Custom Labels/Indicator with events from a strategy

RandomTrader88

I developed a strategy, and i would like to visually view specific "events". The logic of determining the events can be found in the strategy. in other words: In the strategy i have all the if statements in order to determine the exact time that i want to see this event on the graph.

Is it possible to add labels on bars, from the strategy?

Alternatively, and this is not so pretty solution but it will work, i think about developing a custom indicator and change it from 0 to 1 for just 1 bar when this event occurs, but i'm not sure how to do it from the strategy itself (i don't want to duplicate the entire logic of the strategy to a new indicator)

How can i do that?
Do you have any code sample for that?

ab_trader

Write an observer.

Docs - Observers

Code samples -
https://github.com/mementum/backtrader/tree/master/backtrader/observers

RandomTrader88

Thanks, i see that observers can display events like buys/sells. But how do i signal into an observer from the strategy itself? how can i code something like this?

if (condition):
   myobserver.event1[0]=1 #change value in the observer somehow

is it possible to do so from the strategy?
I don't want to duplicate the entire if-logic to the observer just to display a value.

I will also mention that i do see a specific case that it is possible: in the Observers example i see the OrderObserver that displays information about orders, but it is only because the strategy generates orders and the observer catches them using the next() function. but what about just any events, without generating orders? just generating the event from the strategy?

RandomTrader88

OK i found a solution
Observer can access the strategy using self._owner so we can do the following:

in the Observer, we listen to anything inside an self._owner.events array (i just made up the name .events):

def next(self):
    while len(self._owner.events)>0:
        event,value = self._owner.events.pop(0)
        getattr(self.lines,event)[0]=value

and in the Strategy, we just send events:

def __init__(self):
    self.events=[]

if (CONDITION):
    self.events.append(["event_x",1])

Note that event_x must be the name of a line defined in the observer.

ab_trader

if you define self.events in the strategy as bt line instead of the list, than in the observer's next() its values can be accessed simply

last_event_value = self._owner.events[0]

ab_trader

Here is the link to observer I wrote to plot stop and take prices defined as self.sl_price and self.tp_price in the strategy - https://community.backtrader.com/post/1856