Write Lottery Contract 🔗︎

Make sure python contracting package installed before you start this tutorial.

Check this link to get more details.

Here is the completed Git repo for this part

There are only a few things that we need to do in a lottery.

  1. Buy a lottery ticket
  2. Randomly pick the winner
  3. Get payout
  4. Repeat the lottery game

1. Initialization 🔗︎

We need to create some instance variables to manage the state of the lottery. You can understand their role through the comments in the following code.

owner = Variable() # the owner of this contract
current_round = Variable() # Indicates the current lottery round
min_amount = Variable() # Minimum purchase amount for a ticket
interval_seconds = Variable() # 
genesis_round_run =  Variable() # Indicates whether the first round of the lottery has started
total = Variable() # The lottery jackpot
tickets = Variable() # self-explanatory

rounds = Hash() # Lottery info
user_rounds = Hash() # Save the round number of the lotteries we've played so far

Then, we need to seed our lottery contact with some initial states.

def seed():
    owner.set(ctx.caller) # set owner to the submitter of this contract
    min_amount.set(1) # set min purchase amount to 1 tau
    interval_seconds.set(3600) # one hour
    current_round.set(1) # set initial round number to 1
    genesis_round_run.set(False) # default false
    total.set(0) # default 0
    tickets.set(['Banana', 'Grape', 'Lemon', 'Orange', 'Peach', 'Pineapple'])

The seed method with the @construct decorator will execute only once when the contract is submitted. We can use this method to set up some initial states.

2. Run a lottery 🔗︎

Let's run a lottery now! First we need to create a method to end the current lottery round and start the next.

The method run() is used to start next and end the current round. Only owner has the right to call it.

def run():
    # Only owner can call this method
    assert owner.get() == ctx.caller, 'Only owner can execute start method.' 

    current = current_round.get()

    # For genesis round, don't need to end last round and calculate the rewards.
    if not genesis_round_run.get():

    # End last round

    # Calculate rewards

    # Increment current round to next round
    next_round = current + 1

    # start next round

Note that the start_round() and end_round() methods without any decorators are both private methods. They can only be used internally by this lottery contract.

There is a special variable called now in the blockchain which will be the current time in a transaction. Learn more about the now click here.

def start_round(round_num: int):
    # Get current time
    now_time = now
    # Set start time
    rounds[round_num, "startTime"] = now_time
    # Set end time
    rounds[round_num, "endTime"] = now_time + datetime.timedelta(seconds=interval_seconds.get())
    rounds[round_num, "status"] = "Starting"

    for ticket in tickets.get():
        rounds[round_num, "betInfo", ticket] = []

def end_round(round_num: int):
    # Make sure this round can be ended.
    assert rounds[round_num, "endTime"] is not None and rounds[round_num, "endTime"] < now, \
        f"Round #{round_num} not ended"

    # Mark this round is ended
    rounds[round_num, "status"] = "Ended"

As you can see, we use the module random to draw a winning ticket. Note that the method random.seed() need to be run once per transaction. Otherwise, the transaction will fail. Besides the method seed(), the rest of the module follows Python random 1:

  1. Learn more about the random click here.
def calculate_rewards():

    current = current_round.get()

    # Called once per transaction
    # Calculate the winning ticket
    winning_ticket = tickets.get()[random.randint(1,6)]

    winner_bet_amount = 0
    total_amount = 0

    for ticket in tickets.get():
        for value in rounds[current, "betInfo", ticket]:
            if winning_ticket == ticket:
                winner_bet_amount = winner_bet_amount + value.get('amount')
            total_amount = total_amount + value.get('amount')
    # total amount of all winners in this round
    rounds[current, "winnerBetAmount"] = winner_bet_amount
    # total sales this round
    rounds[current, "totalAmount"] = total_amount
    # total prize in this round
    rounds[current, "totalAwards"] = total_amount + total.get()
    # Winning tickets
    rounds[current, "winTicket"] = winning_ticket

    if winner_bet_amount > 0:
        # If winner exists, empty the prize pool
        # If there is no winner, the bonus will be accumulated into the prize pool
        total.set(total.get() + total_amount)

Wow, now we can buy a lottery ticket!

def buy(ticket: str, amount: float, round_num: int):
    # Check whether the ticket is correct.
    assert ticket in tickets.get(), f'Ticket #{ticket} not exists'
    # Ensure the amount is larger than or equal to the minimum purchase amount.
    assert amount >= min_amount.get(), f'At least {min_amount.get()} Taus are required.'

