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PatractPy is a contract SDK to support the development of Python scripts that interact with contracts, including automated scripts to support testing. Unlike PatractGo, PatractPy is mainly for script development, so PatractPy mainly completes contract-related RPC interfaces, and completes contract deployment and instantiation-related operations.

PatractPy will provide support for europa env, which is a good environment for contract exec sandbox, With PatractPy, we can write contract unittest by python, which is more friendly to developer and can easy use other test tools.

PatractPy will be based on polkascan's Python Substrate Interface, which is a Python sdk for Substrate.

Element Group for disscusion:

PatractPy will achieve the following support:

  • Some support that missing in polkascan's Python Substrate Interface, which is needed for contracts
  • Provide Scanning and monitoring support for contract to do statistics and analysis
  • Provide a SDK development example for ERC20 contract
  • Support For unittest to canvas or europa env.

Basic Apis For Contracts#

As polkascan's Python Substrate Interface has provide some support to contract api, so we not need to important the api for contract calls, but there is some api to add:

  • SubstrateSubscriber is a subscriber support to subscribe data changes in chain, for example, the events in chain.
  • get_contract_event_type add event decode support for contracts.

The basic api split into 2 parts:

  • Contract, include:
    • contractExecutor: This api could construct an extrinsic to call a contract, would be packed into a block and change state.
    • contractReader: This api could construct a rpc request to call a contract, do not pack into a block and do not change any state.
    • contractCreator: This api is used for instantiate a contract and holding the WASM code and metadata, receive following parameters:
      • gas_limit
      • endowment
      • deployment_salt (salt parameter in instantiate)
    • ContractAPI: This api is used for call a contract, is a wrapper for contractExecutor and contractReader, developers could use this api to react with contracts. This api could create a instance depends on the metadata, auto generate the contract access functions based on the contract. And the auto-gen functions receive the parameters which defined in contracts, besides receive following common parameters:
      • gas_limit
      • value (notice, if current call's payable is false, this value must be 0)
    • ContractFactory: This api is used for constructing a contract instance in python, and developer could use this instance to access contract. This is a wrapper for contractCreator and ContractAPI
  • Observer, include:
    • ContractObserver: This api is used for listen the events in contracts.

All methods which belong to the instance of ContractAPI and ContractFactory receive a keypair as the first parameter, as the sender for this operation. And from the second parameter, receive the parameters defined in contracts.

ContractFactory and ContractAPI is used to react with contracts#

we add a factory to put code and deploy contracts to chain:

factory = ContractFactory.create_from_file(    substrate=substrate,     code_file=os.path.join(os.path.dirname(__file__), 'constracts', 'ink', 'erc20.wasm'),    metadata_file=os.path.join(os.path.dirname(__file__), 'constracts', 'ink', 'erc20.json'))
res = factory.put_code(alice) # alice is the keypair for `//Alice`print(res.is_succes)
# this api is `ContractAPI`api =, 1000000 * (10 ** 15), endowment=10**15, gas_limit=1000000000000)print(api.contract_address) # contract_address is the deployed contract

The factory will generate constructors from metadata file.

We add api by metadata for Contract, api will auto generate caller for contract from metadata:

# create a ContractAPI from an existed contract addressapi = ContractAPI(contract_address, contract_metadata, substrate)
# api will auto generate caller for contract from metadataalice_balance_old = api.balance_of(bob, alice.ss58_address) # bob is the keypair for `//Bob`
res = api.transfer(alice, bob.ss58_address, 100000, gas_limit=20000000000)'transfer res {res.error_message}')print(res.is_succes)
alice_balance = api.balance_of(bob, alice.ss58_address)'transfer alice_balance {alice_balance}')
bob_balance = api.balance_of(bob, bob.ss58_address)'transfer bob_balance {bob_balance}')

The api will generate exec and read api from metadata file, for example:

      {        "args": [          {            "name": "owner",            "type": {              "displayName": [                "AccountId"              ],              "type": 5            }          }        ],        "docs": [          " Returns the account balance for the specified `owner`.",          "",          " Returns `0` if the account is non-existent."        ],        "mutates": false,        "name": [          "balance_of"        ],        "payable": false,        "returnType": {          "displayName": [            "Balance"          ],          "type": 1        },        "selector": "0x56e929b2"      },

In api, can call by:

bob_balance = api.balance_of(bob, bob.ss58_address)'transfer bob_balance {bob_balance}')

ContractObserver is used to listen contracts events#

ContractObserver can observer events for a contract:

substrate=SubstrateInterface(url="ws://", type_registry_preset='canvas')contract_metadata = ContractMetadata.create_from_file(    metadata_file=os.path.join(os.path.dirname(__file__), 'constracts', 'ink', 'erc20.json'),    substrate=substrate)observer = ContractObserver("0x8eaf04151687736326c9fea17e25fc5287613693c912909cb226aa4794f26a48", contract_metadata, substrate)
# for some handlers (callbacks)observer.scanEvents()

The handler function can take the erc20 support as a example.

