contract TestContract {
    address public owner;
    address public someAddress;

    function TestContract() public {
        owner = msg.sender;
    }

    modifier onlyOwner {
        assert(msg.sender == owner);
        _;
    }

    function setSomeAddress(address _newAddress) onlyOwner public {
        someAddress = _newAddress;
    }
}

transact_params = {
    'from': addr_owner,
    'gas': 21000,
    'gas_price': 1,
}
my_contract_instance.transact(transact_params).setSomeAddress(w3.eth.accounts[1])

File "/usr/local/lib/python3.5/dist-packages/web3/contract.py", line 624, in _find_matching_fn_abi
    raise ValueError("No matching functions found")