I was able to find a solution,

self.data.close.get(size=1, ago=-1)[0]

works right as I want

@backtrader said in Bar data N days back:

It would seem you are unaware of the Python arithmetic with arrays.

I understand the python arrays but this logic is unacceptable here, because we can't get future data, so the -1 has to be the previous day, -2 the day before it and so on and if no such day is available to throw an exception, but not start everything in a circle.

@backtrader said in Bar data N days back:

Using a delayed index provides the proper automatic constraint to the platform.

Can you show me an example of proper way to get data for 2 days back or exeption if no such data is available?

I'm completely confused. Is there a simple way to get data N days back?

@backtrader said in Bar data N days back:

You are simply doing things wrong. But you can disable preloading: Docs - Cerebro

Thank you for your prompt reply. Finally I was able to get it right.

The only moment by disabling preloading I was able to get

next:: current period: 1
date 2018-01-01 23:59:59.999989, current price 50.10, previous price 50.10

it's certainly better, but still not how it should

@backtrader said in Bar data N days back:

Because the data is preloaded

Can this behavior be disabled? It really confuses me.

I want to access the data N days ago. I have written the following code

    def next(self):
        print('next:: current period:', len(self))
        print('date %s, current price %.2f, previous price %.2f' % (self.datas[0].datetime.datetime(), self.data.close[0], self.data.close[-1]))

the dataset I use consists of 3 lines

2018-01-01,50.1,...
2018-01-02,50.2,...
2018-01-03,50.3,...

and the output is

next:: current period: 1
date 2018-01-01 23:59:59.999989, current price 50.10, previous price 50.30
next:: current period: 2
date 2018-01-02 23:59:59.999989, current price 50.20, previous price 50.10
next:: current period: 3
date 2018-01-03 23:59:59.999989, current price 50.30, previous price 50.20

How can day 1 previous price be 50.30? Why it takes the value of the last day and doesn't throws an error?