Trade Size with fixed amount in $ ?

[pliciwebstephane]

Hello,

Is there a way to open a position with fixed amount, for exemple 100 $ .
Because, using size = 1, is to simulate buying one asset (for sample, if BTC is at 35000 $ , is for buying 1 BTC with 35000 $).

100 $ is 0,002857143 BTC

But if i open a position with size = 0,002857143, it is not 0,002857143 BTC but it is 0.2857143% of my wallet.

Is there a way to simulate a fixed amount of buying ?

Sorry for my poor english... :)

[atakanokan]

Seconded.

https://kernc.github.io/backtesting.py/doc/backtesting/backtesting.html#backtesting.backtesting.Order.size states

If size is a value between 0 and 1, it is interpreted as a fraction of current available liquidity (cash plus [Position.pl](https://kernc.github.io/backtesting.py/doc/backtesting/backtesting.html#backtesting.backtesting.Position.pl) minus used margin). A value greater than or equal to 1 indicates an absolute number of units.

which kind of suggests there is no way to trade in equal-dollar positions.

[kernc]

Not sure if the notification comes for this Q&A format. See #401 (comment).

[atakanokan]

Yep yep, received a notification from the below reply too

[kernc]

Buying for fixed $100 is rather easy:

        price = self.data.Close[-1]
        size = 100 // price
        if size:
            self.buy(size=size, ...)

In case of Bitcoin, one would first need to convert that to non-fractional uBTC or satoshis.

[atakanokan]

So the logic is if trade_dollar_amount // price < 1 then convert asset price to a lower value so that trade_dollar_amount // price >= 1? + do the inverse transformation to the Volume?

[kernc]

Integer division (//) will produce only integers (0, 1, ...).

If the price is often high like Bitcoin's, that you might only buy some fraction of units (backtesting.py doesn't support ordering fractional units), convert the whole data to some smaller units like in the example, yes.