In some situations, the cash amount in real brokers may be decreased because the operation on assets includes an interest rate. Examples:

- Short selling of stocks
- ETFs both long and short

The charge goes directly against the cash balance in the broker account. But it
can still be seen as part of a commission scheme. And as such it has been
modeled in *backtrader*.

The `CommInfoBase`

class (and with it also the `CommissionInfo`

main
interface object) has been extended with:

- Two (2) new parameters that allow setting the interest rate and determining if should be applied only to the short side or to both long and short

`interest`

(def:`0.0`

)If this is non-zero, this is the yearly interest charged for holding a short selling position. This is mostly meant for stock short-selling

The default formula applied:

`days * price * size * (interest / 365)`

It must be specified in absolute terms: 0.05 -> 5%

Note

the behavior can be changed by overriding the method:

`get_credit_interest`

`interest_long`

(def:`False`

)Some products like ETFs get charged on interest for short and long positions. If ths is

`True`

and`interest`

is non-zero the interest will be charged on both directions

The default implementation will use the following formula:

```
days * abs(size) * price * (interest / 365)
```

Where:

`days`

: number of days elapsed since position was opened or the last credit interest calculation took place

In order to change the formula subclassing of `CommissionInfo`

is needed. The
method to be overridden is:

```
def _get_credit_interest(self, size, price, days, dt0, dt1):
'''
This method returns the cost in terms of credit interest charged by
the broker.
In the case of ``size > 0`` this method will only be called if the
parameter to the class ``interest_long`` is ``True``
The formulat for the calculation of the credit interest rate is:
The formula: ``days * price * abs(size) * (interest / 365)``
Params:
- ``data``: data feed for which interest is charged
- ``size``: current position size. > 0 for long positions and < 0 for
short positions (this parameter will not be ``0``)
- ``price``: current position price
- ``days``: number of days elapsed since last credit calculation
(this is (dt0 - dt1).days)
- ``dt0``: (datetime.datetime) current datetime
- ``dt1``: (datetime.datetime) datetime of previous calculation
``dt0`` and ``dt1`` are not used in the default implementation and are
provided as extra input for overridden methods
'''
```

It might be that the *broker* doesn’t consider weekends or bank holidays when
calculating the interest rate. In this case this subclass would do the trick

```
import backtrader as bt
class MyCommissionInfo(bt.CommInfo):
def _get_credit_interest(self, size, price, days, dt0, dt1):
return 1.0 * abs(size) * price * (self.p.interest / 365.0)
```

In this case, in the formula:

`days`

has been replaced by`1.0`

Because if weekends/bank holidays do not count, the next calculation will
always happen `1`

trading da after the previous calculation