How to store negative values inside {8||16||32} bits in populus?

Question

I want to store -1 in smallest memory such as in {8||16||32} bits instead of 256 bits.

int i = -1;

When I return var, it returns -1L instead of -1.

I have assigned -1 into a int8 variable. As I know int8 is signed integer.

int8 i = -1;

But when I return output, it returns: '115792089237316195423570985008687907853269984665640564039457584007913129639679L' instead of -1.

[Q] What should I do to obtain -1?

Please note that I have experimented this problem in populus:

As contract I have use @RichardHorrocks's contract: NumTest on the answer.

test.py:

def test_receipt(web3, accounts, chain, unmigrated_chain):
    my_contract       = unmigrated_chain.get_contract('NumTest');

    set_txn_hash     = my_contract.transact().NumTest();
    contract_address = unmigrated_chain.wait.for_receipt(set_txn_hash)

    output           = my_contract.call().number();
    print(output);

Thank you for your valuable time and help.

Answer 1

This code returns -1 for me (for both int8 and int):

pragma solidity ^0.4.6;

contract NumTest {

    int8 i;

    function NumTest() {
        i = -1;
    }

    function number() constant returns (int8 num) {
        return i;    
    }
}

As has been mentioned in previous threads, due to unpacking costs, the int8 case (278 gas) is actually more expensive than the default int/int256 case (224 gas).

For the above code, and for completeness (and because I was interested):

integer bits | gas cost
-------------+---------
     8       |   278
    16       |   278
    32       |   278
    64       |   278
   128       |   278
   248       |   278
   256       |   224

So unpacking costs are the same for anything below the optimal size.