Negative commission isnt properly calculated?

[masafumimori]

Expected Behavior

Increased entry price when selling while it decreases when buying.

Actual Behavior

No matter if it is a positive or negative commission, it is counted as a positive value.
For example, when 0 commission is set, the entry price is 5,429,749 for buying. (calculated with atr values)
When setting 0.04% commission, it becomes 5,431,921, which is 1.0004x than the 0 commission.
When setting -0.04%, the entry price stays the same as 5,431,921. This should be 5,427,577.1004(5,429,749*0.9996)?

For selling, 5,458,457 for both -0.0004 and 0.0004% commission, which is 1.0004x less than commission 0 for 5,460,642.

Steps to Reproduce

1. Create any strategy
2. place order with limit price.
3. run backtest with negative commision like -0.0001
4. run backtest with positive commision like 0.0001
5. compare the result and entry prices in _trade

Additional info

I see the code accepting negative commision til -10% but negative commision doent seem to be working.

class _Broker:
    def __init__(self, *, data, cash, commission, margin,
                 trade_on_close, hedging, exclusive_orders, index):
        assert 0 < cash, f"cash should be >0, is {cash}"
        assert -.1 <= commission < .1, \
            ("commission should be between -10% "
             f"(e.g. market-maker's rebates) and 10% (fees), is {commission}")

Backtesting version: 0.3.3

I'm sorry if i am missing something for negative commission settings.

[mehrnoosh-hk]

@masafumimori
The problem is copysign function behavior, copysing(x,y) returns a float with the magnitude (absolute value) of x but the sign of y. The _adjusted_price method, which adjust the price, uses copysign and therefore always use the absolute value of commission

def _adjusted_price(self, size=None, price=None) -> float:
        """
        Long/short `price`, adjusted for commisions.
        In long positions, the adjusted price is a fraction higher, and vice versa.
        """
        return (price or self.last_price) * (1 + copysign(self._commission, size))

@kernc can I work on it?