Trying to send a token with web3.py, why is the receiver's balance not going up?

Question

When I use web3.py to send some coins from account1 to account2, account2 does not see a balance increase.

from web3 import Web3 ,HTTPProvider, IPCProvider
from solc import compile_source


addr = 'http://127.0.0.1:8545'
web3 = Web3(HTTPProvider(addr))

source_code = ' contract token { mapping (address => uint) public coinBalanceOf; event CoinTransfer(address sender, address receiver, uint amount);  function token(uint supply) { coinBalanceOf[msg.sender] = supply; } function sendCoin(address receiver, uint amount) returns(bool sufficient) { if (coinBalanceOf[msg.sender] < amount) return false; coinBalanceOf[msg.sender] -= amount; coinBalanceOf[receiver] += amount; CoinTransfer(msg.sender, receiver, amount); return true; } }'

web3.personal.unlockAccount('<insert account 1 here>','password!')


compile_sol = compile_source(source_code)
contract_address = '0xC7c1F1129726df4148B756c356Dc64cb27d763AC' #test-net 

Mycontract = web3.eth.contract(abi=compile_sol['<stdin>:token']['abi'], bytecode = compile_sol['<stdin>:token']['bin'],bytecode_runtime = compile_sol['<stdin>:token']['bin-runtime'])
my_contract = Mycontract(contract_address)

print my_contract.call().coinBalanceOf(Account1)
hashs = my_contract.transact({
    'from': web3.eth.coinbase,
    'to': '<insert account 2 here>',
    'value': 231,
    'gas': 702438
}).sendCoin('<insert account 2 here>', 231)

print web3.eth.getTransaction(hashs)
print "----------------------------------\n"
print web3.eth.getTransactionReceipt(hashs)
print  hashs+"\n"
print web3.eth.getTransactionCount(web3.eth.coinbase)

print my_contract.call().coinBalanceOf('<insert account 2 here>')

Account2 doesn't have any tokens. What do I have wrong?

Answer 1

When sending a token, do not set the to or value fields in the transact() method.

Original Code

hashs = my_contract.transact({
    'from': web3.eth.coinbase,
    'to': '<insert account 2 here>',
    'value': 231,
    'gas': 702438,
}).sendCoin('<insert account 2 here>', 231)

New Code

hashs = my_contract.transact({
    'from': web3.eth.coinbase,
    'gas': 702438,
}).sendCoin('<insert account 2 here>', 231)

Note that both of the removed values are specified in the contract method call sendCoin().

Why?

The to key

When you interact with a contract, you send a transaction to it. The receiving account is only mentioned in the call to the contract function.

This is an interesting point that many people don't understand at first: Account 2 does not have a record that it owns the new token. It must query the contract to discover its own token balance.

The value key

Value is always about transferring ether, denominated in wei. When you send the token, the only cost in ether will be the gas paid to run the contract.

Other Issues?

This is a long enough example that there may be other bugs hiding in there. I am not sure I found all the problems. If there are more, feel free to comment on my answer and add more details to your question about what's going on.