Special case: ERC20 API#

Except react contract by ContractAPI, developers could create the wrapper by themself to react with corresponding contract. py-contract create an ERC20 API as an example to show this.

ERC20 api provide a wapper to erc20 contract exec, read and observer events, it can be a example for contracts api calling.

# init apisubstrate=SubstrateInterface(url="ws://", type_registry_preset='canvas')
contract_metadata = ContractMetadata.create_from_file(    metadata_file=os.path.join(os.path.dirname(__file__), 'constracts', 'ink', 'erc20.json'),    substrate=substrate)
alice = Keypair.create_from_uri('//Alice')bob = Keypair.create_from_uri('//Bob')
# erc20 apierc20 = ERC20.create_from_contracts(    substrate= substrate,     contract_file= os.path.join(os.path.dirname(__file__), 'constracts', 'ink', 'erc20.wasm'),    metadata_file= os.path.join(os.path.dirname(__file__), 'constracts', 'ink', 'erc20.json'))
# deplay a erc20 contracterc20.put_and_deploy(alice, 1000000 * (10 ** 15))
# read total supplytotal_supply = erc20.totalSupply()
# transfererc20.transfer_from(alice,    from_acc=alice.ss58_address,     to_acc=bob.ss58_address,     amt=10000)
erc20.transfer(alice, bob.ss58_address, 10000)
# get balancealice_balance = erc20.balance_of(alice.ss58_address)
# approveerc20.approve(alice, spender=bob.ss58_address, amt=10000)
# get allowancealice_allowance = erc20.allowance(alice.ss58_address, bob.ss58_address)

ERC20Observer is a event observer for erc20 contract:

observer = ERC20Observer.create_from_address(    substrate = substrate,     contract_address = contract_address,    metadata_file= os.path.join(os.path.dirname(__file__), 'constracts', 'ink', 'erc20.json'))
def on_transfer(num, evt):"on_transfer in {} : {} {} {}".format(num, evt['from'], evt['to'], evt['value']))
def on_approval(num, evt):"on_approval in {} : {} {} {}".format(num, evt['owner'], evt['spender'], evt['value']))

observer.scanEvents(on_transfer = on_transfer, on_approval = on_approval)

Observer For Contracts#

ContractObserver is a observer to listen events by contract with a given address:

observer = ContractObserver.create_from_address(    substrate = substrate,     contract_address = 'contract_address',    metadata_file= os.path.join(os.path.dirname(__file__), 'constracts', 'ink', 'erc20.json'))
def on_transfer(num, evt):"on_transfer in {} : {} {} {}".format(num, evt['from'], evt['to'], evt['value']))
def on_approval(num, evt):"on_approval in {} : {} {} {}".format(num, evt['owner'], evt['spender'], evt['value']))
observer.scanEvents(from_num, to_num, {    'Transfer': on_transfer,    'Approve': on_approval})

handlers is a hander dictionary by name to hander function.

Unittest Node Environment#

PatractPy can support write contract unittest by node environment.

At First We need install europa.

from patractinterface.contracts.erc20 import ERC20from patractinterface.unittest.env import SubstrateTestEnv
class UnittestEnvTest(unittest.TestCase):    @classmethod    def setUp(cls):        # start env or use canvas for a 6s block        cls.env = SubstrateTestEnv.create_europa(port=39944)        cls.env.start_node()
        cls.api = SubstrateInterface(url=cls.env.url(), type_registry_preset=cls.env.typ(), type_registry=cls.env.types())        cls.alice = Keypair.create_from_uri('//Alice')        cls.bob = Keypair.create_from_uri('//Bob')
        cls.erc20 = ERC20.create_from_contracts(            substrate= cls.substrate,             contract_file= os.path.join(os.path.dirname(__file__), 'constracts', 'ink', 'erc20.wasm'),            metadata_file= os.path.join(os.path.dirname(__file__), 'constracts', 'ink', 'erc20.json')        )        cls.erc20.put_and_deploy(alice, 1000000 * (10 ** 15))
    def tearDown(cls):        cls.env.stop_node()
    def test_transfer(self):        self.erc20.transfer_from(alice,            from_acc=alice.ss58_address,             to_acc=bob.ss58_address,             amt=10000)        # some more test case
if __name__ == '__main__':    unittest.main()

By example, we can use python to write testcase for some complex logics, by europa, we can test the contracts for python scripts.