    return tuple(
        extension for extension in
        nova_list_extensions.ListExtManager(nova_api).show_all()
        if extension.name not in blacklist
    )