[Ethereum] Python and Solidity keccak256 function gives different results

try this snipet

function bytes32ToString(bytes32 x) constant returns (string) {
    bytes memory bytesString = new bytes(32);
    uint charCount = 0;
    for (uint j = 0; j < 32; j++) {
        byte char = byte(bytes32(uint(x) * 2 ** (8 * j)));
        if (char != 0) {
            bytesString[charCount] = char;
            charCount++;
        }
    }
    bytes memory bytesStringTrimmed = new bytes(charCount);
    for (j = 0; j < charCount; j++) {
        bytesStringTrimmed[j] = bytesString[j];
    }
    return string(bytesStringTrimmed);
}

 function someFunction(bytes32 _string) returns(bool){
    bool result = false;
    bytes32 hashparameter = sha3(bytes32ToString(_string));
    bytes32 hashstring = sha3("hello");
    if(hashstring == hashparameter) {
        result = true;
    }

    return result;

}

the first function converts bytes32 to string then passes the result to hashing function. I've tried it gives true for "hello".

Web3.soliditySha3() will give you the correct hash.

The underlying reason is that solidity 0-pads values in arrays, which you won't get by default in most implementations. web3.py has the convenience method just for this purpose.

In this example, you would use:

from web3 import Web3
hash = Web3.soliditySha3(['uint8[]'], [[1, 2, 3, 4, 5]])
assert hash.hex() == '0x5917e5a395fb9b454434de59651d36822a9e29c5ec57474df3e67937b969460c'