web3.py: eth_abi.exceptions.InsufficientDataBytes: Tried to read 32 bytes. Only got 0 bytes

• Version: 3.8.1
• Python: 3.5
• OS: linux
• Parity v1.7.0-unstable-b0e4c91-20170502/x86_64-linux-gnu/rustc1.17.0

What was wrong?

While trying to fix #181, I updated my toolchain and now I’m having problems interacting with my contract.

Setting up web3 works fine

# load contract ABI
contractAbi = json.loads(open('../contracts/MeasPub.abi').read())
measpub = web3.eth.contract(abi = contractAbi)
f=open('MeasPub.address')
measpub.address = f.read().strip()
f.close()
f=open('publisher.address') 
publisher_addr = f.read().strip()
f.close()

web3.personal.unlockAccount(account = publisher_addr, passphrase='*******')

but then I want to call a function

      N = measpub.call({'from':publisher_addr}).getSubscriberCount() 

which fails with:

Traceback (most recent call last):
  File "/home/ethadm/.virtualenvs/dapp/lib/python3.5/site-packages/web3/contract.py", line 767, in call_contract_function
    output_data = decode_abi(output_types, return_data)
  File "/home/ethadm/.virtualenvs/dapp/lib/python3.5/site-packages/eth_abi/abi.py", line 108, in decode_abi
    return decoder(stream)
  File "/home/ethadm/.virtualenvs/dapp/lib/python3.5/site-packages/eth_abi/decoding.py", line 102, in __call__
    return self.decode(stream)
  File "/home/ethadm/.virtualenvs/dapp/lib/python3.5/site-packages/eth_utils/functional.py", line 22, in inner
    return callback(fn(*args, **kwargs))
  File "/home/ethadm/.virtualenvs/dapp/lib/python3.5/site-packages/eth_abi/decoding.py", line 140, in decode
    yield decoder(stream)
  File "/home/ethadm/.virtualenvs/dapp/lib/python3.5/site-packages/eth_abi/decoding.py", line 102, in __call__
    return self.decode(stream)
  File "/home/ethadm/.virtualenvs/dapp/lib/python3.5/site-packages/eth_abi/decoding.py", line 165, in decode
    raw_data = cls.read_data_from_stream(stream)
  File "/home/ethadm/.virtualenvs/dapp/lib/python3.5/site-packages/eth_abi/decoding.py", line 247, in read_data_from_stream
    len(data),
eth_abi.exceptions.InsufficientDataBytes: Tried to read 32 bytes.  Only got 0 bytes

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "publisher.py", line 96, in <module>
    main()
  File "publisher.py", line 62, in main
    N = measpub.call({'from':publisher_addr}).getSubscriberCount() 
  File "/home/ethadm/.virtualenvs/dapp/lib/python3.5/site-packages/eth_utils/string.py", line 85, in inner
    return force_obj_to_text(fn(*args, **kwargs))
  File "/home/ethadm/.virtualenvs/dapp/lib/python3.5/site-packages/web3/contract.py", line 779, in call_contract_function
    raise_from(BadFunctionCallOutput(msg), e)
  File "/home/ethadm/.virtualenvs/dapp/lib/python3.5/site-packages/web3/utils/exception_py3.py", line 2, in raise_from
    raise my_exception from other_exception
web3.exceptions.BadFunctionCallOutput: Could not decode contract function call getSubscriberCount return data 0x for output_types ['uint256']

And here’s my contract, which I did successfully test with truffle/mocca//testRPC:

pragma solidity ^0.4.6;
contract MeasPub         
{
	address public publisher;
	string public description;
	uint public price_per_second; // [wei per second]


	// dictionary that maps addresses to balances
	// always be careful about overflow attacks with numbers
	mapping (address => uint) private balances;

	// dictionary that remebers the timestamp when the last measurement was published to that subscriber
	mapping (address => uint) private last_publish_block;

	// dictionary that provides an iterable list of subscribers
	mapping (uint => address) private subscriber_index;
	uint subscriber_count;

	// dictionary for RSA public keys
	mapping (address => string) private subscriber_pubkey;

	event LogPublished(
	address _subscriber,    // the subscriber
	bytes _pwdenc,          // the encoded password needed for decryption of data stored in ipfs
	bytes _ipfsaddr,        // the ipfs address where the encrypted measurement can be fetched by the subscriber
	uint _cost		 // cost of single publishing item
	);
	
	event LogDebug(
	string _msg
	);

	// The constructor. It accepts a string input that describes the nature of this publishing.
	function MeasPub() public
	{
		publisher = msg.sender;
		price_per_second = 0; // Wei
	}

	// the publisher may change pricing anytime
	function setPricePerSecond(uint _price) public
	{
		if (msg.sender == publisher)
		price_per_second = _price;
	}

	function publish(address _subscriber, bytes _pwdenc, bytes _ipfsaddr) public returns (bool covered)
	{
		if (msg.sender == publisher)
		{
			uint cost;
			cost = (now - last_publish_block[_subscriber]) * price_per_second;
			if (balances[_subscriber] >= cost)
			{
				balances[_subscriber] -= cost;
				// send money to publisher
				if (!publisher.send(cost)) 
				{
					balances[_subscriber] += cost;	
					return false;
				}
				last_publish_block[_subscriber] = now;
				LogPublished(_subscriber, _pwdenc, _ipfsaddr, cost);
				return true;
			}
			LogDebug("subscriber has insufficient funds");
			return false;
			
		}
		LogDebug("only publisher can publish");
		return false;
	}

	function getSubscriberCount() public returns (uint count)
	{
		//if (msg.sender == publisher)
		return subscriber_count;
	}

	function getSubscriber(uint _index) public returns (address _subscriber, string _pubkey)
	{
		if (msg.sender == publisher)
		return (subscriber_index[_index],subscriber_pubkey[subscriber_index[_index]]);
	}

	function subscribe(string _pubkey) payable public returns (bool success) {
		if (last_publish_block[msg.sender] == uint(0x0))
		{
			// subscriber is new to us
			last_publish_block[msg.sender] = now;
			subscriber_index[subscriber_count] = msg.sender;
			subscriber_count += 1;
		}
		subscriber_pubkey[msg.sender] = _pubkey;
		balances[msg.sender] += msg.value;
		LogDebug("new subscription successful");
		return true;
	}
	


	/**********
	Standard kill() function to recover funds
	**********/
	function kill()
	{
		if (msg.sender == publisher)
		suicide(publisher);  // kills this contract and sends remaining funds back to creator
	}

}