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Smart contract in stages

Let's look at the three Python scripts in the contract_in_stages directory in the Python API.

To run the code in this demo, you will need a running node. Details are here.

Details of the Python API are here.

Stage 1

The first script creates an Entity object and inserts the private key from the private.key file in the same directory.

Once this is done, the script gives the Entity 10000 tokens.

def main():
    print('Creating private key...')

    # create our first private key pair
    entity1 = Entity()

    # save the private key to disk
    with open('private.key', 'w') as private_key_file:

    print('Creating private key...complete')

    # build the ledger API
    api = LedgerApi('', 8100)

    print('Creating initial balance...')

    # create wealth so that we have the funds to be able to create contracts on the network
    api.sync(api.tokens.wealth(entity1, 10000))

    print('Creating initial balance...complete')

Stage 2

In the file, we embed the etch smart contract code into the script as a string.

We have an init() function which receives an Address object and sets its balance to 1000000 tokens in a State variable.

The transfer() function grabs or creates the State objects from two Address objects.

After a check to make sure there are enough funds in the from_account, the from_account transfers the amount to the to_account.

The query() function allows you to query the balance of an Address on the ledger via a State construct.

function setup(owner : Address)
  var owner_balance = State<UInt64>(owner);

function transfer(from: Address, to: Address, amount: UInt64)

  // define the accounts
  var from_account = State<UInt64>(from);
  var to_account = State<UInt64>(to); // if new sets to 0u

  // Check if the sender has enough balance to proceed
  if (from_account.get() >= amount)

    // update the account balances
    from_account.set(from_account.get() - amount);
    to_account.set(to_account.get(0u64) + amount);


function balance(address: Address) : UInt64
    var account = State<UInt64>(address);
    return account.get(0u64);

The Python script loads the private key created in the first step, connects to the running ledger node, and deploys the contract to the ledger paying 2000 tokens to do so and designating entity1 as the owner of the contract.

def main():

    print('Loading private key...')

    # load up the previously created private key
    with open('private.key', 'r') as private_key_file:
        entity1 = Entity.load(private_key_file)

    print('Loading private key...complete')

    # build the ledger API
    api = LedgerApi('', 8100)

    # create the smart contract
    contract = SmartContract(CONTRACT_TEXT)

    print('Deploying contract...')

    # deploy the contract to the network
    api.sync(api.contracts.create(entity1, contract, 2000))

    print('Deploying contract...complete')

    # save the contract to the disk
    with open('sample.contract', 'w') as contract_file:

Stage 3

In we load up the demo private key and the smart contract file.


Remember, the smart contract is now on the ledger and can be referenced with the correct hash.

We then perform a transfer using the contract code between the owner and a new user we create as entity2.

def main():
    # load up the previously created private key
    with open('private.key', 'r') as private_key_file:
        entity1 = Entity.load(private_key_file)

    # load up the deployed contract
    with open('sample.contract', 'r') as contract_file:
        contract = SmartContract.load(contract_file)

    # for the purposes of this example create a second private key pair to transfer funds to
    entity2 = Entity()

    # build the ledger API
    api = LedgerApi('', 8100)

    # print the current status of all the tokens
    print('-- BEFORE --')
    print_address_balances(api, contract, [entity1, entity2])

    # transfer from one to the other using our newly deployed contract
    tok_transfer_amount = 200
    fet_tx_fee = 40
    api.sync(contract.action(api, 'transfer', fet_tx_fee, [entity1], Address(entity1), Address(entity2),

    print('-- AFTER --')
    print_address_balances(api, contract, [entity1, entity2])
You should see a result like this:

-- BEFORE --
Address0: 8540   bFET 1000000    TOK
Address1: 0      bFET 0          TOK

-- AFTER --
Address0: 8508   bFET 999800     TOK
Address1: 0      bFET 200        TOK