Web3.py: How to use the function inside a deployed contract? [BadFunctionCallOutput]

[Gingko925]

• 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]

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.

[Gingko925]

@weipin Thank you very much for your patient answer!

I have tried 2 suggested methods to call the function. Both methods except the same Error:

web3.exceptions.BadFunctionCallOutput: Could not decode contract function call getBalance return data 0x for output_types [u'uint256']

Since the return type of getBalance() function is an unit256 type data, I guess there are some problems to decode return data. Is there any other potential solutions to solve this problem?

[Gingko925]

@weipin Your suggestion is so useful! After trying another example, I find that the error comes from contract deploy step.

[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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[pipermerriam]

Hello @Gingko925. If you can try the following modified version of your code. Please check your web3 version and update to the latest if you are not on 3.8.0

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];     } }'

from web3 import Web3  # The `web3.Web3` object is all you should need to import in most normal cases.
web3 = Web3(Web3.HTTPProvider('http://localhost:8200'))
web3.personal.unlockAccount(web3.eth.accounts[0],"123456",1000)

from solc import compile_source
compile_sol = compile_source(source_code)

# If an `address` is not passed into this method it returns a contract factory class.
MyContract = web3.eth.contract(
    abi = compile_sol['<stdin>:MyToken']['abi'],
    bytecode = compile_sol['<stdin>:MyToken']['bin'],   # The keyword `code` has been deprecated.  You should use `bytecode` instead.
    bytecode_runtime = compile_sol['<stdin>:MyToken']['bin-runtime'],  # the keyword `code_runtime` has been deprecated.  You should use `bytecode_runtime` instead.
)

trans_hash = MyContract.deploy(transaction={'from':web3.eth.accounts[0],'value':120})
# wait for mining
trans_receipt = web3.eth.getTransactionReceipt(trans_hash)

# get the contract address
contract_address = trans_receipt['contractAddress']

# now we can instantiate the contract factory to get an instance of the contract.
my_contract = MyContract(contract_address)

# now you should be able to call the contract methods.
my_contract.call().getBalance(obj.eth.accounts[0])

This was of course all typed by hand so there may be some typographical errors. In general this structure should work though. Let me know if it doesn't.

[jeanjluo]

Just wanted to point out the above code example has an issue. There is no "value" defined in the contract.
trans_hash = MyContract.deploy(transaction={'from':web3.eth.accounts[0],'value':120})

it should be:
trans_hash = MyContract.deploy(transaction={'from':web3.eth.accounts[0]})

Also please update the office web3.py doc: http://web3py.readthedocs.io/en/latest/contracts.html. it is not consistent with the instructions here.

[owocki]

Hi folks,

I cannot for the life of me figure out how to run a function inside a deployed contract without deploying the contract myself first.

Hopefully this is considered "inside the scope" of this issue, if not, I am happy to open another issue.

I've got the code in Piper's example running fine, but what I can't figure out is how to execute a method on a contract I do not myself own.

In the example:

trans_hash = MyContract.deploy(transaction={'from':web3.eth.accounts[0],'value':120})
# wait for mining
trans_receipt = web3.eth.getTransactionReceipt(trans_hash)
contract_address = trans_receipt['contractAddress']

It would seem to me that

1. one would be able to execute a contract that they do not deploy themselves, provided they have the abi, bytecode, (or the upstream solidity file).
2. it's horribly inefficient to deploy the contract to the chain _every time_ you want to interact with the contract on-chain

but if i try to run pipers example, minus the deploy:

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];     } }'

from web3 import Web3  # The `web3.Web3` object is all you should need to import in most normal cases.
web3 = Web3(Web3.HTTPProvider('http://localhost:8200'))
web3.personal.unlockAccount(web3.eth.accounts[0],"123456",1000)

from solc import compile_source
compile_sol = compile_source(source_code)

# If an `address` is not passed into this method it returns a contract factory class.
MyContract = web3.eth.contract(
    abi = compile_sol['<stdin>:MyToken']['abi'],
    bytecode = compile_sol['<stdin>:MyToken']['bin'],   # The keyword `code` has been deprecated.  You should use `bytecode` instead.
    bytecode_runtime = compile_sol['<stdin>:MyToken']['bin-runtime'],  # the keyword `code_runtime` has been deprecated.  You should use `bytecode_runtime` instead.
)

#trans_hash = MyContract.deploy(transaction={'from':web3.eth.accounts[0],'value':120})
# wait for mining
#trans_receipt = web3.eth.getTransactionReceipt(trans_hash)

# get the contract address
#contract_address = trans_receipt['contractAddress']
contract_address = 'some_address_which_has_already_been_deployed_to


# now we can instantiate the contract factory to get an instance of the contract.
my_contract = MyContract(contract_address)

# now you should be able to call the contract methods.
my_contract.call().getBalance(obj.eth.accounts[0])

I get an error message:

web3.exceptions.BadFunctionCallOutput: Could not decode contract function call getBalance return data 0x for output_types [u'uint256']

I've poked around the issues board and documentation, and I don't seem to see where this is covered end to end. Can anyone advise?

Thanks
@owocki

[owocki]

I cannot for the life of me figure out how to run a function inside a deployed contract without deploying the contract myself first.

I figured it out. testrpc (which is the RPC provider I am using, instead of localhost:8200 like the demo) resets after after single instanstiation of my script. Thereby, the contract was no longer deployed!

I switched over to a persistent testrpc instance and that did the trick.

Thanks for being my :rubber_ducky:

✌️ !

[pipermerriam]

Closing as I think this is resolved. I'm a big 👍 on having a "this is how you do common contract interactions" section for the documentation. If anyone wanted to write that I'd be grateful. I'm currently swamped with all the other things.

[gitcoinbot]

⚡️ A tip worth 0.08 ETH has been granted to @pipermerriam for this issue from @owocki. ⚡️

Nice work @pipermerriam, check your email for further instructions. | Send a Tip

[pipermerriam]

@owocki I briefly dug through both spam folders in my email accounts and can't find this alleged email.

[owocki]

@pipermerriam looks like it went to your gmail.com address... i just resent it to your pipermerriam dot com one