    # nsure the specified round has started
    assert rounds[round_num, "startTime"] is not None and rounds[round_num, "startTime"] <= now, \
        f'Round #{round_num} not started'
    # Ensure the specified round not ended
    assert rounds[round_num, "endTime"] is not None and rounds[round_num, "endTime"] >= now, \
        f'Round #{round_num} has ended'

    caller = ctx.caller

    # transfer the funds of the caller to this contract
    currency.transfer_from(amount=amount, to=ctx.this, main_account=caller)

    # Store bet info 
    rounds[round_num, "betInfo", ticket].append({
        'buyer': caller,
        'amount': amount
    rounds[round_num, "betInfo", ticket] = rounds[round_num, "betInfo", ticket]

    # Inital
    if user_rounds[caller] is None:
        user_rounds[caller] = []

    # Store the round numbers that the caller played
    if round_num not in user_rounds[caller]:
        # Ensure data stored 
        user_rounds[caller] = user_rounds[caller]

Finally, we can claim our winnings by calling the method claim(). Make sure the current lottery round is ended and the player is eligible for claim.

def claim(round_num: int):
    caller = ctx.caller
    # Check whether the round is ended 
    assert rounds[round_num, "status"] == "Ended", f"Claim failed, round #{round_num} not ended"
    # Check whether you have claimed your winnings
    assert not rounds[round_num, caller, "claimed"], "You have claimed for this round"

    # Do calculations
    winning_ticket = rounds[round_num, "winTicket"]
    bet_amount = 0
    for value in rounds[round_num, "betInfo", winning_ticket]:
        if value.get('buyer') == caller:
            bet_amount = bet_amount + value.get('amount')

    # Check whether you won the prize
    assert bet_amount > 0, "Not eligible for claim"

    winner_bet_amount = rounds[round_num, "winnerBetAmount"]
    total_amount = rounds[round_num, "totalAmount"]
    rewards = (bet_amount / winner_bet_amount) * total_amount
    rounds[round_num, caller, "claimed"] = True

    # transfer the prizes to caller
    currency.transfer(amount=rewards, to=caller)

3. Schedule a job 🔗︎

Unfortunately, we can not schedule a job in Lamden Blockchain at present. In order to run a lottery regularly, we need a script to call the method run() of the lottery contract.

Create file schedule.py. Install python package lamden and requests.

pip install lamden
pip install requests

Next copy the following code into schedule.py. Update the value of these variables my_wallet, masternode_url and contract with you own.

# Import libraries
from lamden.crypto.wallet import Wallet
from lamden.crypto.transaction import build_transaction
import requests

def run():
    # Create wallet
    my_wallet = Wallet('<Private key>')

    # Get Nonce
    # mainnet: https://masternode-01.lamden.io
    # testnet: https://testnet-master-1.lamden.io
    masternode_url = 'Master node url'

    res = requests.get(f'{masternode_url}/nonce/{my_wallet.verifying_key}')

    nonce = res.json()['nonce']
    processor = res.json()['processor']

    stamps = 200 

    # Pushing a transaction is similar to intracting with smart contracts via the client
    contract = 'Your contact name'
    function = 'run'

    tx = build_transaction(

    # You can submit the transaction through any Python HTTP library
    response = requests.post(masternode_url, data=tx, verify=False)

if __name__ == '__main__':

It's time to schedule a job to run this python script every 1 hour. You can choose whatever timing tools you like. For me, I will use crond which is a computer program in Linux that can be used to make a computer do tasks at specific time intervals.

Execute following command in shell. It will open the editing interface.

crontab -e

Add following code to the end. This code will let the system know when to run the schedule.py script.

