Cómo acceder a los atributos de un contrato inteligente implementado usando web3.eth.contract(abi,address) en python

web3 = Web3(provider=HTTPProvider(http://ropsten.infura.io/v4/API_KEY)
abi = '[{"constant":false,"inputs":[{"name":"_utxoPos","type":"uint256"},{"name":"_txBytes","type":"bytes"},{"name":"_proof","type":"bytes"},{"name":"_sigs","type":"bytes"}],"name":"startExit","outputs":[],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":false,"inputs":[{"name":"_cUtxoPos","type":"uint256"},{"name":"_eUtxoIndex","type":"uint256"},{"name":"_txBytes","type":"bytes"},{"name":"_proof","type":"bytes"},{"name":"_sigs","type":"bytes"},{"name":"_confirmationSig","type":"bytes"}],"name":"challengeExit","outputs":[],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":true,"inputs":[{"name":"","type":"uint256"}],"name":"exits","outputs":[{"name":"owner","type":"address"},{"name":"token","type":"address"},{"name":"amount","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":false,"inputs":[{"name":"_token","type":"address"},{"name":"_amount","type":"uint256"}],"name":"startFeeExit","outputs":[],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":false,"inputs":[{"name":"srcTokenAddress","type":"address"},{"name":"destTokenAddress","type":"address"},{"name":"sendToUserAddresss","type":"address"},{"name":"maxDestAmount","type":"uint256"},{"name":"minConversionRate","type":"uint256"}],"name":"depositKyberTokens","outputs":[],"payable":true,"stateMutability":"payable","type":"function"},{"constant":true,"inputs":[],"name":"operator","outputs":[{"name":"","type":"address"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[],"name":"currentChildBlock","outputs":[{"name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":false,"inputs":[{"name":"_token","type":"address"}],"name":"finalizeExits","outputs":[],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":true,"inputs":[{"name":"_blockNumber","type":"uint256"}],"name":"getChildChain","outputs":[{"name":"","type":"bytes32"},{"name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[{"name":"_token","type":"address"}],"name":"getNextExit","outputs":[{"name":"","type":"uint256"},{"name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[],"name":"CHILD_BLOCK_INTERVAL","outputs":[{"name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[],"name":"currentDepositBlock","outputs":[{"name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":false,"inputs":[{"name":"_root","type":"bytes32"}],"name":"submitBlock","outputs":[],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":true,"inputs":[],"name":"getDepositBlock","outputs":[{"name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[],"name":"currentFeeExit","outputs":[{"name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":false,"inputs":[],"name":"deposit","outputs":[],"payable":true,"stateMutability":"payable","type":"function"},{"constant":true,"inputs":[{"name":"","type":"address"}],"name":"exitsQueues","outputs":[{"name":"","type":"address"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[{"name":"_utxoPos","type":"uint256"}],"name":"getExit","outputs":[{"name":"","type":"address"},{"name":"","type":"address"},{"name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":false,"inputs":[{"name":"_depositPos","type":"uint256"},{"name":"_token","type":"address"},{"name":"_amount","type":"uint256"}],"name":"startDepositExit","outputs":[],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":true,"inputs":[{"name":"","type":"uint256"}],"name":"childChain","outputs":[{"name":"root","type":"bytes32"},{"name":"timestamp","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"inputs":[],"payable":false,"stateMutability":"nonpayable","type":"constructor"},{"anonymous":false,"inputs":[{"indexed":true,"name":"depositor","type":"address"},{"indexed":true,"name":"depositBlock","type":"uint256"},{"indexed":false,"name":"token","type":"address"},{"indexed":false,"name":"amount","type":"uint256"}],"name":"Deposit","type":"event"},{"anonymous":false,"inputs":[{"indexed":true,"name":"exitor","type":"address"},{"indexed":true,"name":"utxoPos","type":"uint256"},{"indexed":false,"name":"token","type":"address"},{"indexed":false,"name":"amount","type":"uint256"}],"name":"ExitStarted","type":"event"},{"anonymous":false,"inputs":[{"indexed":false,"name":"root","type":"bytes32"},{"indexed":false,"name":"timestamp","type":"uint256"}],"name":"BlockSubmitted","type":"event"},{"anonymous":false,"inputs":[{"indexed":false,"name":"token","type":"address"}],"name":"TokenAdded","type":"event"}]'
contract_instance = web3.eth.contract(address, abi=abi)
currentChild = contract_instance.CHILD_BLOCK_INTERVAL.call()

Intento conectar mi contrato inteligente en Python y acceder a los atributos implementados en Ropsten pero obtengo un error:

AttributeError: el objeto 'Contrato' no tiene el atributo 'CHILD_BLOCK_INTERVAL'