mapping's used gas changes based on index used as static value or a variable. Probably usedGas returns transaction instead of execution cost.

- platform darwin -- Python 2.7.12, pytest-3.0.2, py-1.4.31, pluggy-0.3.1
- plugins: populus-1.1.0
- OS: Mac OS X

You could see my question on the following link: http://ethereum.stackexchange.com/questions/13433/mappings-used-gas-changes-based-on-index-used-as-static-value-or-a-variable

Other people tried my code and observed that used gas is same for both two codes on solidity environment. But on the populus environment the usedGas differs. I have used `usedGas` parameter to obtain the tx gas value.

[Q] What may be the reason of the problem?

Here is my code:
**Used gas = 86658** (Probably usedGas returns Transaction Cost!)

self.list_[selfSize] = Interval( { num: e - 1, core: c, next: self.head });
self.list_[selfSize + 1] = Interval( { num: 10, core: 10, next: 10 });

//self.list_[selfSize] = Interval( { num: e - 1, core: c, next: self.head });//commented out!!
//self.list_[3] = Interval( { num: 10, core: 10, next: 10 });//commented out!!

-- or
**Used gas = 65220** (Probably usedGas returns Execution Cost!)

//self.list_[selfSize] = Interval( { num: e - 1, core: c, next: self.head });//commented out!!
//self.list_[selfSize + 1] = Interval( { num: 10, core: 10, next: 10 });//commented out!!

self.list_[selfSize] = Interval( { num: e - 1, core: c, next: self.head });//used GAS = 65220
self.list_[3] = Interval( { num: 10, core: 10, next: 10 });

--As you can see both code accessing to index value 2 and 3.

Contract: **Array.sol**

pragma solidity ^0.4.8;
library ReceiptLib {
struct Interval {
uint32 num;
int32 core;
uint32 next;
}

struct intervalNode {
uint32 den;
mapping(uint32 => Interval) list_;
uint32 len;
uint32 head;
int32 coreLimit;
}
function constructReceipt(intervalNode storage self){
self.head = 1; //self.listSize - 1;
self.coreLimit = 128;

self.list_[0] = Interval( { num: 0, core: 0, next: 0 });
self.list_[1] = Interval( { num: 0, core: 0, next: 0 });
self.len = 2;
}
function number(intervalNode storage self, uint32 s, uint32 e, int32 c){
uint32 selfSize = self.len;
//self.list_[selfSize] = Interval( { num: e - 1, core: c, next: self.head });
//self.list_[selfSize + 1] = Interval( { num: 10, core: 10, next: 10 });

self.list_[selfSize] = Interval( { num: e - 1, core: c, next: self.head });
self.list_[3] = Interval( { num: 10, core: 10, next: 10 });
}
}

contract Array{
using ReceiptLib for ReceiptLib.intervalNode;
ReceiptLib.intervalNode receiptList;

function Array(){
receiptList.constructReceipt();
}
function test(uint32 s, uint32 e, int32 c) {
receiptList.number(s, e, c);
}
}

**test.py:**
```
def test_receipt(web3, accounts, chain, unmigrated_chain):
my_contract = unmigrated_chain.get_contract('Array');
set_txn_hash = my_contract_1.transact().test(10, 20, 2);
contract_address = unmigrated_chain.wait.for_receipt(set_txn_hash)
print(contract_address["gasUsed"]);
```

Start with test.py, the Array contract, and the unmigrated_chain.get_contract and wait.for_receipt entry points. Reproduce both mapping-index variants and compare the reported gasUsed values; done means the discrepancy between transaction cost and execution cost is explained and the expected behavior is documented or corrected.

Written by the indexing model from the issue text.