0 */1 * * * python3 /<your_path>/schedule.py

4. Writing Tests 🔗︎

In order to test whether the lottery contract works, we need to write some tests. Here is the entire unit test file:

import imp
import unittest
from contracting.client import ContractingClient
from contracting.stdlib.bridge.time import Timedelta

class MyTestCase(unittest.TestCase):
     # Will be called before per test
    def setUp(self):
        self.client = ContractingClient()

        # Submit the currency contract as the dependency of lottert contact
        with open('currency.py') as f:
            code = f.read()
            self.client.submit(code, name='currency')

        # Submit the lottery contract
        with open('./lottery.py') as f:
            code = f.read()
            self.client.submit(code, name='lottery')

        self.lottery = self.client.get_contract('lottery')
        self.currency = self.client.get_contract('currency')

    # Will be called after per test
    def tearDown(self):
        # Reset the contracting client

    # Test seed method 
    def test_seed(self):
        self.assertEqual(self.lottery.quick_read('min_amount'), 1)
        self.assertEqual(self.lottery.quick_read('interval_seconds'), 3600)
        self.assertEqual(self.lottery.quick_read('current_round'), 1)

    def test_buy_error(self):
        self.currency.approve(amount=1000, to='lottery', signer='dapiguabc')

        # Should raise error if ticket is incorrect
        with self.assertRaisesRegex(AssertionError, 'Ticket #ErrorTicket not exists'):
            self.lottery.buy(ticket = 'ErrorTicket', amount = 10, round_num = 1, signer='dapiguabc')

        # Should raise error if amount is less than the min purchase amount
        with self.assertRaisesRegex(AssertionError, 'At least 1 Taus are required'):
            self.lottery.buy(ticket = 'Banana', amount = 0, round_num = 1, signer='dapiguabc')

        # Should raise error if the lottery round is not started
        with self.assertRaisesRegex(AssertionError, 'Round #2 not started'):
            self.lottery.buy(ticket = 'Banana', amount = 10, round_num = 2)

    # Test whether we can buy a ticket successfully
    def test_buy(self):
        self.currency.approve(amount=1000, to='lottery', signer='dapiguabc')
        self.lottery.buy(ticket = 'Banana', amount = 10, round_num = 1, signer='dapiguabc')

        self.assertEqual(self.lottery.user_rounds['dapiguabc'], [1])
        self.assertEqual(self.lottery.rounds[1, 'betInfo', 'Banana'][0], {
            'buyer': 'dapiguabc',
            'amount': 10

    # Test whether we can end current round and start next
    def test_run_next_round(self):
        self.currency.approve(amount=1000, to='lottery', signer='dapiguabc')
        self.lottery.buy(ticket = 'Banana', amount = 10, round_num = 1, signer='dapiguabc')
        env = {'now': self.lottery.now() + Timedelta(seconds=100000)}
        self.assertEqual(self.lottery.quick_read('current_round'), 2)

    # Test whether we can claim the prize
    def test_claim(self):
        self.currency.approve(amount=1000, to='lottery', signer='dapiguabc')

        # Buy the all tickets to make we can be the winner.
        self.lottery.buy(ticket = 'Banana', amount = 10, round_num = 1, signer='dapiguabc')
        self.lottery.buy(ticket = 'Grape', amount = 10, round_num = 1, signer='dapiguabc')
        self.lottery.buy(ticket = 'Lemon', amount = 10, round_num = 1, signer='dapiguabc')
        self.lottery.buy(ticket = 'Orange', amount = 10, round_num = 1, signer='dapiguabc')
        self.lottery.buy(ticket = 'Peach', amount = 10, round_num = 1, signer='dapiguabc')
        self.lottery.buy(ticket = 'Pineapple', amount = 10, round_num = 1, signer='dapiguabc')

        # Mock the env time to make the current round ended. 
        env = {'now': self.lottery.now() + Timedelta(seconds=100000)}

        self.assertFalse(self.lottery.rounds[1, 'dapiguabc', "claimed"])

        self.lottery.claim(round_num=1, signer='dapiguabc')

        self.assertTrue(self.lottery.rounds[1, 'dapiguabc', "claimed"])

if __name__ == '__main__':

Deployment 🔗︎

After this you should have a contract ready to be deployed on the blockchain. It's easy to manually upload the lottery contract through Lamden Vault.

Open your lamden vault and click the item Smart Contracts on the left sidebar.


Next you should click button SUBMIT TO NETWORK to upload your contract.


Finally, click the button SUBMIT CONTRACT on the bottom of the modal and wait a minute to check the result. Unless something unexpected happens, you'll see that your contract is deployed on the blockchain.

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