Smart contract,in short,is a technology that digitizes the contract in our life and can be automatically executed by the program when a condition is met.For example,you made an agreement with me.We decided on reward and punishment measures,and then entered the agreement into the blockchain in the form of code.Once the agreed conditions are triggered,a program will automatically execute it,which is smart contract

　　從部署的智能合約中讀取數(shù)據(jù)

　　在我們的合約中，有一個(gè)方法getGreeting()可以檢索我們?cè)趨^(qū)塊鏈中添加的問(wèn)候語(yǔ)。我們將使用web3.py調(diào)用此方法打開(kāi)您的Python IDLE編輯器并使用以下代碼創(chuàng)建一個(gè)新文件read.py。運(yùn)行py read.py讀取問(wèn)候語(yǔ)。

　　import json

　　from web3 importWeb3,HTTPProvider

　　from web3.contract importConciseContract

　　#compile your smart contract with truffle first

　　truffleFile=json.load(open('./build/contracts/greeter.json'))

　　abi=truffleFile['abi']

　　bytecode=truffleFile['bytecode']

　　#web3.py instance

　　w3=Web3(HTTPProvider("https://ropsten.infura.io/<ApI Key here>"))

　　print(w3.isConnected())開(kāi)發(fā)功能及流程I59模式2OO7開(kāi)發(fā)3O69

　　contract_address=Web3.toChecksumAddress("<Deployed Contract Address here>")

　　#Instantiate and deploy contract

　　contract=w3.eth.contract(abi=abi,bytecode=bytecode)

　　#Contract instance

　　contract_instance=w3.eth.contract(abi=abi,address=contract_address)

　　#Contract instance in concise mode

　　#contract_instance=w3.eth.contract(abi=abi,address=contract_address,ContractFactoryClass=ConciseContract)

　　#Getters+Setters for web3.eth.contract object ConciseContract

　　#print(format(contract_instance.getGreeting()))需求及案例：yy625019

　　print('Contract value:{}'.format(contract_instance.functions.getGreeting().call()))

　　導(dǎo)入的web3庫(kù)和所有其他必需的模塊

　　通過(guò)指向Ropsten Infura節(jié)點(diǎn)啟動(dòng)web3提供程序

　　通過(guò)指向abi和contract_Address創(chuàng)建合約實(shí)例（來(lái)自上一步）

　　調(diào)用getGreeting()方法并在控制臺(tái)打印結(jié)果