How to get a contract storage data from another contract in smartPy?

Question

I'm trying to access a storage data of a contract from another one using the continuous passing style in smartPy.

import smartpy as sp
class XXIFA12(sp.Contract):
    def __init__(self, admin):
        self.init(balances = sp.big_map(tvalue = sp.TRecord(balance = sp.TNat)), administrator = admin, totalSupply = 0)

    @sp.entry_point
    def mint(self, params):
        sp.verify(sp.sender == self.data.administrator)
        self.addAddressIfNecessary(params.address)
        self.data.balances[params.address].balance += params.amount
        self.data.totalSupply += params.amount

    def addAddressIfNecessary(self, address):
        sp.if ~ self.data.balances.contains(address):
            self.data.balances[address] = sp.record(balance = 0)

    @sp.entry_point
    def getBalance(self, params):
       sp.transfer(
         self.data.balances[params.arg.owner].balance,
         sp.tez(0), 
         sp.contract(sp.TNat, params.target
       ).open_some())

class DIVIDENDS(sp.Contract):
    def __init__(self, admin, xxiAdd):
        self.init(
            xxiAddress = xxiAdd,
            balance = 0
        )

    @sp.entry_point
    def testGetBalance(self, params):
        assertTrasnferRecord = sp.record(arg = sp.record(owner = params.owner), target = params.address)
        entryPointName = "getBalance"
        c = sp.contract(
        t = sp.TRecord(arg = sp.TRecord(owner = sp.TAddress), target = sp.TAddress),
            address = self.data.xxiAddress,
            entry_point = entryPointName
        ).open_some()
        sp.transfer(assertTrasnferRecord, sp.mutez(0), c)

        # this is what I want to do but it doesn't compile
        #self.data.balance = sp.transfer(assertTrasnferRecord, sp.mutez(0), c)
if "templates" not in __name__:
    @sp.add_test(name = "FA12")
    def test():
        scenario = sp.test_scenario()
        scenario.h1("Dividends Contract")
        # sp.test_account generates ED25519 key-pairs deterministically:
        admin = sp.address("tz1g37HrEzgqVtCEs5ea5m3RhL6jfsvTiJfR")
        alice = sp.test_account("Alice")
        bob   = sp.test_account("Robert")
        # Let's display the accounts:
        scenario.h2("Accounts")
        scenario.show([alice, bob])
        c1 = XXIFA12(admin)
        scenario += c1
        scenario.h2("Admin mints a few coins")
        scenario += c1.mint(address = alice.address, amount = 12).run(sender = admin)

        c2 = DIVIDENDS(admin, c1.address)
        scenario += c2

        c3 = c2.testGetBalance(owner = alice.address, address = c2.address).run(sender = admin)
        scenario += c3

I got this error in smartpy.io :

Error: Error in Scenario
Unexpected error in transaction, please use .run(valid=False, ..)
ChannelNotFound: 'default'
default (12 : sp.TNat) line 64
Traceback (most recent call last):

Answer 1

There are several example templates, such as FA2 (real world token contracts) and Collatz (small demo and test template). https://www.smartpy.io/dev/?template=FA2.py https://www.smartpy.io/dev/?template=collatz.py

For your example, the error message is clearly unhelping but you should define an entry point to receive the value in a contract.

You cannot use testGetBalance to both send the request and receive the result of the request so you should define another entry point to receive the result.