How to use the function inside a deployed contract? [BadFunctionCallOutput]

• Version: 0.15.3
• Python: 2.7
• OS: linux

Here is an example to develop a smart contract by python which is written by myself.

Suppose the RPC port of geth node is 8200, and there are at least 2 accounts in keystore. At least 2 python package are required (web3, py-solc)

sudo pip install web3
sudo pip install py-solc

Input following code into python command console:

source_code = 'contract MyToken {     address issuer;     mapping (address => uint) balances;      event Issue(address account, uint amount);     event Transfer(address from, address to, uint amount);      function MyToken() {         issuer = msg.sender;     }      function issue(address account, uint amount) {         if (msg.sender != issuer) throw;         balances[account] += amount;     }      function transfer(address to, uint amount) {         if (balances[msg.sender] < amount) throw;          balances[msg.sender] -= amount;         balances[to] += amount;          Transfer(msg.sender, to, amount);     }      function getBalance(address account) constant returns (uint) {         return balances[account];     } }'

import web3
obj = web3.Web3(web3.HTTPProvider('http://localhost:8200'))
psn = web3.personal.Personal(obj)
psn.unlockAccount(obj.eth.accounts[0],"123456",1000) #replace 123456 to your Ethereum accounts password

from solc import compile_source
compile_sol = compile_source(source_code)

my_contract = web3.contract.construct_contract_factory(
    web3=obj, 
    abi = compile_sol['<stdin>:MyToken']['abi'],
    code = compile_sol['<stdin>:MyToken']['bin'], 
    code_runtime = compile_sol['<stdin>:MyToken']['bin-runtime'],
    source = source_code
)

my_contract.bytecode = my_contract.code
trans_hash = my_contract.deploy(transaction={'from':obj.eth.accounts[0],'value':120})

The contract should be deploy on the blockchain if this transaction is mined.

But when I try to call the function inside the contract, there are some exceptions occured:

my_contract.call({'to':obj.eth.accounts[0]}).getBalance(obj.eth.accounts[0])

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python2.7/dist-packages/eth_utils/string.py", line 85, in inner`
    return force_obj_to_text(fn(*args, **kwargs))
  File "/usr/local/lib/python2.7/dist-packages/web3/contract.py", line 779, in call_contract_function
    raise_from(BadFunctionCallOutput(msg), e)
  File "/usr/local/lib/python2.7/dist-packages/web3/contract.py", line 767, in call_contract_function
    output_data = decode_abi(output_types, return_data)
  File "/usr/local/lib/python2.7/dist-packages/eth_abi/abi.py", line 108, in decode_abi
    return decoder(stream)
  File "/usr/local/lib/python2.7/dist-packages/eth_abi/decoding.py", line 102, in __call__
    return self.decode(stream)
  File "/usr/local/lib/python2.7/dist-packages/eth_utils/functional.py", line 22, in inner
    return callback(fn(*args, **kwargs))
  File "/usr/local/lib/python2.7/dist-packages/eth_abi/decoding.py", line 140, in decode
    yield decoder(stream)
  File "/usr/local/lib/python2.7/dist-packages/eth_abi/decoding.py", line 102, in __call__
    return self.decode(stream)
  File "/usr/local/lib/python2.7/dist-packages/eth_abi/decoding.py", line 165, in decode
    raw_data = cls.read_data_from_stream(stream)
  File "/usr/local/lib/python2.7/dist-packages/eth_abi/decoding.py", line 247, in read_data_from_stream
    cls.data_byte_size,
web3.exceptions.BadFunctionCallOutput: Could not decode contract function call getBalance return data 0x for output_types [u'uint256']

How to deal with this problem? Is there any examples to reference? Thank you very much!

[weipin]

Glad to hear it worked.

@pipermerriam May I suggest to put together a step by step tutorial of web3.py, with code snippets of deploying a contract (likely a standard ERC20 one), executing methods, etc. That would be another issue though.

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[weipin]

my_contract.call({'to':obj.eth.accounts[0]}).getBalance(obj.eth.accounts[0])

Here seems the account address was passed as to, instead of a contract address.

Assuming we use testrpc and the contract address is available immediately, the code snippet of obtaining a contract address and making a call could be something like this:

trans_hash = my_contract.deploy(...)
txn_receipt = web3.eth.getTransactionReceipt(trans_hash)
contract_address = txn_receipt['contractAddress']
my_contract.call({'to':contract_address}).getBalance(web3.eth.accounts[0])

Also, the arg transaction of the method call is optional (default to None), so if we assign my_contract a proper contract address, we can omit the arg transaction.

my_contract = web3.eth.contract(abi=compile_sol[':MyToken']['abi'], address=contract_address)
my_contract.call().getBalance(web3.eth.accounts[0])

Hope it makes sense to you.