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How does one call an entrypoint of an existing contract using SmartPy?

For example, I'd like to be able to write a contract that calls the getBalance method of the existing ETHtz FA1.2 contract and stores the return value in local state.

In other words, I'd like to complete this code:

import smartpy as sp

class MyETHtzBalanceContract(sp.Contract):
    def __init__(self):
        self.init(num_ethtz = 0)

    @sp.entry_point
    def set_balance(self, ethtz_holder_address):
        # The address of the ETHtz FA1.2 contract.
        ethtz = sp.address("KT19at7rQUvyjxnZ2fBv7D9zc8rkyG7gAoU8")
        # How do I get this value for account `ethtz_holder_address`?
        # Note the ethtz contract defines a `getBalance` entrypoint of type
        # `(pair %getBalance (address :owner) (contract nat))`.
        self.data.num_ethtz = ...

BTW, I'm using the SmartPy web IDE.

Thanks!

1 Answer 1

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The getBalance entrypoint is a view entrypoint that requires a parameter of type pair address (contract nat). ETHtz was written back when on-chain read functions were not available on Tezos, so a callback was required to retrieve values from another contract.

class ETHtzBalance(sp.Contract):
    def __init__(self):
        self.init(
            ethtz_bal = sp.nat(0),
            ethtz_address = sp.address("KT19at7rQUvyjxnZ2fBv7D9zc8rkyG7gAoU8")
        )
    
    @sp.entry_point
    def set_balance_callback(self, balance):
        sp.set_type(balance, sp.TNat)

        sp.verify(sp.sender == self.data.ethtz_address, "Not authorised")

        self.data.ethtz_bal = balance
    
    @sp.entry_point
    def set_balance(self, ethtz_holder_address):
        c = sp.contract(
            sp.TPair(sp.TAddress, sp.TContract(sp.TNat)), 
            self.data.ethtz_address, 
            "getBalance"
        ).open_some()
        sp.transfer(
            sp.pair(ethtz_holder_address, sp.self_entry_point("set_balance_callback")), 
            sp.tez(0), 
            c
        )    

Be careful about the SENDER check in the callback whenever you use a view function.

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